GMC Truck Forum banner

1 - 15 of 15 Posts

·
Registered
Joined
·
275 Posts
Discussion Starter · #1 ·
The PCM control's many devices related to the fuel and air system's. In order to make operating decision's the PCM depends on information from a network of sensor's and switches located thoroughout the vehicle.

TPS (Throttle Postion Sensor)
The TP sensor is mounted to the throttle body and operated by the throttle shaft. The sensor uses a 5 volt reference signal to indicate the throttle opening position. When the throttle is closed,the PCM reads a low voltage signal. As the throttle is opened,the PCM reads a higher voltage signal.

CKP(Crankshaft position sensor)
The CKP sensor is a variable reluctance sensor that provides the PCM with the crankshaft speed and position. The PCM uses this information to indicate engine RPM, as well as when to fire the ignition coils and the fuel injector's.

CMP(Camshaft Mean Position sensor)
The CMP sensor identifies when piston no.1 is on the compression stroke. A signal from the sensor is sent to the PCM and is used for synchronizing the firing of the fuel injector's. It is also used to synchronize the firing of the coil's on Coil-on-Plug ignition system's.

MAP(Manifold Absolute Pressure sensor)
This sensor measure's pressure changes in the intake manifold,which also indicate engine load. The PCM uses this information to help calculate fuel and spark timing requirement's. When the key is on and the engine is not running,the manifold is at atmospheric pressure. The MAP sensor is registering this as barometric(BARO) pressure. The BARO reading is used at start-up and high throttle position's for fuel delivery calculation's.

MAF(Mass Air Flow meter/sensor)
The MAF sensor is a hot-wire sensing element placed directly in the air path to the intake manifold,before the IAC and throttle plates. It directly measures incoming air mass entering the engine using the hot-wire sensing element. Air passing over the hot wire absorbs hear from the wire. The MAF circuitry increases or decreases currenty flow through the wire to maintain a constant temperature of 200*C(392*F) above the ambient tempature. The voltage output of the MAF sensor is proportional to the current necessary to maintain the temerature of the hot wire. The MAF sensor is the only input the PCM needs to determine air mass.

IAT(Intake Air Tempature sensor)some call this an intake charge sensor.
The IAT sensor is a thermistor device that is mounted in the air intake assembly or threaded into a cylinder runner of the intake manifold. The IAT recieve's a voltage reference signal from the PCM. The resistance in the IAT changes with temperature, the varying resistance affects the voltage drop across the sensor terminal's and provides a voltage signal back to the PCM. The PCM uses this signal to help proportion the cold enrichment fuel flow and to modify spark advance.

HO2S(Oxygen sensor) 02 sensor
An HO2S sensor is used in the system to measure the amount of oxygen in the exhaust stream. The tip of the sensor is made of zirconium dioxide ceramic, and is covered with platinum. It generate's voltage by the chemical reaction between the two dissimilar materials. Part of the sensor is exposed to outside air as a reference,while the tip of the sensor is exposed to exhaust gases. When the sensor is heated to over 177*C(350*F), voltage is generated by the difference between oxygen content at the tip compared to the outside oxygen. The greater the oxygen content in the exhaust stream,the lower the voltage output(Lean is close to "0" volt's and rich is close to "1"volt.)
*Wideband 02 sensor's have a voltage between 0-5 volt's compared to a narrowband 02 sensor @ 0-1 volt. Stock sensor's are narrowband and are ONLY accurate at 14.7:1 A/F ratio,other than that it can only tell if your richer or leaner but not how much*

The number of 02 sensor's installed in the exhaust system may vary depending on the type of vehicle or engine. Regardless of the type of system at least two 02 sensor's are alway's used. one sensor is placed before the atalytic converter and one sensor is placed after the converter. The 02 sensor before the converter is used to supply input to the PCM for fuel calculation's. The sensor after the catalytic converter is used to reference the emissions to ensure that the converter is functioning properly.

ECT(Engine Coolant Tempature sensor)
The ECT sensor is a thermistor device that measures the tempature of the coolant. The ECT receives a voltage reference signal from the PCM. The resistance in the ECT changes with temperature, the varying resistance affect's the voltage drop across the sensor terminal's and provides a voltage signal back to the PCM. The PCM uses this signal to help modify ignition timing,EGR flow, and air/fuel ratio. The ECT is generally threaded into the coolant passage on the engine.

CHT (Cylinder Head Temperature sensor)
The CHT sensor is also a thermistor device. The CHT is installed in the cylinder head and measures the metal tempature. The CHT receive's a voltage reference signal from the PCM. The resistance in the CHT changes with temperature,the varying resistance affects the voltage drop across the sensor terminals and provides a voltage signal back to the PCM. This signal is used when an engine has both an ECT and a CHT sensor,for the fail-safe cooling strategy. On engine's that have only CHT sensor's,the PCM calculates engine coolant temperatures based on CHT sensor values.

KS (Knock sensor)
The knock sensor is a tuned accelerometer mounted on the engine which allow's the PCM to control the engine ignition timing so that the best possible performance can be achieved while protecting the engine from detonation. The knock sensor does this by converting engine vibration to an electrical signal,which upon detonation is sent to the PCM. If detonation occurs the PCM retards spark timing until detonation stops and then begins advancing depending on the input from the knock sensor.


Hope that give's some people a better idea of what some of the engine sensor's do.
I might add the EGR info on here also,I think i've already posted it on another site somewhere.
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #4 ·
If it's the 98 in your sig then it's under the hood on the driver's side next to the ABS motor.
If it's the deville....good luck with that.
I'll copy and paste some other stuff I have on another forum that goes along with this.

THIS STUFF IS GENERAL INFO,I MIGHT HAVE LEFT STUFF IN THAT DOESN'T APPLY TO GM.
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #5 ·
Since the late 1980’s, the powertrain control system has used On-Board Diagnostics (OBD) to monitor a limited number of systems (fuel, EGR, and O2) and detect input component failures.

Due to increased diagnostic requirements by the California Air Resource Board, OBD II was introduced. OBD II is a real-time on-board monitoring system that is part of the PCM. It consists of software, sensors, and a Malfunction Indicator Light (MIL). OBD II monitors virtually all emission related components and systems as the vehicle is being driven. The objectives of OBD II are to:

reduce high in-use emissions caused by emission-related malfunctions,
reduce time between occurrence of a malfunction and its repair,
assist in the diagnosis and repair of emission-related problems.

Due to increased diagnostic requirements by the California Air Resource Board, OBD II was introduced. OBD II is a real-time on-board monitoring system that is part of the PCM. It consists of software, sensors, and a Malfunction Indicator Light (MIL). OBD II monitors virtually all emission related components and systems as the vehicle is being driven. The objectives of OBD II are to:

reduce high in-use emissions caused by emission-related malfunctions,
reduce time between occurrence of a malfunction and its repair,
assist in the diagnosis and repair of emission-related problems.

Legend
1. PCM with OBD II Software:x
2. Misfire Monitor:The misfire monitor detects variations in crankshaft revolutions using information from the crankshaft position sensor. The acceleration for each cylinder can then be calculated by determining how much time each tooth on the reluctor wheel takes to pass the crankshaft sensor. Cylinders that misfire do not show the expected acceleration.
3. Comprehensive Component Monitor:The comprehensive component monitor looks at any PCM input or output that affects emissions. This refers only to inputs and outputs that do not have their own monitoring requirements. Inputs are monitored for opens, shorts, and an expected value. Outputs are monitored for opens, shorts, and expected response for a particular command.
4. Fuel System Monitor:The fuel system monitor looks for short-term and long-term fuel trim values that have exceeded emission malfunction threshold values. These thresholds are predetermined per vehicle design, and are part of the OBD II strategies incorporated into the PCM.
5. EGR System Monitor:The EGR system monitor checks components for opens, shorts, and expected input values. The monitor also checks flow rate by comparing differential pressures across the flow control orifice.
6. HO2S Monitor:The Heated Oxygen Sensor (HO2S) monitor monitors the output voltage and response rate of the upstream O2 sensors, and the voltage of the downstream sensors for proper operation. The front and rear HO2S heaters are monitored for proper voltage and current.
7. Thermostat Monitor:The thermostat monitor checks engine coolant temperature to verify that it is within 11° C (20° F) of the thermostat regulating temperature. This occurs within a specified time period after cold engine startup.
8. AIR System Monitor:The AIR system monitor checks electrical components and confirms that the pumped air is entering the exhaust stream when it is commanded to do so by the PCM.
The air/fuel ratio is commanded rich, the air pump is turned on, and the time required for the HO2S to go lean is monitored.
9. EVAP Monitor:The EVAP monitor tests the EVAP system for the ability to purge HCs by looking at the HO2S for a lean value and an increase in RPM. The monitor also creates and holds a vacuum on the EVAP system to check for small and large leaks.
10. Catalyst Monitor:The catalyst monitor reads the upstream (A) and downstream (B) HO2S switches and compares them to each other to determine catalyst efficiency.
For this example, an upstream reading of 125 and a downstream reading of 7 gives a catalyst efficiency of 7/125 = 0.056.
11. MIL:Malfunction Indicator Lamp
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #6 ·
NORMAL COMBUSTION

In the combustion chamber air (O2 and N2 ) and hydrocarbon (HC) fuel, which is made up of different combinations of hydrogen (H) and carbon (C), mix, compress, ignite, and burn. The air and fuel molecules recombine into different substances during combustion, and are expelled from the combustion chamber through the exhaust valve.

IDEAL COMBUSTION

The byproducts of the hydrocarbons and air during ideal combustion are carbon dioxide (CO2), water (H2O) and nitrogen (N2

REALISTIC COMBUSTION

If the fuel and air does not burn completely, the air and fuel mixture is too rich or too lean, or cylinder temperatures are too high, then combustion byproducts [carbon monoxide (CO), oxides of nitrogen (NOx), and hydrocarbons (HC)] will form. These byproducts form smog, which is the result of a complex series of chemical reactions that occur in the atmosphere.

The systems used to regulate or clean emissions are shown below

1. Exhaust Gas Recirculation
2. Positive Crankcase Ventilation
3. Air Injection Reaction
4. Catalytic Converter
5. Evaporative Emission

Exhaust and evaporative emissions typically include the elements shown in the graphic below
NOx is a byproduct of combustion at temperatures exceeding approximately 1372° C (2500° F).

Hydrocarbons
Unburned HC comes from evaporation of liquid fuel (evaporative emission) or fuel that is not burned during combustion (exhaust emission).

Carbon Monoxide
CO is formed as a product of the incomplete combustion due to the lack of oxygen

Oxygen
O2 is unused oxygen from the combustion process
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #7 ·
The Positive Crankcase Ventilation (PCV) system (A) vents the vapors from the crankcase and delivers them to the engine intake manifold, where they mix with the intake air for combustion.

BLOW-BY
During normal engine operation, fuel vapor and exhaust gases created in the combustion chamber leak past the piston rings (referred to as blow-by) and enter the crankcase. This blow-by and condensation if not controlled, can contaminate the engine oil, damage seals and gaskets, and eventually enter the atmosphere.

PCV System Function
On most engine applications, a vacuum operated valve (A) is used to keep crankcase vapors in check. Fresh air is added to the crankcase through a remote air cleaner (B), mixes with the crankcase vapors, and is redirected back into the intake manifold to be burned in the combustion chamber. This is referred to as crankcase scavenging.

PCV Valve
The PCV valve consists of a housing with openings on both ends and a spring-loaded plunger. The plunger moves up and down to seal the crankcase, seal the intake port, or allow crankcase pressures to purge.

PCV VALVE OPERATION
At idle, manifold vacuum is high. Vacuum draws the plunger up to seal the intake port of the valve and restrict vapor flow.
At cruise speeds, vacuum decreases. Spring pressure overcomes vacuum and the plunger moves downward and allows vapor to be drawn into the intake port at a moderate rate.
Under heavy load or wide-open throttle, engine vacuum is low. This allows spring pressure to open the valve even more, which allows vapor to flow at a higher rate.
If the engine should backfire, intake pressure closes the valve to prevent excessive pressures and flame from entering the crankcase.
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #8 ·
Evaporative Emission Systems


Fuel or Hydrocarbons (HC), if not contained, will evaporate into the atmosphere and contribute to photochemical smog, ozone deterioration, and eye irritation. In the fuel tank, raw fuel (when heated) evaporates and may cause HC pollutants to escape into the atmosphere. The Evaporative Emission Systems (EVAP) system collects these vapors, stores them, and then disposes of them through combustion.


Fuel vapors rise to the top of the fuel tank and out the vent valve. They are routed through tubes to the fuel vapor control valve where any raw fuel is separated from the vapors and sent back to the fuel tank. From there, the vapors collect in the charcoal canister until the powertrain control system commands the purge solenoid to open and allow the gases to be sent to the intake manifold, where they are directed to the combustion chamber and burnt. This only occurs when drivability and emissions will not be affected, but a varying percentage of purge can occur under most closed loop operating conditions.

The early EVAP system includes the following basic components:
1. Purge valve:The purge valve is electronically controlled by the Powertrain Control Module (PCM). During conditions where engine emissions are least affected, the valve opens and allows fuel vapors to be drawn into the intake manifold.
2. Intake
3. Charcoal canister:The charcoal canister stores fuel vapors until the powertrain control system opens the purge valve or vapor management valve and allows them to be drawn into the intake manifold.
4. PCM
5. Fuel tank
6. Rollover/vent valve:The rollover/vent valve prevents raw fuel from entering the charcoal canister, and directs the vapors out of the fuel tank. It also prevents fuel from leaking out of the tank if the vehicle rolls over.
7. Fuel cap


The Non-Enhanced EVAP system is a flow monitoring system. It includes the components of the EVAP system plus the purge flow sensor (A) or the vapor management valve (B).

NOTE: the Non-Enhanced EVAP system includes either a purge valve and purge flow sensor or the vapor management valve, but not both.


The Enhanced EVAP system is a leak monitoring system. It includes the components of the EVAP system plus the vapor management valve (B), service port (C), canister vent solenoid (D), fuel tank pressure sensor (E), and fuel level signal to the PCM.

TIP: Typically, if the vehicle has a service port then it uses an Enhanced EVAP system

The On-Board Refueling Vapor Recovery (ORVR) seals the fuel tank filler neck at the fuel nozzle during refueling. This causes the air in the fuel tank to be displaced through the charcoal canister, trapping any HCs it contains.
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #9 ·
The Exhaust Gas Recirculation (EGR) system (A) is used to lower combustion temperature to reduce NOx emissions.

As an engine reaches normal operating temperature, the combustion chamber can reach very high temperatures. When temperatures begin to approach 1,372º C (2500º F), nitrogen and oxygen start to chemically combine to form Oxides of Nitrogen (NOx). To reduce NOx formation, the combustion temperature must be lowered.

Enriching the fuel mixture, lowering the compression ratio, recirculating exhaust gases (EGR), or retaining exhaust gases (variable cam timing) are all ways to reduce combustion temperature.

Enriching the fuel mixture:
Although the PCM can control air/fuel ratio, it is not recommended to enrich the fuel mixture to control combustion temperatures. This reduces fuel economy and produces higher Hydrocarbon (HC) and Carbon Monoxide (CO) emissions.

Lowering compression ratio:
It is not recommended to lower compression ratios because fuel economy and engine performance are reduced under all conditions.

Recirculating exhaust gases:
EGR is used because it can be controlled. Exhaust gas is an inert gas and does not support combustion. If the air/fuel mixture is diluted with small amounts of EGR (between 6 and 10 percent by volume), then combustion temperatures are lowered and NOx is reduced. Fuel economy increases because ignition timing is advanced with the addition of EGR. EGR is introduced by the powertrain control system when engine performance or driveability is not compromised.

Variable cam timing (VCT):
VCT is used to adjust exhaust valve timing. Closing the exhaust valve early allows a small amount of inert exhaust gases to be retained in the combustion chamber, which helps lower combustion temperatures.

Components of the EGR system
1. Vacuum EGR Valve:The vacuum EGR valve is a sealed component that has a spring-loaded, vacuum operated diaphragm.The diaphragm controls a pintle valve regulating the flow of exhaust gas into the intake manifold.When vacuum is applied to the diaphragm, it draws the diaphragm up and opens the port to the intake manifold. Exhaust gas is then redirected into the intake manifold. Vacuum to the diaphragm can be regulated to control pintle valve position. When vacuum is removed from the diaphragm, spring pressure closes the port to the intake manifold.
2. Stepper Motor EGR Valve:The stepper motor EGR valve is electronically controlled by the PCM. The PCM sends a signal that extends or retracts the pintle valve and then the EGR valve sends a return signal back to the PCM to indicate valve position.
3. EGR vacuum regulator (EVR) Solenoid:The EGR Vacuum Regulator (EVR) valve is a normally closed valve that, when commanded by the powertrain control system, opens and regulates the flow of vacuum to the EGR valve.

The EGR feedback type's were non-GM specific so I left that out.
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #10 ·
The secondary AIR system, whether belt-driven mechanical or electrical air pump, is used to reduce the amount of HCs and CO by injecting air directly into the exhaust system. The air can be directed either upstream into the exhaust manifold or downstream into the catalytic converter depending on the system used.

The mechanical AIR system includes the following components.

1. Mechanical AIR pump:The mechanical AIR pump is a belt driven, positive displacement, vane-type pump.
2. AIR check valves:The one-way check valve is used to prevent exhaust gases from backing up in the AIR system and melting components with hot exhaust gases.
3. Normally closed AIR bypass solenoid:pCM controlled solenoids are "on" or "off" devices that direct the flow of vacuum.
4. Normally closed AIR bypass valve:The bypass valve is used only on the mechanical AIR pump system. This valve directs airflow to the exhaust stream, the diverter valve or to the atmosphere
5. Diverter valve:The diverter valve receives air from the bypass valve, on mechanical systems, or directly from the electric pump. The diverter valve directs airflow "upstream" or "downstream".
6. Normally closed AIR diverter solenoid:pCM controlled solenoids are "on" or "off" devices that direct the flow of vacuum

Mechanical System:
A typical mechanical AIR pump belt-driven system constantly supplies air to the normally closed bypass valve. When AIR is needed, the PCM sends a ground signal to the bypass valve solenoid. The bypass valve solenoid sends a vacuum signal to the bypass valve, opening the valve and routing air to the exhaust manifold through the one way check valve.
Adding air into the exhaust manifold (upstream) continues combustion. This reduces the levels of HCs and CO in the exhaust. The additional heat generated is used to help the catalytic converter come up to operating temperature.

If the vehicle is equipped with both a Three-Way Catalyst (TWC) and a Conventional Oxidation Catalyst (COC), a diverter valve and diverter solenoid are added to the system between the bypass valve and the check valves.
After the PCM grounds the bypass solenoid, the solenoid turns on, directing vacuum to the bypass valve, which then routs air to the diverter valve. The PCM then turns the diverter solenoid off, eliminating vacuum to the diverter valve. The diverter valve then directs air downstream into the COC.

Electrical System:
A typical electric AIR pump system does not require a bypass valve, and only pumps air upstream to the exhaust manifold on a cold start. The PCM commands the AIR pump "on" and "off" via the SSR as needed so there is never excess air to bypass to the atmosphere
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #11 ·
Exhaust And Catalytic Converters


With increasing exhaust emissions, a catalytic converter (A) was added to the exhaust system in the mid-1970s. Catalytic converter technology has steadily improved, and as a result, the modern day catalyst has become the cornerstone of emission control devices.

The function of the catalyst is to chemically alter or "convert" HCs, CO, and NOx gases in the exhaust to environmentally safe gases by heating precious metals such as platinum, palladium, rhodium, and ceria.

1. HCs, CO, NOx
2. HCs + O2 => CO2 + H2O Imagine #1. gases going into a cat
3. CO + O2 => CO2 and #'s 2. 3. and 4. coming out.
4. NOx + H2 => N2 + H2O

As the catalyst heats up, converter efficiency rises rapidly. The point at which conversion efficiency exceeds 50% is called catalyst light-off. For most catalysts this occurs at 246-301° C (475-575° F) (A). Once the catalyst lights-off, the catalyst will quickly reach the maximum conversion efficiency for that catalyst
The amount of emissions created in the combustion chamber is effected by the air and fuel ratio. For diagnostic purposes, by looking at either the emissions (using a 5 gas analyzer) or the air/fuel ratio (using a scan tool) you may be able to better understand where the concern is.

In general, there are three different catalyst system types. The Conventional Oxidation Catalyst (COC), Three-Way Catalyst (TWC), and TWC + COC. The only type of catalyst that is currently monitored by On-Board Diagnostics II (OBD II) is the TWC.

COCs were some of the first catalysts used on vehicles. These catalysts converted only HCs and CO. Secondary air injection was used to provide the extra oxygen needed for proper conversion (A). Since a COC does not provide NOx conversion, most vehicles controlled the air/fuel ratio slightly rich to reduce NOx while using the COC to control HCs and CO.

TWCs combine an oxidation function to control HCs and CO and a reduction function to convert NOx. The reduction function is made possible by adding materials like ceria. Ceria has the ability to store and release oxygen, thus eliminating the need for pumped in air. TWCs are used on almost all vehicles today because they are capable of performing conversion for all three regulated emissions.

For some vehicle applications, both the COC and the TWCs were used. Some used separate catalysts and some combined both catalysts into one housing. Secondary air was injected upstream into the exhaust manifold during startup and injected downstream into the middle of the catalyst system. The downstream air provided the extra oxygen required by the COC while allowing the TWC to operate using a stoichiometric air/fuel ratio (14.7:1)
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #12 ·
Ignition/Fuel and Air Inlet System Relationships"

------------------------

The IAC valve uses an adjustable bypass port to meter air around the throttle plate during engine cranking and to control idle speed.

The IAC valve action closely follows the throttle. Whenever the throttle is depressed rapidly, the IAC will open with it. When the throttle is released rapidly, the IAC will "feather" back to idle to prevent stalling due to the sudden absence of intake air.

During engine crank and at wide open throttle, the PCM commands the IAC to a pre-determined duty cycle. The PCM is initially calibrated with a duty cycle based on engine configuration. As the IAC valve is operated over time, changing conditions (such as engine wear) require adjustments to the duty cycle. The PCM monitors engine data and adjusts the duty cycle as necessary to maintain the target idle speed.

-------------------------

The amount of air entering the engine must be determined so that the proper amount of fuel can be delivered. The quantity of air entering the engine is known as air mass. The two systems used to measure air mass are the Speed Density system and the Mass Air Flow system.

In the speed density system, the PCM uses many sensor inputs to calculate incoming air.
In the mass air flow system, the PCM calculates air mass using only the MAF sensor to directly measure incoming air.


-------------------------------------

In the speed density system, the PCM determines air mass using various inputs. A formula typically used by the PCM to calculate air mass is shown below.

MAP, RPM, EGR mass, and volumetric efficiency affect the fuel calculation. IAT and ECT inputs also affect the fuel calculation, but to a much lesser extent.

Click on each element in the equation to learn how that factor affects the air mass measurement.


PCM
K
Air Mass = ( K x MAP x RPM x Volumetric Efficiency x ECT correction x IAT correction ) - EGR Mass

------------------------

During engine cranking, the air mass is not calculated (MAP is not used). Instead, the PCM commands the IAC valve to a predetermined duty cycle of 50 to 100%. The amount of air that enters the engine is determined by the size of the idle air passage, the small amount of air that goes past the closed throttle plate, and a stored barometric pressure value

--------------------------

The Mass Air Flow (MAF) system directly measures the incoming air mass using a sensor placed in the incoming air stream. The MAF sensor is the only input the PCM needs to determine air mass
======
The MAF sensor measures the mass of air as it enters the engine using a hot wire-sensing element (A). Air passing over the hot wire absorbs heat from the wire. The MAF circuitry increases or decreases current flow through the wire to maintain a constant temperature of 200°C (392°F) above ambient air temperature. The voltage output of the MAF sensor is proportional to the current necessary to maintain the temperature of the hot wire.

The rate at which heat is absorbed by the air is directly related to the air mass.


TIP: If the wire becomes contaminated or obstructed, MAF readings will not be accurate.
======
During engine cranking, the MAF sensor is not used. Instead, the PCM commands the IAC valve to a predetermined duty cycle of 50 to 100%. The amount of air that enters the engine is determined by the size of the idle air passage, the small amount of air that goes past the closed throttle plate, and a stored barometric pressure value.
======
The MAF signal is also used to calculate load. Load is determined by the amount of air entering each cylinder per intake stroke compared to the cylinder displacement. Load is a numerical representation of volumetric efficiency, and typically ranges from 0 to 100% on naturally aspirated engines.

The PCM uses the load calculation to help schedule:

EGR delivery
Spark timing
Desired air/fuel ratio

The mass air reading requires no compensation for altitude (BARO) or EGR

-------------------------

Once the air mass has been determined, the amount of fuel needed by the engine must also be calculated. The fuel delivery system delivers fuel under pressure to the injectors. The fuel control system uses injector pulse width to control the amount of fuel delivered.

The fuel delivery process takes into account a wide range of factors and inputs:

The desired air/fuel ratio changes with engine operating conditions.
The PCM selects open-loop or closed-loop fuel system operation based on the desired air/fuel ratio for the engine operating conditions.
Fuel trims are used to maintain the target air/fuel ratio.
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #13 ·
The purpose and function of the fuel delivery system is to draw fuel from the fuel tank and provide filtered fuel at regulated pressure to the fuel supply manifold for delivery by the injector.

Types of fuel delivery systems include:

Return-type
Mechanical Returnless
Electronic Returnless
In return-type fuel systems, excess fuel not used by the engine is returned to the fuel tank. In returnless fuel systems, fuel is delivered to the fuel supply manifold with no engine-heated fuel returned to the tank. This helps to lower fuel tank temperatures and reduce evaporative emissions.

-----------------------------------
The amount of fuel to be delivered by the injector is determined by the fuel control system.

Fuel control depends on:

How much air enters the engine (air mass),
The desired air/fuel ratio for the engine operating conditions,
How much fuel is needed to achieve the desired air/fuel ratio (fuel mass), and
The injector pulse width required to deliver the correct amount of fuel to the proper cylinder.
===========
The PCM uses the air mass measurement and desired air/fuel ratio to calculate the fuel mass. It then uses the fuel mass calculation to determine the appropriate injector pulse width. The pulse width is the length of time the PCM turns the injector on, and is measured in milliseconds.

Air Mass-->Fuel Mass

Fuel Mass-->Pulse Width
==============
Although stoichiometric is considered the ideal air/fuel ratio, there are many operating conditions in which a stoichiometric ratio is not desired. When operating conditions require an air/fuel ratio other than stoichiometric, or the oxygen sensors are not at operating temperature, the fuel system is commanded to open loop mode.

When the engine is operating in open loop, the PCM ignores oxygen sensor input and commands an air/fuel ratio that is typically richer than stoichiometric. Once the desired air/fuel ratio and the air mass are determined, the PCM calculates the appropriate injector pulse width. The PCM commands open loop operation under the following conditions:

COLD ENGINE STARTUP

During cold engine start-up, the oxygen sensor does not produce an accurate signal because it has not reached operating temperature. The PCM is programmed to wait a certain amount of time after starting before attempting to go to closed loop operation. During warm-up operation, air/fuel ratio is commanded rich to aid in engine and catalytic converter warm-up.

HIGH LOAD OR WOT

During high load or wide-open throttle conditions, the air/fuel ratio is commanded rich for maximum power.

CATALYST OVERTEMP PROTECTION

During catalyst overtemp protection conditions, the air/fuel ratio is commanded rich because a richer air/fuel ratio will burn cooler. This lowers the catalyst temperature to prevent damage.

==========
Once the oxygen sensor has reached operating temperature and open loop conditions are not demanded, the PCM commands a stoichiometric air/fuel ratio and the engine operates in closed loop. In closed loop operation, the PCM calculates air mass to determine injector pulse width. Feedback from the oxygen sensor indicates if the mixture is rich or lean. The PCM uses this information to make constant adjustments to the commanded injector pulse width to achieve a stoichiometric air/fuel ratio.
===========
Fuel is injected into the engine, burned, and exits to the exhaust stream. As the exhaust passes the oxygen sensor, the sensor reads the oxygen content in the exhaust and sends a signal to the PCM.

The oxygen sensor can only indicate if the mixture is richer or leaner than stoichiometric. During closed loop operation, short term fuel trim values are calculated by the PCM using oxygen sensor inputs in order to maintain a stoichiometric air/fuel ratio. The PCM is constantly making adjustments to the short term fuel trim, which causes the oxygen sensor voltage to switch from rich to lean around the stoichiometric point. As long as the short term fuel trim is able to cause the oxygen sensor voltage to switch, a stoichiometric air/fuel ratio is maintained.

Short term fuel trim operates within pre-calibrated limits. If short term fuel trim is not able to cause the oxygen sensor to switch, a DTC will be set.
====================
Fuel Mass = [air mass/14.7] x short term fuel trim x long term fuel trim


Short term fuel trim (SHRTFT) values are displayed on a scan tool as a percentage of fuel added or subtracted. Typically, the short term fuel trim value switches above and below zero percent. Zero percent on the scan tool means there is no adjustment and the PCM multiplies the air mass by 1. If the percentage is positive, the PCM multiplies by a value greater than 1 to increase the fuel mass calculation, and if the percentage is negative, the PCM multiples by a value less than 1 to decrease the fuel mass calculation.
SHRTFT Scan Tool Reading-SHRTFT Multiplier Value-Fuel Impact
0% 1.0 No adjustment

15% 1.15 Increase fuel mass

-15% 0.85 Decrease fuel mass
===========
In open-loop operation, short term fuel trim values are taken from a table in the PCM and are typically positive. When initially entering closed loop fuel operation, short term fuel trim begins adding or subtracting fuel in order to make the oxygen sensor switch from its current state.
============
If the oxygen sensor signal sent to the PCM is greater than 0.45V, the PCM considers the mixture rich and short term fuel trim shortens the injector pulse width. When the cylinder fires using the new injector pulse width, the exhaust contains less fuel and more oxygen. Now when the exhaust passes the oxygen sensor, it causes the voltage to switch below 0.45V, the PCM considers the new mixture lean, and short term fuel trim lengthens the injector pulse width. This cycle continues as long as the fuel system is in closed loop operation.
=============
As components of the fuel, air, or engine systems age or otherwise change over the life of the vehicle, the PCM learns to adapt fuel control. This is known as adaptive fuel strategy. Corrections are only learned during closed loop operation, and are stored in the PCM as long term fuel trim values. Long term fuel trim values are only learned when short term fuel trim corrections are able to cause the oxygen sensor to switch.

If the short term fuel trim average remains above or below zero percent, the PCM learns to use a new long term fuel trim value, which allows the short term fuel trim to return to an average value near zero percent. There is a different long term fuel trim value stored for various RPM and load operating conditions. Long term fuel trim values are displayed on a scan tool as a percentage of fuel added or subtracted.

Long term fuel trim also operates within pre-calibrated limits. When those limits are reached, a DTC will be set.
TIP: The long term fuel trim value displayed on the scan tool is the value being used for the current operating condition.
==================
During open loop operation, short term fuel trim values come from lookup tables in the PCM. The specific value selected is based primarily on RPM, load, and engine coolant temperature. In open-loop operation, short term fuel trim values are typically a positive percentage, resulting in a rich air/fuel ratio. Although the oxygen sensor is not used for fuel control, it will reflect this rich condition.

Long term fuel trim values learned during closed loop operation are used in both open and closed loop modes
-------------------------


After air measurement and fuel delivery are complete, the air/fuel charge needs a quality spark delivered at exactly the right moment to ignite the compressed air-fuel mixture.

The cylinders must fire in the proper order and at the precise time, depending on engine speed and load. Proper ignition is critical to engine performance, fuel economy, and exhaust emissions.

The PCM uses inputs from sensors of both the air and fuel systems to help in calculating spark advance
==================
The PCM calculates spark advance based primarily on RPM and load in an effort to achieve the best engine performance for current operating conditions.

Spark is less advanced for the following conditions:

Rich air/fuel mixture
Low RPM operation
Heavy load
High intake air temperatures
High engine coolant temperatures

Spark is more advanced for the following conditions:

Lean air/fuel mixture
High RPM operation
Low intake air temperatures
Low engine coolant temperatures
EGR flow
============

If load remains constant and RPM increases, timing advance increases because the piston is moving faster.

If RPM remains constant and load increases, timing advance decreases because there is more fuel and higher compression pressure in the cylinder, which causes the mixture to burn faster.

If both load and RPM increase, the PCM will select the appropriate timing advance for that combination
=========
Various inputs to the PCM, such as ECT, IAT, and EGR mass, may result in minor timing adjustment
=========
There are other spark control strategies to stabilize engine idle. Spark timing, also known as ignition timing, may be advanced or retarded to assist in keeping idle speed controlled to a target idle RPM, because spark timing can be changed faster than the IAC valve position
------------

The Positive Crankcase Ventilation (PCV) system vents fuel vapors and exhaust gases from the crankcase and delivers them to the engine intake manifold, where they mix with the intake air for combustion. The amount and content of vapors purged from the crankcase affects the fuel system. If a condition occurs that results in high fuel content in the vapor, the oxygen sensor will indicate a rich condition and short term fuel trim and long term fuel trim will go negative to compensate.

=====================
The amount of PCV system purge can affect the IAC system. If a condition occurs that results in a high PCV flow rate, the IAC valve will decrease to reduce the amount of intake air. The high flow rate will also cause the oxygen sensor to indicate a lean condition, and short term fuel trim and long term fuel trim will go positive to compensate
==============
The Evaporative Emission (EVAP) system collects the fuel vapors that evaporate from the fuel tank, stores them in a charcoal canister, and then disposes of them through combustion. The vapors collect in the canister until the powertrain control system commands them to be sent to the intake manifold, where they are directed to the combustion chamber and burned.

The amount of fuel vapor in the canister at the time it is purged affects the fuel system. A high concentration of vapor in the canister causes the oxygen sensor to indicate a rich condition, and short term fuel trim will go negative to compensate for this condition.

If there is little or no fuel vapor in the canister at the time of the purge, the purge flow acts like extra intake air and causes the oxygen sensor to indicate a lean condition. This causes short term fuel trim to go positive to compensate.

The fuel control system tries to separate fuel errors from purge flow. Therefore, long term fuel trim is not learned while purge is flowing so that the PCM does not learn purge flow as a fuel error.

============
One method of cooling the combustion chamber is through Exhaust Gas Recirulation (EGR). The air/fuel mixture is diluted with small amounts of EGR to lower combustion temperatures and reduce NOx. Too little EGR can cause high NOx. EGR flow takes up space in the cylinder and therefore affects the amount of air and fuel entering the cylinder, but it does not affect the actual air/fuel ratio.

The PCM commands EGR under conditions where high combustion chamber temperatures are likely. EGR flow is not not used at idle because it causes unstable combustion. EGR flow is most beneficial during cruise conditions. The PCM may advance ignition timing when EGR is flowing, which can improve fuel economy.

EGR flow at the wrong time or in the wrong amount can cause incomplete combustion (misfire) and affect driveability and idle quality. Exhaust/emissions can also be affected because unburned hydrocarbons and oxygen end up in the exhaust stream and the oxygen is detected by the oxygen sensor.

The PCM uses a feedback sensor to measure EGR flow at all engine operating conditions.

===========
The air, fuel, ignition, and exhaust/emissions systems all interact with each other. Understanding the relationships among the systems is key to proper diagnosis, because a change in one system can influence the operation of other systems. For example, lean air/fuel mixtures require peak ignition system performance.
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #14 ·
The purpose of the ignition system is to provide a spark to the correct cylinder at the correct time to ignite the air/fuel mixture. The cylinders must fire in the proper order and at the precise time, depending on engine speed and load. Proper ignition is critical to engine performance, fuel economy, and exhaust emissions. Improper ignition can cause driveability symptoms and create excessive emissions.

All ignition systems operate in a similar manner. Current flow in the coil primary circuit is interrupted to produce a high voltage in the secondary coil circuit, which fires the spark plug. The differences are in how the ignition system controls primary circuit current flow and how the secondary high voltage is distributed to the spark plugs.



--In early vehicles, a magneto furnished ignition. A magneto is a kind of DC generator that produces an extremely high voltage spark. The magneto was connected to a crude distributor that routed the spark to the correct spark plug at the correct time. There was no need for a battery, generator, or body wiring as we know it today.

The Ford Model T used a low voltage magneto that sent timed pulses to four coils mounted to the rear wall of the engine compartment. The coils, each fitted with armatures and contact points, alternately opened and closed their own circuits, which produced the high voltage secondary current

--In the breaker point system, the points would close, completing the circuit and allowing battery current to flow through the ignition coil, energizing the primary side. When the points opened, the ground path would be interrupted, stopping current flow through the coil. When the current flow is stopped, the magnetic field collapses, inducing high voltage in the secondary side of the coil.

The distributor routed the high voltage to the correct spark plug. The distributor was connected, or geared, to the camshaft so the distributor would be in the correct position for each piston for correct timing of the spark.

A capacitor was installed across the points to prevent arcing that would cause the points to quickly pit and degrade

Breaker point systems had their drawbacks. They wore out, both mechanically and electrically. Spring tension, gap, alignment, and wear on the rider were critical for proper operation. Breaker points were susceptible to damage from water, electrical and mechanical loads, improper adjustment, and service

--Electronic or solid-state ignition systems were an early replacement for breaker point ignition systems.
Duraspark systems:
With the introduction of electronic control modules, all of the coil-related functions that used to be done mechanically on the distributor can now be done with electronics. On Duraspark II systems, a magnetic signal generator, armature, and pickup coil replaced breaker points in the distributor. An ignition module controlled the dwell, but timing advance and retard mechanisms were still incorporated in the distributor.
With Duraspark III systems, a crankshaft position sensor and a crankshaft-mounted pulse ring are used in place of the magnetic signal generator, armature, and pickup coil in the distributor. Centrifugal and vacuum advance mechanisms were eliminated from the distributor.

Engine timing was not controlled by the distributor, but by the Powertrain Control Module (PCM). With this system, the only job of the distributor is the distribution of secondary high voltage from the coil to the spark plugs.

Thick Film Ignition (TFI) Systems:

Thick Film Ignition (TFI) systems were introduced in the early 1980’s. Electronically, the system operates just like the Duraspark II system.

The distributor contains a CMP sensor (stator) and vane cup, and timing is controlled by the PCM. In addition, the TFI system has a more efficient coil with a higher energy capacity
Distributorless Ignition Systems (DIS) were introduced in Ford vehicles in 1989. DIS eliminates the need for a distributor by using multiple coils. Each coil within the coil pack fires two spark plugs at the same time. The spark plugs are paired so that one spark plug fires during the compression stroke and the other spark plug fires during the exhaust stroke.

The next time that coil is fired, the spark plug that was on the exhaust stroke will be on the compression stroke, and the one that was on the compression stroke will be on the exhaust stroke.

Distributorless Ignition Systems (DIS) were introduced in Ford vehicles in 1989. DIS eliminates the need for a distributor by using multiple coils. Each coil within the coil pack fires two spark plugs at the same time. The spark plugs are paired so that one spark plug fires during the compression stroke and the other spark plug fires during the exhaust stroke.

The next time that coil is fired, the spark plug that was on the exhaust stroke will be on the compression stroke, and the one that was on the compression stroke will be on the exhaust stroke.

Another variation of the distributorless ignition system is the Coil-On-Plug (COP) ignition system. Each spark plug in the COP ignition system has an individual coil, and the coils and spark plugs are no longer fired in pairs. This system has the advantage of eliminating the secondary spark plug wires.
 

·
Registered
Joined
·
275 Posts
Discussion Starter · #15 ·
That's all I got for now,should help some people out.
Remember,never listen to the guy at autozone,just get the code from him to give to someone knowledgable.
 
1 - 15 of 15 Posts
Top