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1989 C1500 5.0L Intermittent running problems

3K views 4 replies 1 participant last post by  FederovJones 
#1 ·
Hey so I've been trying to solve this problem for a bit now, and I'm try to go back to the basics and I hope someone here can give some insight. 1989 GMC C1500 5.0L Auto 2WD

Symptoms:
-Hesitation right when you press the throttle(mostly noticeable from a stop)
-Abnormal idle surge/stalling for about 5 seconds
-High Idle(anywhere from 800-1300 rpms)
-Loss of power underload(sort of like if the ignition advance curve stops)

As to when these things happen it is very intermittent and very hard to reproduce on command. I notice it most after about 20-30min of driving, however it sometimes doesn't happen at all.

Things I do know:
-There are no vacuum leaks(with the IAC fully closed idle is perfect)
-Timing chain slack is in specification
-The IAC is functioning, however as far as the high idle goes it seems as if the ECU is telling it to raise the idle for a reason I'm not quite sure of.
-I have tried 2 different TPS and MAP sensors that I had laying around with no noticeable change in the symptoms.
-My fuel pressure is in specification.
-Distributor Shaft does not have any noticeable radial play.
-Coolant sensor seems to have the appropriate resistance values.
-The injectors have a clean spray pattern.
-I have tried a second ECU I have, however it literally just wanted to high idle all of the time, even as high as 1600rpms.


Items that I think may be of concern at this time:
-The IAC(not sure how they work internally but maybe its missing a few teeth on a gear or something?)
-The distributor cap and rotor are not in the best shape.
-The ignition coil(its about 1-2 years old but seems like it could cause some of these symptoms.
-The Ignition control module(It is new from napa but the lack of power I feel relates to this part the most)
-The TPS(maybe the voltage setting isn't perfect, but I don't see it causing this much of a problem, especially intermittently)
-Lack of grounds from the engine to the chassis(just a theory)


So if anyone can give any input from their own experiences or even just refresh my memory on how each part works I would appreciate any information regarding this issue.

Thank you
 
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#2 ·
****************Update*************

Symptoms I have corrected:
-Hesitation when you press the throttle(The TPS was operating out of range from 1-3.7V in the throttle sweep-Should be 0.5-4.5V)
-Abnormal idle surge and High Idle(The IAC was unable to fully seat while bridging A+B terminals at the Diagnostic connector; it would continuously go in and out)

Symptoms I have yet to fix:
-Intermittent Loss of power under load.

It seems that the vehicle will run fine until its fully warmed up after a good drive. You will floor it and it will have good power and then instantly you feel about a 20% loss of power but it is still accelerating.

Items of concern:
-Ignition control module(It is new from napa, however I have dealt with a few bad new ignition control modules in the past)
-Knock Sensor(read below)

So I have not have my knock sensor hooked up for a few years. From what I can gather the ESC(Electronic spark control module), located on the front right of the engine, simply gets battery voltage and sends 8-10V back to the ECU as the signal. The knock sensor, if sensing a knock, then reduces that 8-10V and the ECU will retard the timing. Now I have also found that during a drive cycle once the coolant temperature reaches 95 degrees Celsius the ECU will perform a test where it will advance the timing until it sees a signal from the knock sensor.

*As I was typing this I found that if the ECU does not detect a spark during this test it will set code 43 and if that code is set in a drive cycle you will lose your timing advance. I believe I have found my issue. I will reconnect my knock sensor and come back with an update.

Hope this thread can help anyone in the future.

Thanks again.
 
#3 ·
*****Update*****

After hooking up my knock sensor the loss of power problem was fixed. Now runs better than ever. HOWEVER.

Returning Symptom:
-Surge/High idle(Randomly it will high idle 900-1400 RPMS and sometimes the ECU will try to bring the rpms down but it will sputter and raise the idle back up)


I do have a new IAC, and as far as Ive tested I do not have any vacuum leaks. I am going to try and pull some codes when it happens again but its hard to duplicate.

I think it might be an ECU issue but I will definitely have to look into it further and if anyone has any ideas or experience with this issue, again that would be much appreciated.

Thank you
 
#4 ·
***Update***

Symptom:
-Surge/High idle

What I have done:

So I pulled a code 22(TPS Signal Voltage low). Basically if the ECU sees a voltage of 0.2V or less for a certain amount of time over 1250 RPMs it will set this code.

I tested my new TPS to make sure it was working correctly and it was. I then probed the TPS signal line at the ECU and started playing around with the wires and TPS connector where I then noticed the signal voltage started jumping around.

Basically I have a loose connector I believe to be right at the connector itself, so I have temporarily zip tied it down nice and tight until I can get a new connector tomorrow.


I will return with a hopefully finally update.

Thanks
 
#5 ·
**Update***!!!

So I replaced the TPS connector. I noticed that the signal voltage was only getting to 3.8V at WOT so I changed the closed throttle signal voltage to 0.9V from 0.6V to get a better top end. HOWEVER, after letting it run for a few minutes the High idle came back...sigh.... So I brought it back in the shop to check things again.

Started off by double checking the TPS voltages, I set the IAC to the fully seated position to check the base idle again(should be 400-500rpms). It turned out to be all good.

At this point I figured I might as well triple check for vacuum leaks and BEHOLD I found one at the rear of the intake manifold...sigh....I don't have the time to replace the gasket on that so I just splooged some RTV around the spot I figured to be leaking. Turned it back on and everything seemed good. Drove it around all day and it was fine.

Later in the evening after turning it off and on again the idle went high once again....WOW....and then for some reason the car stalled. Started right back up with a high idle again. At this point I'm thinking theres no way a small vacuum leak at the intake manifold could cause this high of an idle and this random of an idle and not to mention why the heck would it make it stall?

Well, I decided to swap my ECM out with a 350 ECM I had and since then it has been running flawlessly with even more power than it ever had.

For now I'm assuming that the 305 ECM is starting to degrade internally, which most likely explains the intermittency of the issue.

Until a new problem arises this should be it. I hope that this information can help anyone in the future.

Thanks.
 
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