2003 silverado, chasing mysterious power loss

Discussion in 'Maintenance' started by L_Dillinger, Dec 21, 2016.

  1. L_Dillinger

    L_Dillinger GMFS Member

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    Hello, ive been after the culprit to my loss in power for awhile to no avail. Truck is 2003 silverado rcsb z71 4.8 5 speed w/180 k miles. When i give it much over ¼ throttle in mainly 3rd ,4th an 5th gear it jus flatlines. No sputter or miss, jus wont go any faster, back off the gas some and it will go. Its down on power in the lower gears as well. I can hear it suckin air the more gas i give it so i know the tb is opening up. I had it tuned, mild 91 octane tune as well b4 this got bad but im thinkin ot migjt be the tune? Ima call the guy soon and see what he thinks. And ill hook it up to a scanner with live data this weekend and see what my trims and timing look like. I was gettin a P0101 for some time, usually popped up when it would hesitate, and recently got lean codes on both banks, ive replaced both knock sensors, bout a year ago. Last week replaced fuel pump, regulator and filter, as well as plugs and wires and all codes are gone. Cleand the dirty MAF and did nit fix it. Pulled the known to be good MAF off my wifes truck and put on and did not fix it. I no longer get any lights or codes after the parts i put on last week so i am stumped. Best way i know to describe it is it feels like the pcm is pulling a crap load of timing when i throttle it very much. It wont pull hills in 5th anymore either and ist real consistent, over all it does it alot but out of 10 hills it might pull 1 or 2 in 5th ok. I know its a 4.8 and shouldnt expect a ton of power out of it but dang, my ol 4.3 silverado would smoke this thing like it is. Any help at all is appreciated greatly cuz im pulling my hair out, thanks guys!
    ,Lee
     
  2. L_Dillinger

    L_Dillinger GMFS Member

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    And forgot to add, the truck idles perfect, starts up pefect, doesnt use oil or smoke either and has no cats. Everything else seems to function as it should.
     
    Last edited: Dec 21, 2016
  3. John Clark

    John Clark GMFS Member

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    When you floor it the O2 sensors should immediately go rich. A good scan tool with some graphing capabilities would be helpful here. Fist to check the O2 sensors for proper operation. Then, if they appear to be working right, if the O2 sensors are going lean when you floor it then it's a fuel delivery or MAF issue. The same scan tool can watch your MAF data, as well.
     
  4. L_Dillinger

    L_Dillinger GMFS Member

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    I had it on a good scan tool today. O2's seem fine, its getin plenty of fuel i believe, long term fuel trims were between 8-9% after a few mile trip. Pump, regulator and filter are new. My ignition timing at 55mph in 5th on level ground was about 30° advance, when i mash the pedal a little and make it do its boggin the timing drops to about 5° instantly. Im pretty sure i have some leakin intake gaskets, which i think could cause the issue in queastion. IAT was right and the MAF readings seed right. Was runnin 0.60 lb/mim at hot idle, mid to high 20s at ¾ throttle and on. MAF is clean amd knock sensors are maybe a year old. I will change the intake gaskets in a few days and see where that goes.
     
  5. Phildirt

    Phildirt GMFS Member

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    Dat boggin and leakin do
     
  6. John Clark

    John Clark GMFS Member

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    What does "O2's seem fine" mean? Pay attention to the front sensors (Banks 1 & 2, Sensors 1.) Are they cycling rich-lean-rich-lean? They should cycle from between 0.100V to 0.900V at around once per second or so, and faster at higher RPM's. If the tool can graph then it should look like a jagged sine wave. Then when you floor it they should go rich, or to 0.900V and stay there for a bit until things even out and then cycle again. When you let off the gas and close the throttle they should go lean, or 0.100V for a moment or two until it evens out and starts cycling again. If the sensor goes lean (0.100V) when you step on the gas there is a fuel delivery or possibly MAF issue.

    A vacuum leak like an intake manifold would likely give you lean codes at idle when cold (cold is usually when they leak most.) So, if you get a lean DTC (P0171 and P0175 I think) the first thing you'd want to do is check the freeze frame data for the code and see when it set. A vacuum leak is usually worse at idle and less of a problem at higher RPM's. You would also see higher fuel trims at idle than at, say, 3000 RPM. Remember, with fuel trims you need to look at both short term and long term and add the two to get total fuel trim.
     
  7. ron7000

    ron7000 GMFS Member

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    did you tune or modify the programming in the engine computer, i.e. PCM or ECM ?

    from your description i would begin to look at bad catalytic converters, and if you had them i'd suggest measuring exhaust pressure before the catalytic converter.
    http://www.aa1car.com/library/exhaust_backpressure.htm
    but even without cc you might want to check this if you've exhausted all other possibilities

    for a 2003 with 180k miles, i would say replace the MAF with a new delphi one for your year and model truck, don't substitute it with a used one. and replace both front O2 sensors, and spark plugs.
    After those big three, if it's still not putting out power and not throwing any engine codes then do a compression test.
     
  8. L_Dillinger

    L_Dillinger GMFS Member

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    If i remember the O2 sensors were runnin bout .7v average at hot idle but were goin up and down some, not much tho. , dont have engine light anymore, but had p0101, p0171, & p0174 stored, which is MAF and lean on both banks, strongly implying an vac leak. Dude that tuned my truck was sayin when u got an air leak, if big enough, all that unmetered air it can pull timing back crazy which is the only thing i know to describe what its doin. Any load or more than ¼ throttle it jus flatlines...no sputter or poppin. Truck idles fine, cold and hot, runs bout the same cold or hot as well. I was by myself while i was drivin and lookin at the data so i couldnt really tell what it was doin every second. The scan tool i was usin im not real familiar with so ill see if can do a graph and what not.
     
  9. L_Dillinger

    L_Dillinger GMFS Member

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    It dont use a drop of oil, i did have it tuned, has no cats, plugs are new ac delco 41-110 irudiums. i put the MAF off my wifes 2006 silverado 5.3 which runs perfect on it and her truck is all stock with a clean MAF, barely over 100k miles and the original delphi unit with exact same numbers, it didnt help much, maybe a lil but still ran like crap. Ima propane check or smoke check the intake for leaks which im almost sure it has now. Has those crappy leakin orange stock gaskets, ima out the teal green ones on it when i swap em. I thought about doin a comp test but i cant imagine it bein low and has never and still dont use a drop of oil and has no blow by. Hell i can park it on a hill in 3rd gear and it wont even roll, btw i dont ever leave it parked in 3rd. I guess intake gaskets are next, if that dont work ill do front O2's and MAF. Shits getin expensive
     
  10. John Clark

    John Clark GMFS Member

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    You should check those O2 sensors close. If the front sensors are not cycling between 0.100V and 0.900V and stuck at around, say 0.700V that's not good. The rears should be somewhat steady if one had cats but if you don't have cats then the rears are going to do the same thing as the fronts unless you have an adapter on them to keep them out of the exhaust stream.

    If you don't do the right checks you'll just throw parts at it, spend a ton of money, and possibly never fix it.

    You can do snap throttles (momentary wide open) and watch the sensors, too. They should go high voltage (0.900V,) indicating rich, then back to lean (0.100V) and cycling again.
     
  11. L_Dillinger

    L_Dillinger GMFS Member

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    The O2's are cycling up and down, i didnt know at first if it was normal but seems it is. Ive got the non foulers in the rears. The plugs and wires and stuff im sure it needed anyways, had 180k original miles on em, ive never had to hardly turn a wrench on the truck b4 except for knock sensors. Cant complain too much. After christmas ill get back to it. Thanks for the input guys, i really appreciate it, i havnt been diagnosing with live data much b4 so your help has really helped me. I'll try and post an update when i get it fixed.
    Thanks!
     
  12. John Clark

    John Clark GMFS Member

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    So, if they're cycling they're likely working. The front should cycle and the rear should stay relatively steady, compared to the front.

    When you floor it or do a snap wide open throttle what do the front O2 sensors do? Take it for a test drive, with a graphing scan tool and watch the front O2 sensors. When you floor it the front sensors should go rich, or high voltage (around 0.900V.) When you let off, they should go lean, or low voltage (around 0.100V.) It shouldn't go lean immediately when you floor it but based on the hesitation you describe it might be.

    If you want real help, go sign up on the forum at ScannerDanner.com. If you can manage to get some help from Paul Danner on his forum you'll save yourself a ton of money. He's the master of diagnostic and teaches this stuff for a living. I have watched all his free channel videos (highly recommended) but haven't bought his book or signed up for his premium channel.
     
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  13. ron7000

    ron7000 GMFS Member

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    those codes to not necessary mean vacuum leak as in intake manifold gasket. I had all 3 codes when I bought my 2006 which is the 8.1L. Ahole previous put on a K&N cold air and had the filter exposed to the wheel well and it caught all kinds of water and i found water stains within the MAF sensor. More times than not you need to replace the MAF, not clean it.
    p0171/4 is often caused by the evap solenoids not sealing closed and letting small amounts of air in to the intake, this will cause p0171/4 which is lean condition for too long a duration sensed by the PCM. On my mine it was the "fuel vapor purge valve" which sits on my intake manifold took 2 minutes to replace for less than $20, that's on the 8.1L i don't know the 4.8L so you'll have to research what evap solenoids are where.

    :hung:
    and with that said, best of luck. I have NEVER heard, read, nor seen someone who had some dude tune a late model EFI engine and make a street vehicle run better than factory settings.
    What credentials did this dude have?
    Don't bother throwing anything else at the engine until you put back the factory programming on the computer
     
  14. John Clark

    John Clark GMFS Member

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    That's all correct. Vacuum leaks can come from many sources, intake gaskets just being one of the more common ones on GM V8's. An intake vacuum leak will be evident on the fuel trims when the engine is cold. Fuel trims will be high at idle as the computer compensates for the vacuum leak. When you rev the engine to 2500-3000 rpm the fuel trims will come down as the computer doesn't need to compensate as much. Often as the vehicle warms up the vacuum leak will often times go away. A lean code set when the vehicle is cold and at idle is a good indication of an intake vacuum leak. That's where looking at your freeze frame data can be really helpful.

    If something like the purge valve is bad the vacuum leak will be there all the time, cold or warm. Using pliers to clamp off vacuum lines and then watching the fuel trims will find a problem like a leaking purge valve. If you clamp off the hose to the purge valve and the fuel trims begin to come down you've found your problem. However, on some vehicles the purge valves are duty cycle controlled so the valve will always be open a slight amount when the engine is running depending on what the PCM is calling for. I found that to be the case on my 08 Honda Odyssey.
     
  15. L_Dillinger

    L_Dillinger GMFS Member

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    Thanks for the replys guys. I should rephrase what i said about "some dude" tuning my truck. The guy is very good and very smart. Here is his webite, lt1swap.com
    He does tuning for a living mostly mail order stuff, and there's a ton of good LS LT swap and wiring info on his page, i jus so happen to live about 30 mins away from him so drove to his shop and he did it in person. I will check the purge valve solenoid on the intake which i understand should be closed, as well as the vent valve which should be open. I guess what i should have asked to begin with is what do you guys think could yank 25-30° of timing back when i press the pedal down some jus to accelerate, whether it be slowly or quickly? I understand the pcm uses the maf to calculate engine load, and if its bad or reading improperly thinking its under a huge load itll pull timing, or alot of unmetered air maybe? I cleaned the maf awhile back and it was filthy, idk how it read anything, after cleaning i didnt notice any difference. Putting the oem delphi unit off my wifes truck made a slight difference but did not fix the issue. I know i need to get a new one to really know i guess. The lb/min readings seemed normal on my "cleaned" maf. 0.60 at idle and 20+ at wot. IAT also seemed correct. So basicly its yanking timing back like crazy and i need to know why. I wasnt able to scan the truck cold, so i cant comment on the fuel trims while its cold. Usually intake gaskets make them run worse when cold and thay get better when warm or at higher rpms. Mine is pretty much all the time. Some days are a lil better than other but could be the air temp or something. I need to ditch that cai thats been on here since i bought it. Find some kid and swap it for a stock air box as mine is not an enclosed filter one.
     
  16. John Clark

    John Clark GMFS Member

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    The timing coming back could just be the symptom of something else. If the MAF was that bad then it's definitely suspect. A bad/dirty MAF will do exactly what you describe--it'll fall flat on its face when you try to accelerate. It's going to take some good analysis of the scan data to find the problem without just throwing parts at it.
     
  17. Keeler

    Keeler unɾuı ʇuǝpısǝɹ sɟɯƃ

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    Check your fuel pressure. Quick and easy, don't check it at just idle, you need to watch the gauge while you're driving. Bet you have a weak fuel pump. If your FP is good, find someone with a smoke machine and smoke the intake. Easiest way to reveal vacuum leaks.
     
  18. John Clark

    John Clark GMFS Member

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    He said he replaced the fuel pump, regulator and filter. I've been trying to get the O2 sensor information from him when stepping on the gas which would be a good indicator of whether the problem is fuel starvation. I've been claiming fuel delivery or MAF since my first post. Both will cause the exact symptoms he describes.

    To the OP, did you check the tank for sediment when you replaced the pump? If the tank had crap in it and you put the new pump in you may have just plugged up a new screen or filter. Since you said it was fine for a while that sounds suspect. You'll need to get a scan tool and watch those O2's as I suggested a couple times here or a fuel pressure gauge to find out what's going on.
     
  19. L_Dillinger

    L_Dillinger GMFS Member

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    Tank was suprisingly clean. I havnt been able to run the scan tool on my way to work lately to see whats goin on with the O2's . Yea i suspect the timing issue is being caused by the malfuntuon of another part. On the bright side i ditched that open filter element and installed a stock air box. Didnt have a stock tube but made it work with my chrome cai tube, that and unhooking (for the moment) my purge solenoid on the intake made id say a 50% difference. I figure my vent valve solenoid is bad and stuck open, when the purge opens it makes a vac leak simce the vent valve should close when tbe purge is open. Its better but not fixed. I likely have a plethra of issues goin on, i wouldnt be suprised if the maf was bad and the intake gaskets were leaking. I dont believe fuel pressure is an issue, fuel parts are new, and i have checked the pressure. Guage read bout 58psi key on engine off, 54psi at idle. Rev it up it jumps to 58psi or so. I know i didnt run it while driving, i was using a buddy's gauge and it was not long enough to reach up to where i could see it while drivin and close the hood. It was a harbor freight gauge so idk how accurate it was . Possibly this weekend i will check and see what the O2 sensors are doing while driving. I know at idle and quick rev they seemed to cycle like you guys had described they should but idk bout goin down the road. I know the IAT gets its reading from the maf, and it seemed correct for the conditions, as well as the lb/min reading, i think it uses it to calculate engine load as well and the scan tool showd the engine load %, it was 2.0% at warm idle but idk what it was driving, i guess if its calculating the load wrong it could pull timing, but if the IAT and lb/min readings were correct i would think the load % would be as well since that info comes from the maf. I dont have acsess to a smoke machine but it sure would be helpful. I will try my best to get back asap with some more data, been busy with work lately. Thanks for your input guys
     
  20. L_Dillinger

    L_Dillinger GMFS Member

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    Now i got a pending p0327 code but no engine light. #1 knock sensor
     
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