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'94 Chevy c2500 5.7 Exuast/Engine Timing Problems

This is a discussion on '94 Chevy c2500 5.7 Exuast/Engine Timing Problems within the Technical / Maintenance forums, part of the General Discussion category; I have a 1994 Chevy c2500 TBI 5.7 v8 that is having major issues. On a cold start everything is ...

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    '94 Chevy c2500 5.7 Exuast/Engine Timing Problems


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    I have a 1994 Chevy c2500 TBI 5.7 v8 that is having major issues. On a cold start everything is fine, but after it hits operating temperature it bogs really low and blows out black smoke. So far we've switched out the ECM, (didnt work) and the Spark Timing Module in the distributor (also didnt work). The previous owner had wrapped the wires going into the timing module with a lot of electrical tape. Also the knock sensor wasn't working until the timing module was replaced. My grandpa (the mechanic) degreed the engine to run in 10 degrees adnvance. Since that didnt fix the problem either, now he is unable to retard the engine back to its original setting for some reason. Only idea that he has left is the PROM in the ECM. CODE 42 SHOWS UP ON SCANNER! PLEASE HELP!!
    Last edited by dcole25; 07-31-2012 at 01:44 PM.

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Your coolant temp switch in the intake manifold may be going bad as it controls fuel mixture as it gets warmer. That may explain the black smoke. The timing is controlled by the ecm. But you can set initial timing by disconnecting the single brownish wire behind/under the glove box. That seperates it from the ecm so you can set timing by the distributor. Book says to set it to 0 degrees. I have better luck at 4 degrees btdc.
    HTH
    Last edited by pima.1971; 07-31-2012 at 11:50 AM.

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Def sounds like the Coolant temp switch.

    Basically it tells the ECM what temp the coolant is, and it adjusts the fuel trim accordingly, because a cold engine needs a little more fuel to run right, than a hot engine does.

    When they go bad, they tend to go bad to the cold side, for instance, telling the ECM its -60*F, the ECM responds by dumping in fuel.
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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    if your grandpa was messing with timing i would start there.
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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Ill let him know. atleast it gives him another idea of what to look at. Think the PROM in the ECM would do anything?

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Quote Originally Posted by NEUMANNZZ View Post
    if your grandpa was messing with timing i would start there.

    Hes now able to set the timing. could the PROM be the problem?

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    If you replaced the ECM then no, unless you have two bad ecms, as others have said check the coolan temp switch
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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Quote Originally Posted by 305guy View Post
    If you replaced the ECM then no, unless you have two bad ecms, as others have said check the coolan temp switch
    The ecm i ordered came without the prom so it has the original prom just a different ecm

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Quote Originally Posted by pima.1971 View Post
    Your coolant temp switch in the intake manifold may be going bad as it controls fuel mixture as it gets warmer. That may explain the black smoke. The timing is controlled by the ecm. But you can set initial timing by disconnecting the single brownish wire behind/under the glove box. That seperates it from the ecm so you can set timing by the distributor. Book says to set it to 0 degrees. I have better luck at 4 degrees btdc.
    HTH
    Checked the coolant temp switch and it was working the other day. checked it again and now it isnt working. no clue why it was working awhile ago and now its not unless we tested it wrong but were switching it with a new one and we'll go from there. Code 42 also shows on the scanner.
    Last edited by dcole25; 07-31-2012 at 07:11 PM.

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    42 is from disconnecting the ESC module while setting the timing.
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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    code 42 is timing bypass wire disconnected. disconnect battery to reset ecm and clear code.
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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    switched the coolant temp switch and it helped a little bit. doesnt run as rich but still isnt running lean enough. gonna put in different spark plugs and see what happens.

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    remove and clean ground wire connections at thermostat housing stud. repair grounds if broken or missing.
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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Quote Originally Posted by NEUMANNZZ View Post
    remove and clean ground wire connections at thermostat housing stud. repair grounds if broken or missing.
    Already cleaned the grounds for the thermostat AND the knock sensor. were just guessing on things to replace now. Probably gonna sell the truck for parts and ill have to start looking on craigslist for something to drive
    Last edited by dcole25; 08-02-2012 at 09:01 PM.

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Checked for a bad 02 sensor?
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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    I had one where the 02 sensor had come out, but it didnt sound different somehow, started belching black smoke. so much fuel was spraying out the injectors that it was reverbing off the plates.

    Found the o2 sensor out, put it in...and bang, ran like a top.

    And it makes sense in your case, because when cold, it runs in closed loop mode, under a preset timing and fuel trim, and when warm, it starts open loop and begins taking info from the sensors, and a bad 02 sensor will surely make it dump in fuel.
    Last edited by 454/4X4; 08-02-2012 at 09:47 PM.
    97 K3500 dually Sierra 4X4
    Vortec 454/4L80E
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    95 K3500 Silverado
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    4.10's


    95 K2500 Silverado
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    6.5 Turbo Diesel,
    4.10's, 4" straight pipe


    94 K1500 Sierra Z71 OFF ROAD
    Reg Cab, Stepside
    350 TBI
    2" lift, 33's

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Quote Originally Posted by 454/4X4 View Post
    I had one where the 02 sensor had come out, but it didnt sound different somehow, started belching black smoke. so much fuel was spraying out the injectors that it was reverbing off the plates.

    Found the o2 sensor out, put it in...and bang, ran like a top.

    And it makes sense in your case, because when cold, it runs in closed loop mode, under a preset timing and fuel trim, and when warm, it starts open loop and begins taking info from the sensors, and a bad 02 sensor will surely make it dump in fuel.
    That was one of the first thing he checked. It was working fine. He even disconnected one fuel injector and it still was dumping in too much fuel. Already bought a 1997 saturn sc2 so i gave up on this truck. Hes been a master mechanic for over 50 years and this is the first truck he hasnt been able to fix.

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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Did you check fuel pressure? Maybe a bad FPR? Did you change the right coolant temp switch? 1 is for the gauge (in the head) and one is for the computer (intake). You must c
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    Re: '94 Chevy c2500 5.7 Exuast/Engine Timing Problems

    Quote Originally Posted by Blue 04 Z71 View Post
    Did you check fuel pressure? Maybe a bad FPR? Did you change the right coolant temp switch? 1 is for the gauge (in the head) and one is for the computer (intake). You must c
    Yes. weve checked both and still no luck. deciding to sell the truck for parts

 

 

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