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91 GMC 5.7 Timing help

This is a discussion on 91 GMC 5.7 Timing help within the Technical / Maintenance forums, part of the General Discussion category; Hello everyone, new here. My truck sounds and runs terrible. It always pops out the exhaust and doesn't have much ...

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    91 GMC 5.7 Timing help


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    Hello everyone, new here. My truck sounds and runs terrible. It always pops out the exhaust and doesn't have much power. I have had the distributor timing set all over the place. I know to unhook the connector on the firewall for the timing control before setting the timing and I have set it to between 0 and 4 degrees (other times way ahead and way behind and everything in between). I have taken it to a local shop and the mechanic says the only thing he can think of would be a broken wire somewhere. It always sounds like it is advanced way to far and the mechanic said he confirmed this with "his computer". Also, I have the egr unhooked (had it capped off at one time) and pluged on both sides of the solenoid and have changed the coolant temp sensor, the map sensor, the iac valve, the knock sensor (I get this code when checking the OBDI), throttle position sensor, rebuilt the tb with new regulator, new distrubutor and module, and also swaped ecm and spark module. That's about all I can think of, maybe more. Just want to know if anyone here has any ideas for me to try before I spend an arm and a leg more on this truck taking it to a dealer to diagnose. Thanks in advance. Chris.

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    Re: 91 GMC 5.7 Timing help

    Do a compression test and see if there are bad valves.
    04 K1500 Silverado EXT 3:73 gears, 5.3L 265/75-16,Pace Edwards tonneau,

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    Registered User ToyHauler's Avatar
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    Re: 91 GMC 5.7 Timing help

    whats the number of the code? did you check the wiring from the knock sensor to the pcm? bad knock retard signal will throw the timing off
    99 GMC Yukon SLE 4dr
    KYB monomax, air lift, tekonosha, reese dual cam, bfg at/ko

    87 Firebird, 07 Monte Carlo lt, 98 Bonneville SSE, 04 F550 CC 4wd service body

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    I'll have to double check but I think it was either 34 or 43 but I remember it was the code for the knock sensor. Would the bad knock signal throw the timing off bad enough to pop out the exhaust and run bad throughout the throttle range? I figured some other problem was making the knock sensor throw the code because of the rough running and timing being off. Does that make sense? Thanks for the reply by the way.

    Sent from my ADR6300 using Tapatalk 2

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    GMC RULZZ NEUMANNZZ's Avatar
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    Re: 91 GMC 5.7 Timing help

    yes a bad knock sensor or esc circuit will cause timing issues.
    same for a worn out engine and lack of fuel pressure.
    Last edited by NEUMANNZZ; 09-13-2012 at 02:50 PM.
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
    rebuilt/beefed up 4L60E(at 43,000miles)
    Matching Ninja 500r


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    Well the shop I took it to said the fuel pressure was good and the engine is a reman with maybe 30,000 on it. Does that one wire go all the way back to the ecm? What is an easy way to check that it is good? Thanks.

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    Registered User Srabai47's Avatar
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    Re: 91 GMC 5.7 Timing help

    not sure about 1991 but in my 94 if i set the timing with the timing wire unplugged until it sounds decent, then shut off the engine and plug the wire back in, then drive it for a week, the computer actually adjusts the timing to the correct specifications.
    1994 Chevrolet C1500 Suburban 5.7 260,xxx miles.

    1994 Chevrolet C1500 SCLB W/T 4.3 TBI NV3500-(Rebuilt 02/23/12) 3.42 --268,xxx miles -Blew motor (3/20/13)



    1992 Toyota Celica (182,000miles) - Sold

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    GMC RULZZ NEUMANNZZ's Avatar
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    Re: 91 GMC 5.7 Timing help

    typical knock sensor circuit diagram:
    http://images.search.yahoo.com/image...mb=xMjOia4saPR
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
    rebuilt/beefed up 4L60E(at 43,000miles)
    Matching Ninja 500r


 

 

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