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Vortec 350 idles way too high

This is a discussion on Vortec 350 idles way too high within the Technical / Maintenance forums, part of the General Discussion category; I am working on a 1999 GMC 3500 for a friend with the Vortec 350, the problem is that the ...

  1. #1
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    Vortec 350 idles way too high


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    I am working on a 1999 GMC 3500 for a friend with the Vortec 350, the problem is that the truck idles too high. As soon as you put the truck in gear, it takes off and will go about 35 mph without touching the accelerator. If you shift into neutral at 35 mph, the engine will run about 1600 rpms and stay there until you stop, at that point it idles down to 1000 rpms.

    I have checked for vacuum leaks, found one at the PCV valve elbow but fixing it didn't cure the idle problem. Throttle body seems to be working fine, I cleaned it with throttle body cleaner and that seemed to make the problem worse.

    I tried starting the engine with the idle air control valve unplugged to see what it would do, the engine raced to 2500 rpms and stayed there until I shut it off.

    The truck has been well maintained and has roughly 120k on it, the idle problem has come on gradually. The check engine light is not on.

    Any suggestions?
    Last edited by battyice; 09-30-2008 at 06:22 PM.

  2. #2
    silverado96
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    Re: Vortec 350 idles way too high

    Maybe a tps. that would explain the high idle. and why it takes off like that.
    1985 k-10 on 33's. Build in progress....

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    Registered User 4x4k20's Avatar
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    Re: Vortec 350 idles way too high

    replace the coolant temp sensor they will cause this problem also replace the wiring connector that hooks to it napa sells them with the wiring end harness ..oh and check for codes agian
    Last edited by 4x4k20; 09-30-2008 at 06:31 PM.
    4x4k20 if money was trouble i would have plenty of it

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    Re: Vortec 350 idles way too high

    yes coolant sensor, it is probably internally open, which would make the computor see an extremely low temp, therefor always running in cold start enrichment. replace then scan for hidden codes.

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    Re: Vortec 350 idles way too high

    Does anyone know the voltage/resistance spec for the ECT? If possible I would like to test the sensor rather than just replace it and find out that wasn't the problem.

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    loose nut behind da wheel JeremyB's Avatar
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    Re: Vortec 350 idles way too high

    At 5 degrees F 21450
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    Registered User justinp95's Avatar
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    Re: Vortec 350 idles way too high

    Tps
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    Re: Vortec 350 idles way too high

    Update: I hooked up a Scanguage II to the truck and discovered that the TPS reads "0" at all throttle positions, if you move the throttle enough once and a while I would get a brief reading of "20". The wires leading to the sensor check out fine so I will replace the TPS this weekend and hope that is the end of it.
    Last edited by battyice; 10-07-2008 at 03:56 PM.

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    Re: Vortec 350 idles way too high

    looking at that info it would indicate your tps is bad, but a real voltage would be nice, if you have that info it would really help, get back to me if you need help with it
    mechanic by trade, enthusiast by hobby

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    87 monte ss, 355 ci. vortec carbed

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    Re: Vortec 350 idles way too high

    The plot thickens. I replaced the TPS, and nothing changed. The truck still idles too high, the TPS output on the scanguage still reads "0". I checked the wireharness for broken wires and didn't find any. With the truck at operating temperature, I tried unplugging the TPS to see if anything changed...nothing, I unplugged the MAF sensor...nothing changed, and no check engine light. Does it make any sense that the truck would run the same with the MAF, IAT, and TPS sensor unplugged and not throw a check engine light????

    I am wondering if the PCM has a problem.

  11. #11
    I Hate You All, Equally.
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    Re: Vortec 350 idles way too high

    Truck should have run like A$$ with all those thing unhooked. sounds like something is definitely not right. Do you have a spare ECM you can try to test out?


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    Re: Vortec 350 idles way too high

    at least swap the coolant temp sender it not but about 30 bucs
    4x4k20 if money was trouble i would have plenty of it

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    Re: Vortec 350 idles way too high

    The coolant temp. sensor output was accurate on my scanner. Started out at ambient temperature and leveled out at ~195 when the engine was warmed up.

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    Re: Vortec 350 idles way too high

    did you remove and clean the i a c valve. sounds like it may be stuck, what kind of scanner are you using , is your e.g.r valve stuck open even just a little bit? clean the throttle plate?
    mechanic by trade, enthusiast by hobby

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    Re: Vortec 350 idles way too high

    might still be a vacuum leak, i have had a few with intake gaskets causing a running problem before they leaked antifreeze, the gaskets warp and the center ports suck external air, still doesnt explain the no check eng light, i will look in to it a little tomorrow when i go to work
    mechanic by trade, enthusiast by hobby

    2000 tahoe z-71,5.7 vortec obs
    87 monte ss, 355 ci. vortec carbed

 

 

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