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Author Topic: 1998 Chevy silverado High Idle Issues  (Read 9422 times)

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1998 Chevy silverado High Idle Issues
« on: January 09, 2010, 01:00:37 PM »
  • I know some of you on here are mechanics so here it goes.  I have a 1998 Chevy 1500 with a 5.7L.   When i start it up it revs up to 2400 rpms and slowly drops to 1300-1500 rpms and that is what it idles at.  I have cleaned the IAC valve and the throttle body.  Is there a way to test the IAC valve?  Or do you have any other suggestions to what it might be, I would replace the IAC but its over $100 and i would like to exhaust all other possibilities before i spend that kind of money. 


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    Re: 1998 Chevy silverado High Idle Issues
    « Reply #1 on: January 09, 2010, 01:17:28 PM »
  • you could try re-flashing the ecu. in the engine bay unplug the 3 wire harness's on the top computer. leave them unplugged for 15 min, and then plug them back in, then start it and drive for 15-20 min.

    this is a free test to see if the ecu is what is controlling the high idle.

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    Re: 1998 Chevy silverado High Idle Issues
    « Reply #2 on: January 09, 2010, 02:56:02 PM »
  • Check vacuum hoses first   spray carb cleaner around intake to check for any leaks. If idle drops doing this its intake casket. I've never heard of ecu being the culprit.
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    Re: 1998 Chevy silverado High Idle Issues
    « Reply #3 on: January 09, 2010, 08:59:26 PM »
  • I would also say vacuum leak....plastic manifolds tend to shrink way different compared to aluminum/steel, there-for causing a leak at the gasket surface.  Best way to find out is watch fuel-trims on scan tool while spraying eco-friendly brake cleaner at head/intake flange...the #'s will jump radically if there is a vac. leak present.
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    Re: 1998 Chevy silverado High Idle Issues
    « Reply #4 on: January 12, 2010, 05:12:10 AM »
  • If idle drops doing this its intake casket.

    I know its older but don't kill it off yet!!   :lol:  Great advice Scott...wish I would have talked to you about 4 years ago!!  I have a 1992 K1500 Chev Blazer that has the 350 in it.  At about 160k miles I had the intake gasket replaced.  After that I noticed that the idle was off & there were some issues with it just plain running.  It would not idle at times & just plain kill at a stop sign for no reason I could find.

    With my knowledge of auto mechanics I knew it was some sort of a vacuum leak but I couldn't figure out what it was.  I should have done the test you talked about.  But I didn't & ended up replacing the motor because it was about to spin the main bearings & throw a rod.  I found out as we pulled the old one out that when the intake was replaced there was a large amount of silicone gasket sealer that squished out & fell into where the distributor meets the oil pump drive/camshaft.  So the oiling of the cam/crank were compromised.

    I also found out that the same problem I had before the motor swap was STILL there as far as the run ability & idle issues.  So I finally took it in & they found the carb gasket was leaking which was causing the vacuum leak as well as the run ability issues.  If I had checked that earlier I may still be driving on the original motor!  Expensive lesson learned!!

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