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Poor Idle 1990 350 TBI

Discussion in 'The Garage' started by Polaris, Aug 19, 2004.

  1. Polaris

    Polaris 1/2 ton status

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    Maybe some of you have run into this before. When my truck is cold, it likes to hang at higher RPM's. One time it nearly stopped idling while I was letting it warm up, then the RPM's came up & it sort of saved itself. When it's warm, it can't seem to idle consistently. It goes from about 500-750RPM and feels a bit rough. It's mostly stock, but I have had a TBI spacer on since last year. This problem started this summer.

    What I've tried so far:
    Spraying brake cleaner near the base of the TBI, suspecting a bad gasket. Didn't change the idle.
    Swapping out MAP sensors.
    Replaced a cracked/rotten vacuum hose.
    Inspected the throttle cables. It does seem that they are not closing all the way when it's cold, hence the hanging at higher RPM's.

    I'm not getting any trouble codes whatsoever.

    Thanks for any advice!
     
  2. Thunder

    Thunder 3/4 ton status

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    You might try pulling the IAC assembly off and cleaning it.
    Sometimes the valve gets carboned up and causes idle problems.

    Also pull the EGR valve off and clean it too.
     
  3. mouse

    mouse 1/2 ton status

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    Look for vacuum leaks. TBI base is a good start.
     
  4. jekquistk5

    jekquistk5 Weld nekid Premium Member GMOTM Winner

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    I had the same problem with mine, it was the egr valve. it wasn't releasing/contracting correctly. Try tapping on it, or even press it in to see if it comes back out.
     
  5. Derf00

    Derf00 1/2 ton status

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    I would assume you probably have cruise control on a '90. Carefully inspect the linkage from the cruise control diaphram and make sure it is not binding. I had various idle problems and it turned out to be the cruise contro.
     
  6. woody9

    woody9 1/2 ton status

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    with my 91 sub, it was the throttle position sensor. (forget what miles they're good for??), but it wasn't throwing a code either. If you replace it, i believe you need to install, disconnect batt for 5-10 (to clear computer), key on (NOT STARTED), then depress gas 1-3 times, then start.... (this is from memory, but a search on here may yeild more DETAILED, but the Memory reset is essential to doing it right! The idle/WFO cycle is to reset the throw it has...)
    Found plug wires doing that to me once as well.. would do it on the humid days as well as the wetter mornings!
     
  7. Polaris

    Polaris 1/2 ton status

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    Thanks, guys. I'll try some of this out after work. No cruise control to be concerned with. As an ex-sheriff's patrol vehicle, it's the basic of the basic!
     

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