GM Shop Stumped Idle Issue Up Down... Up Down

Discussion in 'Classic GM' started by Earl Phillips, Nov 2, 2016.

How to resolve an idle surge (up and down)

  1. yes and here is the fix

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  2. we are stumped too

    0 vote(s)
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  1. Earl Phillips

    Earl Phillips New Member

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    Nov 2, 2016
    1987 GMC v-2500 Sierra Classic 5.7 TBI

    For years I have had a up and down idle issue with my truck and I like most rebuilt TBI, replaced many parts as I don't have the technical background to diagnose the problem but I do have the shop repair book and wiring book for my truck from GM. I also had a issue with losing heat in winter too and a local garage told I had an internal intake vacuum leak. Both issues did resolve themselves after replacing many parts to an acceptable degree.

    I just brought my truck home from the GM repair shop to replace the motor and to fix the idle issue and the alleged internal intake vacuum leak. This is the 2nd GM crate motor and my idle problem developed several years after the first engine replacement. The new set up is from Jeg's which was a package deal which was GM crate motor, Edlebrock Performer intake, wires, plugs, coil, distributor, cap & rotor etc. The truck was in for three weeks... yes three weeks. The motor job was straight forward but the idle issue came back with a vengeance. This truck has a 4 speed Muncie trans and should idle at 650 rpm, yet at an idle it would go from almost stalling to 900 (-+) RPM. The shop went through everything from the charcoal canister, ECM, TPI, IAC, MAP sensor, EGR, Fuel Pressure & volume, replaced and rebuilt TBI to name a few items and everything they could think but to no avail. In the end it was not stalling but the idle is not right.

    I see they did turn up the idle stop. The truck starts right way and idles about 1100 RPM (+-) 200 or so RPM. When warmed to OT, the idle is around 1000 RPM but still fluctuates up and down.

    As they were trying to figure out the issue, it was odd that if you pulled the vacuum line from the vacuum booster, the idle surging would stop and smooth out but then they said it was running lean. I was told that something seemed to be turning on and off at least as far as they could tell.

    In the end with many new parts I still have an idle issue. Does anyone have the same problem or a direction we have not tried?
     

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