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Bob, Auto Tech
Category: Chevy
Satisfied Customers: 4087
Experience:  Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)
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my 1997 chevy truck idles too high when i start it

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5.7 L idles up and sometimes, though rarely, misses and backfires. it always idles back down after a while but i would like too fix it. also when i put it in gear it idles too high again until i hold it stopped for a few secs. if i take off before doing this it will stay idled up until i come to a complete stop for a few secs. what is it?

Hello, as a rule the first thing to suspect with this kind of a condition is a problem with the IAC (Idle Air Control). Sometimes they will get carbon build up on the pintle and that can keep the pintle from moving in and out as it should. The IAC is what controls the engine idles speed by allowing more or less air into the system as needed. But it can also sometimes be just a bad IAC or the IAC could be not getting the correct signal to it from the computer because of another sensor that may be defective and giving the computer false information. The only way an IAC can be reset is with the use of a class 2 scan tool. But you already have a pretty good idea of the things to check for if you have been looking for vacuum leaks. I would inspect the pintle for carbon build up and if that doesn't appear to be the case you can take a look for some of the other things that GM suggests posted below. Other than that, you will need to have the IAC tested with a scanner to see if it needs reset or is getting the wrong signal to it. Thanks

  • Inspect the engine mounts. Damaged mounts may cause tension on the throttle linkage.
  • A fuel system that is too lean or too rich.
  • Inspect the throttle body bore for foreign material.
  • Gasket leaks
  • IAC reset is done with the scan tool.

An intermittent may be caused by any of the following conditions:

  • A poor connection
  • Rubbed through wire insulation
  • A broken wire inside the insulation

Idle Air Control Valve Replacement

Removal Procedure

    Caution: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.

  1. Disconnect the negative battery cable.
    Object Number: 312513  Size: SH

  2. Disconnect the IAC valve harness connector.
    Object Number: 12909  Size: SH

  3. Remove the IAC valve attaching fasteners.
    Object Number: 12910  Size: SH

    Notice: If the IAC valve has been in service: DO NOT push or pull on the IAC valve pintle. The force required to move the pintle may damage the threads on the worm drive. Also, DO NOT soak the IAC valve in any liquid cleaner or solvent, as damage may result.

  4. Remove the IAC valve assembly.
  5. Remove the O-ring.

Cleaning and Inspection Procedure

  1. Clean the IAC valve O-ring sealing surface, the pintle valve seat, and the air passage.
    Use carburetor cleaner and a parts cleaning brush in order to remove carbon deposits. Follow instructions on the container.
    Do not use a cleaner that contains methyl ethyl ketone, an extremely strong solvent and not necessary for this type of deposit.
    Shiny spots on the pintle or seat are normal. They do not indicate misalignment or a bent pintle shaft.
    If air passage has heavy deposits, remove the throttle body for a complete cleaning. .
  2. Inspect the IAC valve O-ring for cuts, cracks, or distortion. Replace if damaged.
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Customer: replied 9 years ago.
i took that thing off and a piece of carbon fell out that i thought was the o ring!!! it was filthy. cleaned and replaced it. didn't completely fix problem but greatly improved it. at least i know i'm on the right track. thanks, XXXXX XXXXX saving me days of searching. i will be a repeat customer.
Glad to hear your making progress, keep me informed. I will be here most of the time if you need me, just drop a note. Thanks