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  1. #1

    Default 350 TBI Idle Issue

    Hey everyone, new member here!

    My truck in question is a 1988 GMC Suburban V2500 350 TBI / TH400.

    I have had consistently high idle since I bought the truck in may. Not sure what it is exactly, but sounds to me like around 1200 RPM. Other than the idle, the truck runs fine. No power loss or hesitation. I have seen the service engine soon light on twice, just came on while driving normally and shut off next time I started the truck. I checked the timing today and it seems to be at about 12 degrees with the tan wire CONNECTED. If anyone could tell me exactly where that wire is that would be a help. I am going to check my IAC tomorrow and see if having it out changes anything. As far as vacuum leaks go, I found one behind the throttle body (not exactly sure where it went, just saw a hole in the line and snipped the end off, reconnected it). Fixing the leak didn't change the idle at all.

    Any suggestions please let me know.


  2. #2
    Sr. Mechanic Ethan's Avatar
    Join Date
    Sep 2009
    Cedar Park, TX


    Theres a few things that will give you high idol on that kind of truck, one is a vacuum leak, like, if you pull the vacuum line out of the brake booster, it'll bring your idle speed up, so check all your vacuum lines for leaks, or just go ahead and replace them all to be extra sure.

    Second thing is the IAC valve, taking it out will give you no idle, cause that's what dictates your idle speed. Usually they don't go bad though.

    Third thing would be a leaking injector, could cause a high idol.

    I wouldn't mess with your timing at all, it's rare for the timing to go out on these kind of trucks.

    The truck should have an idol speed of around 550 to 600 rpm, from factory it's set at 750 rpm, but a TSB tells you to reset it at 550 rpm. Might take it to Autozone and have them hook up the scan tool to it and play with the IAC valve to make sure it works, and check the O2 sensors and reset your idol speed.
    1994 Chevy Silverado 1500 Ext Cab 350 cuin TBI in dark Chevy blue
    160,000+ miles
    Original paint
    Original interior
    Original engine and tranny
    wrecked twice stolen once
    Diamond Plate bed rails
    Power windows
    Original Alloy wheels
    Original fancy smancy cup holder
    Tow package
    Original plastic bed liner
    K&N Cone Air Intake
    Dual MagnaFlow exhaust(not true dual) with chrome tips
    MSD Street Fire Performance Ignition coil

  3. #3
    stephan's Avatar
    Join Date
    Sep 2010
    Far West, Oregon USA,
    Blog Entries


    88 Burb, the wire you're looking for is behind the black plastic cover that's on the firewall passenger side. It's a tan/light brown wire with a black tracer & has a disconnect plug. There's a mess of wires behind the cover but it's in that wad.
    1988 Chevy C-3500 2wd (no pic)
    350 c.i. 5.7 L Stock Block, 4 Bolt Mains
    L-31 Vortec Heads, Edelbrock Cam & Intake,
    Holley 650, Flowtech Headers, Magnaflow exh.
    Jet Trans 700R4, B&M Ratchet, 4:10 gears,
    3" susp. lift kit "shadetree"
    No rev limiter, No speed limiter lol

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