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Thread: IAC runs up to 30% and engine races up.

  1. #1

    Default IAC runs up to 30% and engine races up.

    After rolling into the throttle while test driving, I put in the clutch and the engine is revving about 3000 RPM and the IAC reads 28 or 30%. When I pull the revs down with the clutch, it seems to idle normal, but the IAC will still read high. I've reset the TPS Autoset, and I'm using the 4 BBL MPFI Idle strategy. IAC Hold Position - 10%. Ramp Decay Time - 1.5 Sec. RPM Above Idle To Start Ramp - 1000 RPM. RPM Above Idle To Re-enable Idle Control - 50 RPM. Any Ideas?

  2. #2


    Your IAC Hold is 10%? If so, then that's probably your problem. Try IAC Hold of 30%.

  3. #3
    Join Date
    Dec 2009


    Usually a higher IAC Hold Position % is what causes problems.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  4. #4


    What's the part number of the EFI system you're using? Looks like your IAC might be wired wrong for the harness you have.

  5. #5


    Yes that is another very good possibility. If the IAC is wired wrong then the behavior can be baffling. Sometimes the factory harness is wrong which can drive you crazy trying to figure out the problem.

  6. #6


    I don't have the part number,But i did order it from Holley[Richard]. It's the billet body, the pig tail fits the stepper motor plug!
    I was wondering if as I rev, the 4554 non resistor plugs could be an issue? I'me using MSD 8.5 wires. I'me using a GM style one wire Alternator.
    There was a Boat Guy who posted a link to Amazon for Ferrite clips and a alternator noise suppressor,I thought I book marked it. Thanks for the replies

  7. #7


    Non resistor plugs are a bad idea with EFI. You need to replace those since they can cause random issues that you'll never solve. The MSD 8.5 wires should be okay and the one wire alternator should work as long as it is well grounded. Everything needs to be well grounded and that means a very clean connection back to the battery not just tied to some random part of the chassis.

  8. #8


    Should I double check the color code with the published HP Harness diagram?

    The ECU has dedicated terminals on the Battery. The MSD 6AL is connected to the Battery switch in the trunk. I did cheat and connect the MSD ground with every thing else at a lug welded to a frame connector. This is where the ground cable from the battery mounts, and a heavy cable goes from there to the Bellhousing, and from there a 10AWG goes to each head.

    I've checked the purple wires the Blue, Black, White and Yellow stripes are in the correct order. I couldn't verify A, B, C or D terminals on the plug. Has A & D as well as B & C been inverted in some cases?

  9. #9


    First find out of you have a GM IAC or the Holley IAC. I explain this at the bottom of my Holley Tech page, but here's a summary. Unplug the IAC and measure between the middle 2 pins (B & C). A GM IAC will be open and a Holley (or Chrysler) IAC will show resistance under 100 Ohms.
    Click image for larger version. 

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  10. #10


    I got my answer looking at the diagram. Thanks.
    Fastmanefi, I had a couple good test runs with no IAC issues. Thanks for sharing your knowledge.

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