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Thread: IAC goes to 0% at idle, what can I modify to help it stay in play?

  1. #1

    Default IAC goes to 0% at idle, what can I modify to help it stay in play?

    I'm working with an Enderle 8 Stack on a 462 BBC. Marine hydraulic roller on a 112° centerline. This cam is designed to make vacuum. As it's warming up, the IAC slowly moves from 35-40% until it's at 0% when it reaches operating temperature. When it's done, it's idling at 900 RPM with that 0% reading.

    What are some things I could modify in the tune to allow the IAC to get involved? I have done a warm "butterfly" adjustment procedure as instructed by folks that set this intake up for EFI. However, there must still be enough airflow moving past the butterflies that the IAC doesn't need to open to help achieve the idle speed.

    Perhaps less timing? More fuel? Change the Target A/F Ratio? I'm a newbie at this, so I'm learning as I go. I plan to have Rich at do an online tuning session once I'm ready to drive the car, but right now I'm working on my startup and idle settings.

  2. #2
    Join Date
    Dec 2009


    Typical IAC Control/Ramp Down Parameter Settings
    Advanced Idle Control: ......................... Slow (Usually the best control.) Sniper EFI users select "Sniper TBI".
    IAC Type: .......................................... Stepper (4-wire), PWM (2-wire)
    IAC Hold Position: ................................ 10%-30% (Usually 15%-20%.)
    Ramp Decay Time: ............................... 1.0-3.0 sec (Usually 2.0 sec.)
    RPM Above Idle To Start Ramp: .............. 1000 RPM (Or higher.)
    RPM Above Idle To Re-enable Idle Control: 50-200 RPM (This setting can be finicky.)
    Startup IAC Position - Hold & Decay Time: 1.0-1.5 seconds (Less is typically better than more.)
    Not IAC, but Idle Spark control usually works well at: 30-40 P Term & 50-60 D Term. (General IAC Information - Read "IAC NOTES") (Initial Checks & Adjustments - Read Step #3!)
    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.

  3. #3


    Things you can do:
    Raise the Target Idle RPM.
    Decrease idle timing.
    Work more on the throttle plates when warm. For manifolds that don't control each butterfly separately, use the old "bend to fit" approach using a flow meter.

    I would not change the Target A/F Ratio. Once you find the engine's "sweet spot". That's where it would run better (and longer too).

  4. #4
    Join Date
    Nov 2014
    Ont. Canada


    If your Target Idle RPM is 900 and it's idling at 900, out of gear with 0% IAC, what's the problem? When it goes into gear and the RPM drops, the IAC should open a little.

  5. #5


    I actually have not had the car in gear yet, the rear end is out of the car for a gear change. What you said makes sense. The load on the engine with the car in gear and the brakes on, might be enough to make the IAC get involved to maintain the idle speed.

  6. #6


    Hi Fastmanefi. It's your customer, Rick C. Although my screen name here is 427bbc, that's a left over from a Corvette that I had in the past. Back then, I was on this forum for some carb help.

    The tune you sent back to me made for a smoother startup. I let it get up to temp and run for 15 minutes or so, enough for the fan to cycle on & off a couple of times. It still managed to get down to that 0% on the IAC Position. I haven't tweaked anything on your tune, I'm going to leave it alone for now.

  7. #7


    With a race engine, and especially with an 8 stack system, if the idle speed is stable with the IAC at 0% then you might not have a problem. On a daily driver you would want the IAC to have a little more control at idle, but with a race engine it isn't unusual for the IAC to at zero at hot idle. It won't be at zero when the engine is cold which is the whole point. The IAC can help a race engine get started and warmed up. When you get the IAC and the cold start enrichment working correctly you can cold start a race engine by just leaning in the window and touching the starter button. That's the great thing about EFI and the IAC. If it starts and runs and has a stable idle, then that's what's important. The fact that the instruction book says you should have X at idle doesn't really mean much. The instructions were not written for 8 stack engines.

  8. #8
    Join Date
    Jan 2010
    Brisbane, Australia


    427bbc, What size throttle bores?
    How do you have your remote IAC plumbed to each runner?
    Photos would be good. Gary
    Regards, Gary

  9. #9


    Mine seems to run best when the IAC is setup such that it goes to ~0% when the engine is fully warmed up and it's been idling for maybe 5 minutes. It creeps down when warm and ends up at 0%, though if I zoom in on a datalog, it's not really 0%, I can see it ticks above 0 occasionally. Idles really nice considering 900 RPM, but only 8" vacuum.
    427W 10.7:1 250 duration(0.050)/0.600 lift cam Supervic/elbow 85mm Ford TB/IAC controlled by Holley HP.

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