Results 1 to 7 of 7

Thread: Surging Idle

  1. #1

    Default Surging Idle

    When the engine is idling in park or neutral, the idle fluctuates from ~770-930 RPM. When I put the car in gear, the idle gets much smoother. Very close to target. The idle even gets smoother when the dual electric fans come on, I guess placing an alternator load on the engine. Target Idle Speed is 850 RPM. Engine is 376 cubic inches with a small cam (210°/230°@ .050). I took a datalog. MAP is 34-38 kPa at idle. The ECU seems to be modulating Idle Spark properly (retarding as RPM increases and advancing as RPM decreases). The IAC also seems to be functioning properly (going from 6-10% trying to stabilize the idle). Target A/F Ratio in that area is 12.6:1 (E85 on a Gasoline scale). Actual AFR varies from ~12.2-13.1. Any ideas what may be causing this? Thank you.
    Last edited by Danny Cabral; 10-17-2019 at 11:56 AM.

  2. #2
    Join Date
    Dec 2009


    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 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, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 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 tires.

  3. #3


    Injector PW ranges from 1.05-1.2 ms where this idle surging occurs. I use a sequential injection strategy. I have 160 lb/hr Holley injectors. Is this pulse width too low?

  4. #4


    It might be, but you should also make sure that the timing is stable in the idle area. Andrew
    IG @projectgattago

  5. #5


    Try leaner. My BBC idles 14.2 AFR with E85. ~65 kPa 800 RPM.

  6. #6


    Thanks guys.

  7. #7


    Make sure you're using some Idle Spark control. Also, Injector End Angle tuning can help with this.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
About us
Holley has been the undisputed leader in fuel systems for over 100 years. Holley carburetors have powered every NASCAR® Sprint® Cup team and nearly every NHRA® Pro–Stock champion for four decades. Now, Holley EFI is dominating the performance world as well as our products for GM's LS engine. Holley's products also include performance fuel pumps, intake manifolds & engine dress–up products for street performance, race and marine applications. As a single solution, or partnered with products from other Holley companies - Hooker Headers, Flowtech Headers, NOS Nitrous, Weiand, Earl's Performance Plumbing, or Diablosport - Holley products can give you the edge you need over the competition.
Join us