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Thread: High Idle, Probably Intake Vacuum Leak?

  1. #1

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    I've got about two days of intermittent driving on my new-to-me Terminator setup. Right away the ECU couldn't get the idle speed down below 850-900 RPM. It's even worse now, about 1100 RPM. Backing off the primary & secondary blades completely and covering the IAC hole seemed to lower the idle by maybe 50 RPM. The IAC value never leaves 0%. Now it would seem evident to me that air is getting into the engine from somewhere. Being confident in my vacuum lines, it stands to reason that I'm having an intake manifold vacuum leak. Anything else I should be taking a look at as well?

    Additionally, engine warm in gear I see erratic idle speeds around 850-900 RPM. Going pack to park/neutral, the idle revs up to around 1300 RPM then settles to 1150-1200 or so.

    AMC 360, Air-Gap manifold with 1" spacer, lightly ported factory heads, 1-5/8" primary long tube headers, 204°/214° @ 0.50, .472"/.448", 110° LSA, TF727 1800-2000 stall converter.

    Additionally, I've been reading every thread I can find here on idle issues and tuning as well as IAC issues. I've familiarized myself with the Holley EFI manual as well. That said, assume I'm an idiot and talk to me like a child, LOL.

  2. #2
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    Either a vacuum leak somewhere, or a inoperative IAC valve (since it never changes from 0% IAC Position).
    http://forums.holley.com/showthread....neral-IAC-Info (General IAC Information - Read "IAC NOTES")
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, FRPP 3.31 gears, Moser 31 spline axles, Cobra Trac-Lok differential, 4-disc brakes/Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  3. #3

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    Well to clarify, the IAC does work. It goes to the Hold Position and when I let off the throttle, it decays back down to 0%. I’m going to go ahead and do a cam swap and will be pulling heads to inspect valve seats, so I will be resealing the intake manifold soon. It’d better run well enough to break in the new cam. I’m bumping up duration about 10° and lift a few thousands. Probably going to be working with a new base Global File.

  4. #4
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    You would be better off finding the intake leak first, before trying to run-in a new flat tappet cam.
    The intake is getting air from somewhere, if not plumbing to the throttle body, then it could be via the crankcase breathers. Gary
    Regards, Gary

  5. #5
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    Plug all the vacuum ports on the TB to eliminate it as a cause.

  6. #6

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    Quote Originally Posted by Gaz64 View Post
    You would be better off finding the intake leak first, before trying to run-in a new flat tappet cam.
    The intake is getting air from somewhere, if not plumbing to the throttle body, then it could be via the crankcase breathers. Gary
    I don’t disagree with you. It’s always smart to do one thing at a time. The only thing I haven’t checked is the brake booster line or the booster itself. I’ve never had as much vacuum assist as I feel I should. But, I’m pretty confident it’s at the intake runners to heads. Need to do a leak down test. Thanks for the input, guys!
    Last edited by Red20; 01-12-2019 at 01:15 AM. Reason: *leak down

  7. #7

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    You know, I'm getting some pretty powerful suction when I put my hands over the air horn. And I can almost stall the engine. If I have the throttle blades shut and the IAC is closed, how is it pulling so much air? I'm still pretty confident on an intake manifold leak, but I wanna rule out everything I can.

    If anyone cares to take a look, here's a cold startup .hefi and .dl:
    https://www.dropbox.com/s/qaiukbei89...idle.hefi?dl=0
    https://www.dropbox.com/s/lqg73qzl90...192801.dl?dl=0
    I didn't turn the car off; it just suddenly died. If it'll stay running, I'll datalog it at warm idle and share as well.

    I pulled & plugged the PCV line and the car seemed to be idling a bit lower. Still have the throttle stop screws all of the way out though. Here's a datalog of it idling, a rev or two, and putting the car in and out of gear:
    https://www.dropbox.com/s/a9bdw7dvkx...0noPCV.dl?dl=0
    Right after that, I plugged the PCV back in and datalogged a hot start (it was slow) and idle, in & out of gear, etc:
    https://www.dropbox.com/s/nl9so5pjba...20wPCV.dl?dl=0

  8. #8
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    The two most common places for a vacuum leak are, between the TB and mounting pad, and between the intake manifold and the head. Which intake are you using? Is it a spread bore or square bore mounting flange? It can also be hard to find the leak if it is at the lower portion of the gasket between the intake and the head. It sucks air from inside the lifter valley. You would need to remove the intake and look at the impression the intake leaves in the gasket to determine if this is your problem.

  9. #9
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    What are you using for crankcase ventilation? Remove all fittings from the valve covers and see if there's vacuum in the crankcase with the engine running. Seal all fittings except one, that's where you can plug it with your hand to feel suction or not. If you do feel heavy suction, the vacuum leak will be from the intake gaskets. Was the manifold verified for the EXACT same angles as the heads? Gary
    Regards, Gary

  10. #10

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    Gaz64, PCV valve in rear of intake manifold to TB port. No valve cover breathers on AMC. Good idea on leaving a single open intake manifold source to crankcase and looking for vacuum. There’s a thousandths difference top to bottom of intake mounting flange. I’m using fiber gaskets to make up for it, instead of the valley pan style that’s common on these AMCs. Also, using a THIN bead of RightStuff around water ports and liberal application of Indian Head or Aviation Form-a-Gasket (don’t recall which I used last) around intake ports. I think it just sucks the non-hardening type of sealant in on the gap side. Might have to try a different fuel resistant sealer, or run by the machine shop.

    81 TransAm, thanks. I’m pretty solid on this engine. I built it. And as mentioned in a previous post, it’s an Edelbrock Air-Gap manifold. Square bore with 1” spacer. Terminator throttle body.

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