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Thread: High Idle, Probably Intake Vacuum Leak?

  1. #11
    Join Date
    Jan 2010
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    Brisbane, Australia
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    Did you understand what I meant remove all the fittings on the valve covers?
    I should have said seal up the engine, valve covers, etc., sealed including NO PCV, then run the engine.
    If you can't remove the oil cap, there is a large vacuum in the crankcase.
    If the engine idles slow with cap on, then idles fast with cap off, the intake manifold is the cause. Gary
    Regards, Gary

  2. #12

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    Yep, fully understand. Makes good sense. I’m with the family at Disneyland, but I’ll be trying that when I get home.

  3. #13
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    That's good. Hopefully something stands out. Gary
    Regards, Gary

  4. #14
    Join Date
    Oct 2016
    Location
    Buenos Aires, Argentina.
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    46

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    Check the power brake hose, the boost port in TBS and manifold.
    1970 Dodge GTX - Small Block Mopar 418ci - HP EFI Terminator TBI

  5. #15

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    Breaking all of the “make one change at a time” rules, I just pulled the intake manifold, swapped cams, and buttoned it all back up last night. I dry fit the intake and I have a .0025" gap at the top of the intake ports, bottom flush. Not flat as it should be, but not as bad as I’ve seen before. I was meticulous with cleaning and properly resealing it, and I feel confident with the sealer & gasket set I’m using that I will get a good seal.

    Tonight I’ll do a once over, retorque everything, and break-in the cam. Then I’ll let it settle into an idle, Learn a bit, and hopefully get some response out of the IAC valve and be able to set the idle properly. I look forward to cruising & tuning and enjoying the car again.

  6. #16

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    Well, the intake manifold is sealed up and I’m still getting a high idle & 0% IAC. It does move to the IAC Hold Position with throttle input, so I know it physically moves and functions. Idle won’t go below 950. I did notice when I had the throttle body off that even with the screw backed all the way out, the secondary throttle blades don’t completely close. There’s maybe a 2-3 thousandth gap that I can see through when I hold it up to the light. Also, this thing linkage is progressive. I didn’t check yet to see at what TPS % it starts to move, but it is definitely delayed.

  7. #17
    Join Date
    Dec 2009
    Location
    Connecticut
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    19,570

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    Quote Originally Posted by Red20 View Post
    I did notice when I had the throttle body off that even with the screw backed all the way out, the secondary throttle blades don’t completely close.
    Terminator TBI users may need to also adjust the secondary shaft screw (LINK), so the throttle blades are equally open.
    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.

  8. #18

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    This is a used Terminator throttle body and it looks like the secondary bores and throttle blade shafts were a little dirtier than I thought. It was just gunked up enough to keep the blades from closing properly. I can push the butterflies closed with my fingers and it idles down a good 100 RPM or more. Some cleaner and working it back & forth repeatedly for a bit made a big difference. It’s coming back down the my 750 RPM idle speed now and IAC is hovering around 2-5%. Works for me.

    I datalogged with the ignition in the run position, engine off, and opened the throttle until the secondaries almost started to move several times and determined they start moving at 15% TPS, so I set it up for that. I also added 50% fuel prime which cured a long cranking time I was having.
    I have a ticking lifter after swapping cams and adjusting preload, so tomorrow I’m going to pull the covers and adjust them while idling like I usually do and I’ll start driving it. Just a quick lap around the block though was awesome. It already feels great.

    Thanks everyone for the thoughtful responses and help getting this thing setup. I’ll be around the forum often as I begin to hone in on my tune with time. Hopefully we’ll be chatting about TCC control and nitrous setup in the future. Can’t beat a daily driver with a 150 shot & overdrive.
    Last edited by Red20; Today at 04:06 AM.

  9. #19
    Join Date
    Dec 2009
    Location
    Connecticut
    Posts
    19,570

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    I had a suspicion you'd find a secondary shaft/throttle blades problem. Glad you found it.
    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.

  10. #20

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    Thanks Danny! I’m glad I went ahead and resealed the intake manifold as well. The gaskets I was using before were too thick and alignment wasn’t what it should have been. Plus I’m still fairly certain it was leaking. Micing & inspecting the gasket confirmed some issues. Plus it gave me an excuse to install a cam I had sitting on the tool cart waiting for this car. And clean & regap plugs, clean distributor cap contacts, deal with a leaking timing cover gasket, replace old valve cover gaskets, etc., etc.

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