Upper intake vacuum leak - Ford Mustang Forum
 
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post #1 of 9 (permalink) Old 01-18-2011 Thread Starter
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Upper intake vacuum leak

I believe I have a big vacuum leak between my upper and lower intake, would this cause the engine to idle at high revs?

1988 GT 5.0, MAC Fender Well CAI, MSD Ignition, Ron Davis radiator, Ford Racing 3.73 gears, 5 lug conversion, tubular control arms, fuel pressure regulator, Cobra upper and lower intake, high torque starter, full length headers.
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post #2 of 9 (permalink) Old 01-18-2011
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In my case just the opposite.

Every vacuum leak I've ever had when the engine is cold it doesn't want to say running and dies, and if you get it too much gas you can hear it backfire/pop inside the intake.
If you can slowly give it gas and once the engine warms up it sounds like you have a big cam, is about the best I can explain it.
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post #3 of 9 (permalink) Old 01-18-2011 Thread Starter
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I can't figure it out, I took off the upper intake to replace the fuel pressure regulator, while I was at it I noticed one of the fuel rails was not bolted down, so I pressed it down and bolted it in, replaced the regulator, put the upper back on and now there is a loud hissing noise and she idles at 1500rpms, then when I shut her off there is loud sound like air being sucked or blown out...I dont need to say I'm pretty concerned lol

1988 GT 5.0, MAC Fender Well CAI, MSD Ignition, Ron Davis radiator, Ford Racing 3.73 gears, 5 lug conversion, tubular control arms, fuel pressure regulator, Cobra upper and lower intake, high torque starter, full length headers.

Last edited by dragonride; 01-18-2011 at 10:41 AM. Reason: grammar
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post #4 of 9 (permalink) Old 01-18-2011
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Quote:
Originally Posted by dragonride View Post
I can't figure it out, I took off the upper intake to replace the fuel pressure regulator, while I was at it I noticed one of the fuel rails was not bolted down, so I pressed it down and bolted it in, replaced the regulator, put the upper back on and now there is a loud hissing noise and she idles at 1500rpms, then when I shut her off there is loud sound like air being sucked or blown out...I dont need to say I'm pretty concerned lol
make sure your upper to lower intake gasket is positioned properly, and you have your pcv hose plugged back into the intake.

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post #5 of 9 (permalink) Old 01-18-2011 Thread Starter
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Ok, so I just went to check the torque on the upper intake bolts and some were off so I re-torqued them, unfortunately one of them broke at just past 10lbs guess I better take the whole thing off and start from scratch with a new gasket. I tried to start her anyways and she still rev'd up to 1500rpm, even with the bolt missing, then gave a loud pop so I decided to let it be for now.

1988 GT 5.0, MAC Fender Well CAI, MSD Ignition, Ron Davis radiator, Ford Racing 3.73 gears, 5 lug conversion, tubular control arms, fuel pressure regulator, Cobra upper and lower intake, high torque starter, full length headers.
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post #6 of 9 (permalink) Old 01-18-2011
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The ol snaping the upper intake bolt....the stock bolts are crap. Look for stronger ones at your local hardware store, or online. I think stock torque for upper bolts are like 8-12 lbs or something like that. You shouldnt need to crank the hell outa them. Usually a vacuum leak will cause a fluctuation in your rpms, the computer keeps trying to compensate for the leak. Are you sure the leak is from the upper intake? It usually sits well. Have you checked you lower manifold? I had to retorque those bolts on occasion at the beginning of a new season. Whats your fuel pressure at? Should be lower 40's with vacuum hose disconnected and blocked. I keep mine at about 43 lbs. What about something as obvious as the idle itself? Pop you heard may be a timing issue, as could be the idle problem. Have you properly connected all the hoses to the upper manifold? Did you check the o rings on the injectors where the fuel line was loose? Just throwin stuff at you....and one more unknown trick that did wonders for my mustang, get some di electric grease, pull all your engine sensors apart and clean them with it. Good luck, youll find the problem if you keep tinkering.

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post #7 of 9 (permalink) Old 01-19-2011
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I was having trouble with my idle and stalling problems with the initial startup of the car for a while. Replaced gasket on my iac which was one culprit. Started spraying carb cleaned around gaskets and when spraying around the upper intake your can hear the idle change when spraying. I was getting the backfire in the intake and it also sounds like there is a huge cam. Id say replace your gasket..
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post #8 of 9 (permalink) Old 01-19-2011
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Hissing is a vacuum leak you can pinpoint it with TB cleaner if the new gasket doesnt fix it.

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post #9 of 9 (permalink) Old 01-21-2011
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Quote:
Originally Posted by ALMOST STK View Post
In my case just the opposite.

Every vacuum leak I've ever had when the engine is cold it doesn't want to say running and dies, and if you get it too much gas you can hear it backfire/pop inside the intake.
If you can slowly give it gas and once the engine warms up it sounds like you have a big cam, is about the best I can explain it.
Not meaning to hijack this thread, but your description describes what my car is doing exactly. Thanks. And thanks to all who help on this site.

BBK CAI, 70mm MAF, TFS Street Heat kit(intakes, cam TW heads), long tubes, off road h-pipe, Flowmasters, Aluminum driveshaft, Pro 5.0 shifter, 3:73's, 255lph, a few suspension mods.
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