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Old 03-19-2011, 01:18 AM   #1
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Default Mystery Vacuum Leak

I need some advice. Got a really rough idle with exhaust fumes that burn my eyes. Vacuum is reading about 8-10 inHg at 1000RPM, which is at least 50% of normal as I understand it. Prior to this it was running a solid 21 inHg at 1000.

It started a few weeks ago. I had been running a GT2860RS for about a month with a Hydra 2.5. No problems. Tune was conservative and limited to 5 psi until I could sort everything out. Coming home one night I ran the car up to 5 psi, not even over 4K rpm and i felt a stutter... came to a stop and the car dies. It restarted and ran rough at idle but at about 2K would come alive and make some really nice power. Idle was terrible and rich smelling, vacuum low, as stated. Boosted fine, though.

Thing is, I can't find the damn vacuum leak to save my life. I've used the "propane" bottle method, checked all the hoses, removed the intake manifold and throttle body and inspected the gaskets. Siliconed everything back together. Put new seals on the injectors. I've tried to eliminate every potentially bad vacuum hose by systematically removing it and plugging the IM, just to see what effect this might have. EGR is blocked off. I pinched the hose to the brake booster and PCV. Just to be sure I changed the plugs, which were wet when removed, and wires. Tested spark at the coils. Temporarily blocked off the IAT. I tested the compression and found it rock solid across all four cylinders.

I went so far as to reinstall the stock ECU to run the engine at idle. Same symptoms.

A few specs... 2001 engine, Hydra 2.5, GT2860RS, 3" exhaust, No MAF.

Any ideas? If I can't figure this out I'm gonna have to find a smoke machine or a mechanic who has one. It's got to be a big damn leak to cause a 50% loss of vacuum.

Does anyone know which component(s) might be most susceptible to ruptures under boost that could cause such a big vacuum leak? Something in the EVAP emissions system or the stock FPR? I've tried to eliminate that possibility, but who knows. Thanks for considering the problem...

James
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Old 03-19-2011, 02:59 AM   #2
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Try this:

unplug the isc, see what happens to the idle
close off the throttle body idle adjust screw. see what happens to the idle.

report back
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Old 03-19-2011, 10:27 AM   #3
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Sounds like something has gone out of time to me. Check the timing belt for jumping. Vac that low is not a leak that you cant hear, a leak that high would give high RPM, not a low idle.
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Old 03-19-2011, 03:12 PM   #4
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I just checked the ISC in combination with the idle speed adjustment screw.

TB idle screw out 100% and ISC plug removed = engine dies
TB idle screw out 50% and ISC plug removed = engine dies
TB idle screw out 0% and ISC plug removed = engine dies

Checked the timing belt. All three marks are dead on at TDC. Belt has less than 20K and tension looks good.

Still running really bad at idle. Smooths out over 2K.
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Old 03-19-2011, 08:48 PM   #5
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how much vacume at 2000 and 4000 rpm?
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Old 03-20-2011, 01:21 AM   #6
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I had the same problem and found that my intercooler has a crack at the bar that could not be detected on visual inspection.
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Old 03-20-2011, 03:22 AM   #7
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Quote:
Originally Posted by jimmyturbo View Post
I just checked the ISC in combination with the idle speed adjustment screw.

TB idle screw out 100% and ISC plug removed = engine dies
TB idle screw out 50% and ISC plug removed = engine dies
TB idle screw out 0% and ISC plug removed = engine dies

Checked the timing belt. All three marks are dead on at TDC. Belt has less than 20K and tension looks good.

Still running really bad at idle. Smooths out over 2K.
?????

Hit your ******* ISC with a hammer?

Pull valve cover and rotate to crank TDC

make sure camshaft lobes are both pointed outwards?
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Old 03-20-2011, 03:22 AM   #8
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Quote:
Originally Posted by nismo502 View Post
I had the same problem and found that my intercooler has a crack at the bar that could not be detected on visual inspection.
Seriously how did you figure this out sable I mean nismo?

Last edited by Faeflora; 03-20-2011 at 12:21 PM.
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Old 03-20-2011, 06:35 AM   #9
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Check the seals at the front of the injectors. Check the injectors to make sure one is not hung open.
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