F1nd3r05

Hi guys. My 944 turbo has a strange vacuum leak. While it normally sits at 19in/hg, after revving it, or going into boost, at idle it will go to 15in/hg and then slowly work its way back to 19in/hg. It is also preventing me from achieving full boost. Even when I tried clamping the wastegate line off the intercooler piping, the car still cuts out at a certain boost pressure. I am assuming the boost cut is due some rubber or silicone piece opening after a certain amount of boost pressure is introduced/it sealing up after it is reduced. I just figured I'd ask if anyone has an idea if this is specifically what is happening, if they themselves have had it happen and know what it is, or any advice that is useful. Yes i tried pressurizing the system and havent been able to find anything that stood out. Thanks and let me know what you think!

Dougs951S

do a simple test, once the car is good and warmed up, unplug the Idle control valve from under the intake manifold and then rev it and see if it still does it. It may be getting stuck, which wouldnt show up under a pressure test. 19" is otherwise a nice healthy number for a 951 to pull.

F1nd3r05

so when I unplug the ICV, what will the result of the test look like on my aftermarket boost/vacuum gauge? with it unplugged should the problem go away or remain?

Dougs951S

if the car is completely up to operating temp, the ICV is not being used and the computer should put it in the closed position when at idle. (since it is a PWM controlled device, when no power is applied it should default to closed) If my theory is correct and it is sticking when you give the throttle a jab, doing teh same thing with it unplugged should yield a quick return of your normal 19" vacuum value after the throttle is closed.

F1nd3r05

okay thanks for your help. One more question. Can i drive the car with it unplugged? In otherwords can i go into boost with it unplugged? can that harm the car? Whatever the issue at hand is also causes the car to not boost fully so i would like to test if that is the direct cause.

Dougs951S

Yes you can drive the car with it unplugged but seeing as how its january and I'm assuming its cold wherever you are, the car will idle like *** till it warms up. I run with no ICV in my car at all, but I live in south Texas and my car is set up and tuned to run like this. By the way I guess my reading comprehension sucks because the ICV has nothing to do with the car not boosting properly, its simply a bypass to allow a small amount of air to get around the closed throttle blade and into the intake to help a cold engine idle. If you are having boost issues, that is very unlikely the cause. It really does sound like you have a leak source that is opening up under pressure. How did you pressurize the intake to test for leaks? You need to put at least 15 lbs of pressure into to test it correctly. As is all too common with these cars, you may very well have a multitude of issues. Any exhaust leaks including in the wastegate is a common cause of boost issues.

F1nd3r05

I removed the afm and hooked up a compressor to a plate where that goes into the jboot. The only air i really hear is the air that comes from the aos. i will block off the aos next time i try and hopefully something else will be heard easier. wish me luck.

F1nd3r05

so I pressure tested the car and heard next to nothing. something faint under the intake manifold so i took it off and the only tear i found was the rubber piece that goes from the air oil seperator to the intake. specifically the rubber piece from the aos tip on top to the metal venturi pipe which then, on the other side, goes into the jboot. i actually took that out and plugged it when doing the test, so i am curious if this actually would have any affect on my boost loss, or do i need to continue to look for something. also i found a slight tear in the vacuum line that goes from the throttle body to the heat exchanger. INTAKE MANIFOLD IS OFF so now would be a great time for suggestions

OnlineAlias

Replace all the things!

Carlsbergas

best advice you can get

Dougs951S

Are you still running the stock cycling valve? If so, I'd bet its cracked.

Van

I bet it's your check valve either to the brake booster (are your brakes sometimes hard?) or to the vacuum reservoir. If either of these goes bad, they will "collect" boost pressure, and then when the intake generates vacuum (throttle closed) there is a lag before all the air is sucked out of those places.

F1nd3r05

The brakes are fine. The boost comes on perfectly as well until about 12-14 psi and it just cuts like its blowing out somewhere. I know this sounds weird but if i keep my foot a hint less than that amount while watching it on the gauge, the car accelerates perfectly. the second i go to push over that rate it just like stops. the car never stops completely though. i can literally just lift my foot up slightly and it will keep accelerating through the gear as long as i stay below the limit of boost.

Dougs951S

Fuel pressure issues?

F1nd3r05

I don't think its fuel pressure. I upgraded to a 3.0 fpr and have lindsey chips in my dmd and klr. I also have a lindsey super 75 if that makes a difference to you guys. I will check the check valves when i get a chance.