Thread: Vac loss, Power loss, strange bov whistle..WTF!

Ok so I upgrade from t25 to t28 and then my clutch starts slipping. No problem, I got a new clutch and installed. Around the same time my clutch went out I had a power loss and low vac issue. I waited until the clutch was done until I tried to figure it out and now It's become an issue I can't figure out for the life of me. The power/torque feels much lower, the vac sits around 15-16 as opposed to 18-19, and the BOV makes a constant whistle due to it being partly open for whatever reason. It idles ok and drives normal but breakes up @ wot around 5k.

Ive triple checked vac lines, replacing some that looked questionable. Removed the TMIC and put it back together checking all couplers. Swapped the Greddy BOV back to stock, no change there. Swapped intake pipes and ran both circ and uncirc. Sprayed brake cleaner over anything that could have had a leak with zero results.

Im to the point where Iv'e checked everything 20 times and had other people look at it and they found nothing eigther.


Tonight I depressed the brakes at idle 3-4 times and watched the idle drop even lower to the point that it nearly stalled. Turned the car off and removed the hose to the brake booster (heard the rush of air) and checked the check valve which seems to be operating fine. Any ideas on this? I guess from here I should just replace the whole hose from the intake mani to the booster and see if that helps

You sure you're getting ignition and firing on all cylinders? Breakup sounds like either fuel injectors or ignition... I'm thinking if you have a cylinder out the car could still run but you feel loss of power.

A non firing cylinder wouldn't cause loss of vac though, would it?

I would even say go so far as to comp test it.... I don't know.

I've got the same exact vacuum problem, as far as depressing the brake pedal repeatedly and watching the vacuum drop the car into a stall.

Let me know what you find on this one. Subscribing.

Hope you get it figured out, though!

i was on the same boat as you were with the last motor. not sure if the problem will remain on the new one. everytime i come to a stop the vac would drop low and sometimes stall not all the time, i didnt feel no power loss or cutting up top like yours. but i could sit there at idle and press the brake and watch the vac go from 13 (S4 cams) to under 10... i checked and rechecked everything and could not figure out what it was. i couldnt hear any vac leaks. im thinking maybe brake booster. i would say cap off the hose to the brake booster and start the car and see if the problem persists with low vac.

that was my next plan until the motor gave out, haha good luck knuckleduster. seems to be a problem that haunts many of us.

Ill cap it off and see what happens. I was running the wastegate and bov off the booster line, im going to remove those and run from the intake manifold instead to see if anything changes.

I also thought it could have been a leak in the charcoal canister somewhere but removed it and there was no change. Ill post up any positive/negative results tomorrow.

man, sounds like what happened to me when my motor first started dying, but i hate to say that. give yourself a little comfort by checking compression. If you don't have a tester, I have one i'll send to you if you want to borrow it. lemme know. good luck man.

I would think that if it was compression it would get progressively worse over time not just all of the sudden.. but I don't know that for sure.

I pressurized the brake booster and that held up fine.

wierd... if you sprayed carb cleaner over everything the idle would have gone up quick if there was a leak

I can remember a lot of cars where the idle would respond to pumping the brake pedal like you are describing.

I'd do a quick comp test (just to be safe) and then pressure test and/or smoke the system out for leaks.

There is a check valve in the brake booster line. If it wasn't working properly, it seems like it could cause some serious problems (like affecting the idle when you hit the brakes). It could also be bleeding off boost, which would keep the engine in vacuum, which would keep the BOV open, which would make it even more difficult to build boost.