samseed101

Well I thought I had a vacuum leak. At idle the best I can get on the vacuum gauge is 15 inches. When in park, it seemsto be a steady 15 inches and the engine runs smooth with no problems. But as soon as I step on the brake and put it in reverse or drive, the RPMs drop to the point where it is ready to stall and the vacuum drops to 10 or below.

Now here's where it gets weird. If I put it back to neutral or park while keeping my foot on the brake, it seems to surge and nearly stall out. If I take my foot off the gas it seems to slowly build back up to 15 inches and run smooth again. If I'm in drive or reverse and I keep my foot on the brake while giving it some gas, the vacuum level rises and the car runs fine.

I was thinking that it may be a vacuum leak, possibly the brake booster, but I'm not 100% sure. The brake thing could just be a coincidence.

Now I know it's not the PCV because that has been essentially eliminated. The PCV now simply acts as a breather. The valve is hooked to a hose that vents into the atmosphere and the line on the intake has been capped off. I also have two valve cover breathers to help relieve crankcase pressure. So there is no vacuum working there and no unmetered air would enter the system that route.

While it was at idle and in park, I sprayed carb cleaner around the intake gaskets, TB gaskets, vacuum tree, brake booster, IAC, and behind the intake manifold where the vacuum source is. Nothing happened and the idle stayed smooth.

I just can't seem to figure out what this is. Was I on the right track with the brake booster thing? Or is this not even a vacuum leak? After all, if the RPM did drop that would cause the vacuum level to drop as well. I figured that the carb cleaner would give it away, but no dice.

On, and in case you are going to say "just listen for a hiss"...that's next to impossible because all I hear is a loud *** exhaust mixed with the loud *** whine of a Procharger supercharger, lol.

Thoughts? Ideas?

samseed101

You know...the more I think about it, the more I think I may have a check valve or something leaking in the brake booster... Too bad it's not something I can really chck myself. I imagine I would need someone to step on the brake to see if I hear anything.

With a bad check valve or bad brake booster, I would be able to hear the air leaking or escaping right?

AdderMk2

remove the brake booster hose from the booster and find something to block it off with.

as long as your not going anywhere, and dont step on the brakes, there wont be any problems. If you block that hose and you still have low vac... your problems is elsewhere

samseed101

Problem is... the surge / idle only happens after I've stepped on the brake so I'd have no way to tell.

Although I imagine that if I did that abd blocked off the line, the vac would increase past 15 inches (am I correct in assuming that?) so that may be one way to tell.

TwistedMetalMan

Have you checked your air bypass valve?
Does the problem present inself when you actually step on the brake or when reverse engages?
Also, is the same problem present when you subject the car to other loads at idle, ie. AC or hard Power steering cranking?
It is possable that it's a leak but I have seen the same thing with a faulty bypass with a load.

samseed101

Hrm.. I'll have to check into that to see. Unfortunately I won't be able to mess with anything until about 6ish Eastern tomorrow when i get home from work. I'll check those out and post results.

samseed101

I was able to mess around with it last night, but forgot to check it under other loads. It doesn't seem to be brake related, at least not directly. I disconnected and plugged the hose from the brake booster and nothing changed... the vacuum stayed at a constant 15 inches. Actually it was about 14.5 inches, but close to 15. It didn't surge or anything.

Again, when I put my foot on the brake, it was fine until I put it in D or N, at which point it would begin to surge and the car would flluctuate from 300 or so RPM to around 1100 or so. (hard to get exact numbers because it was constant movement.)

I did drive it a little to see what woudl happen. It seemed to want to idle at around 600 RPM when in gear and stopped, which seems too low to me. If I held the gas and got it to arounf 900 or so it seemed fine. I only drove for about a mile, but when i stopped it seemed fine. Only wanted to surge and stall when i tried to make a sharp turn while keeping my foot on the brake and traveling at a slow speed (which makes me think vacuum again.) But as soon as I gave it a little gas that solved that problem.

So right now I don't know what the hell this could be. I'm thinknig I should just reset the idle speed and see if that helps. Should be able to do that with the Tweecer when it gets here.

I can't even think of where else to check. bypass valve looks okay. EGR is blocked off. IAC looks fine. Brake Booster and vacuum tree looks fine. Intake gaskets look fine, PCV is eliminated and it's basically just a breather setup now.One thing I didnt think of checking... the line that connect to my vacuum / boost gauge itself. But I don't think that will be an issue.

frdmstng1986GT

maybe it is internal in the brake booster itself?

samseed101

Forgot to mention, pumping and holding the brakes does nothing when in park or neutral. The gauge still reads a steady 15.

Most likely is a leak somewhre, but I sure the hell cant figure out where, lol.

frdmstng1986GT

ok, well i am not up on my aod stuff but is there any vacum operated controls in the aod? if there is that would be where i would start.