Well, as some of you know, ever since the day I put my SuperCharger in, my boost and vacuum haven't been what I expected.

This is what I have done to troubleshoot it, and what I have found on the way: (Stillen SC, 3.25" Pulley, MEVI)

June -
ISSPRO Gauge I bought with the SC says 12Hg vacuum and no more than 6PSI of boost.

Dynoed at 231hp without the MEVI. The previous owner, with a 5spd, Dynoed at 290. Others who have Auto and 3.25 tend to Dyno at around 250-260.

Resealed all gaskets on the piping with gasket seal. (MAF/Ambient Air Temp, and TB)

Checked for hose leaks everywhere. Disconnected MEVI system completely and tested. No difference.

July -
Removed manifold, reseated, and re-installed.

Bought an AutoMeter Boost Gauge and hooked it up. Vacuum read better at 15Hg, but boost was still only 6PSI.

August -
Went to Czars house for a day of troubleshooting. After moving the boost gauge around, messing with hoses all day, and doing general diagnostics, I am getting 7-8PSI of boost. Checked vacuum against 2 diagnostics gauges. One said 15-16, the other said 20Hg. As the boost gauge also read 15-16, it was assumed gauge #3 was optimistic.

Hit 9PSI once or twice over the next couple of weeks, vacuum still low.

September -
Removed entire manifold, replaced TB and EGR gaskets, and sealed on top of that with gasket seal. Sealed top of MEVI with gasket seal as well. Car idles far better, but no improvement on vacuum.

While the manifold was off, went through every hose in the FSM vacuum diagram and inspected. No leaks could be found. Used a 4th vacuum gauge to test. This gauge said 17Hg. Now it is verified that vacuum is always at 15-17Hg at idle.

Compression Test with Jeff92SE -
Front Bank - 190 190 190
Rear Bank - 210 210 210

This morning at 3:00AM -
(I worked graveyard this week, and happened to be up, what better time to work on my car!?)

Removed the SC piping so that the only thing in line to the TB was the MAF. Vacuum unchanged.
Tested Boost gauge with a vacuum pump. It would pump up to 10Hg, then release quickly any air. Is this normal?

Question:
Any other ideas or tests I could run to try and figure out why my vacuum is lower than it hsould be. Everyone here I've spoken to with SC (Including MardisGrasmax, who has a MEVI) gets 20-25Hg.

Thanks!
IanS

 

Just so we are talking apple to apples here. When I say I get ~20hg that is with the idle around 800-900rpms, the AC off. on a 70 degree day average baro pressure at sea level or close to it.

Are your readings done in D, P or N?

 

I tested his car, in 'P', idle was around 750, about 75-80 out, AC off, pretty close to sea level. Took readings with an OBII reader, and pulled everything from timing - water temp.

 

I'm at idle around 650-700RPM, AC off, 60 degrees, average day, 300 yards from the waterfront. (sea level)

My last reading (3:00am this morning) off of my diagnostic pressure gauge was 17Hg in P. My boost gauge reads 15.5Hg, and I'm not sure at this point which one is closer to the truth. Either way, it's still low.

So to be safe, I'll say I'm getting 16.3Hg.

IanS

 

OK, I don't really know anything technical about superchargers or VIs or vacuum or forced induction in general, but when you say this, do you mean you bought the supercharger used? Do you think its possible the supercharger itself is damaged in some way? That seems like the only thing that hasn't been checked.

 

Like I said:

"Removed the SC piping so that the only thing in line to the TB was the MAF. Vacuum unchanged."

Basically, I unhooked the SuperCharger itself from the air system.
The Supercharger was taken out of the equation in this way.

So the SC is most likely fine.

IanS

 

IMHO 650-700 is a bit low for idle on a modified Max. Try and turn it up to ~850 and see how the vac looks then.

 

Did that. I adjusted the IAC to 900 actually. It didn't change anything more than 1Hg....so about 17.

IanS

 

Ah sorry. Oh well it was worth a try. Good luck troubleshooting man! This would really drive me nuts!

 

there must be something wrong with your engine if your leakdown test are 190 on front bank and 210 on rear bank. I think it should be with in a certain percent of each other. 210-190=20 is alot of pressure difference

 

According to the FSM, within spec for the VQ Engine is 142-185.

Which means I'm getting better than that, and if spec is 142-185 (a difference of over 40), then a difference of 20 doesn't worry me.

IanS

 

Don't they sell those dye so you can detect where the vacuum leak is coming from?

 

I think you mean A/C leak. If you were to put dye, or any other liquid on a car's Manifold vacuum, the water would probably get sucked into the engine, thereby screwing you.

IanS

 

Ian, I took at look at my vacuum for you. Mine seem to be normal.




With A/C off:

in Park, I got 21hg

in Drive, I got 18hg

Also, my idle was at about 850 rpms


Just to let you have a comparison because if you remember, I said I might have some boost problem too. However, my vacuum seem to be normal. I closed down the BOV some more, about 5 threads left including the hex nut on it. I noticed that no matter if I close or open up the BOV, my vacuum stay about the same. Well, hope this helps a bit.

 

hmm...with the car off...couldn't you somehow pressurize the manifold system and see where they dye is coming out from? There is gotta get a better way to look for a vacuum leak.

 

You are right but what i meant to say is that isn't it that both banks be about the same and not vary so much like your engine. I think it should be like front- 195 and rear 198 or something. That's what my mechanic friend (20 years and still working) told us prior to rebiulding my friend's Buick turbo V6

 

Nothing in the FSM mentions anything like that, so I don't know.

I do know that on Jeff92SE's 3rd Gen, he had the exact same kind of differences between Front and Rear banks.

IanS