boost lag?

birdboy89

Registered User
I have a '94 sc in the shop and I just got the motor swaped out. However the problem existed before the new long block. Ok so the car runs and drives fine but when under boost sometimes it will go to about 5psi and hang there for about another 2,000rpms. Then the boost will just kick and it will peg to 14psi right away. Sometimes there is no lag at all. Again I got a newer long block (less than 20,000) and my problem existed before the swap. Could it be a slipping belt?
 
Dump Valve

I'm no expert, but I would think you'd hear a slipping belt, or at least see evidence of it in the form of rubber dust deposited on the pullys, etc. Something else you might check is the "dump valve" (don't know the correct name) located between the inlet and outlet plentums of the supercharger. It's mounted behind the supercharger, and actuates a butterfly valve via a vacuum motor and external linkage. I suppose that if this actuator or it's hose was leaking, or the linkage or valve were hanging up, it could cause the condition you mention. However, I'll defer to the more seasoned experts that will respond...

Good Luck,

JD
 
Any other suggestions? I'd really like to finally get my car back so any help, thoughts or ideas are greatly appreciated.
 
what TXSC is talkin about is the air/bypass valve. if its staying open when ur under boost i believe it would cause that problem. its supposed to close completely and put all incoming air into the SC. yours may be "stuck open" momentarily after boost. u can check for belt slippage also. it would be at the crank to jackshaft belt or the jaskshaft to SC belt.

good luck.
 
The bypass valve needs atmospheric pressure to close, and is usually connected to a bunch of other stuff through a check valve, it's this check valve that accumulates vacuum pressure and prevents the bypass valve from closing in a timely manner. Its not the check valve's fault, its just doing its job.

Try connecting the bypass valve's vacuum hose directly after the throttle body, but before the blower, tee'ing and plugging what you have to. Get good vacuum plugs, I sucked the center out of one once at the most inconvenient time EVER.


Let me know how it works out.
 
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The shop also informed me that there is no "check engine" light on. Should it be for this type of problem?
 
There would be no sort of Check Engine light for this problem, thats why its usually very flustrating for mechanics to find.

Mine did the same thing and it was that the 2 little screws that hold the valve in place had come loose cause the vavle to move back in forth and not set like it should. Also sounds like a belt slipping issue also...

-Tim
 
wouldn't a belt slip the whole time it was getting boost? Also, its a new belt... but I guess thats not sayin they put it on right. First thing to check is the valve. Where is that valve located exactly?
 
The valve everyone is refering to is inbetween the back of the blower top and the intake plenum. Its actually attached the to blower itself with a nut a 2 tiny screws. have a very small vaccum hose attached to it the runs back into the intake plenum also.

Should be able to understand what I'm refering to with that...if not I can post pictures and mock them up for you.

-Tim
 
Finally got it back from the shop today....still running bad after the swap. They told me that they couldn't read my computer.Once it gets warmed up it runs fairly well. But when its cold it completely bogs down when boosted is introduced and the engine light eventually comes on too. Any diff thoughts now fellas?
 
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