Intermittent Boost leak?

Helltime

Helltime

Registered User
I've just recently started gathering parts for my car and got it legal to drive today, i noticed that sometimes it only boost to about 3psi, other times it will go to 8psi (and there is definitely a noticeable difference in power so its not just the gauge). It will do this even with identical situations such as, completely warmed up, 2nd gear pull from about 2k rpm til redline foot to the floor and I'm sitting at 3psi the whole time, next time in the same situation it will pull to about 8psi. Where should i start looking? Why would it be intermittent? How does the boost system work? I know on my GTI turbo if it did this i would be looking towards the n75 valve or the diverter valve, is there something similar on a supercharged system?
 
Helltime said:
I've just recently started gathering parts for my car and got it legal to drive today, i noticed that sometimes it only boost to about 3psi, other times it will go to 8psi (and there is definitely a noticeable difference in power so its not just the gauge). It will do this even with identical situations such as, completely warmed up, 2nd gear pull from about 2k rpm til redline foot to the floor and I'm sitting at 3psi the whole time, next time in the same situation it will pull to about 8psi. Where should i start looking? Why would it be intermittent? How does the boost system work? I know on my GTI turbo if it did this i would be looking towards the n75 valve or the diverter valve, is there something similar on a supercharged system?
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could be a belt slipping or the bypass valve could be acting up
 
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I think it is the bypass, i've read threads that say you can see it closed when engine is off and open when engine is on, it stays the same with engine running or not.
 
bypass.jpg

If its the piece in the pic then its always open. Engine running or not its still open. If the bypass is something else then show me a pic? (i'm not familiar with supercharged engines, only other f/i engine i've had is my gti turbo.)
 
If thats the position its slways in then it is stuck closed. The sc will make a whining noise all the time instead of just under boost. That thing you have circled is the vacuum actuator. The little throttle body it attaches to is the bypass valve. There is a vacuum line that goes from the back of the actuator to the intake side of the plenum. About 8" long. Check and see if its broke. I would bet it is. They get brittle
 
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The line runs clear over to the firewall on the drivers side, really small line, (think i read on here about 89s having a fail safe built in, something like a boost controller, think i should relocate it to a better vacuum source? Line seems to be intact all the way though.
 
My car is a '94. I dont know that it has changed but i could be wrong. The only line that goes to the firewall is the boost gauge line. I see it in the picture. Look on the other side of the rubber hose that connects the plenum. There should be a vacuum nipple sticking out. Thats where its supposed to be run to
 
mmathews said:
My car is a '94. I dont know that it has changed but i could be wrong. The only line that goes to the firewall is the boost gauge line. I see it in the picture. Look on the other side of the rubber hose that connects the plenum. There should be a vacuum nipple sticking out. Thats where its supposed to be run to
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I think some early sc's did not have that nipple on the intake plenum like our 94's do
 
mmathews said:
My car is a '94. I dont know that it has changed but i could be wrong. The only line that goes to the firewall is the boost gauge line. I see it in the picture. Look on the other side of the rubber hose that connects the plenum. There should be a vacuum nipple sticking out. Thats where its supposed to be run to
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I don't see one. it was on the 89s only (as far as i can tell) where they ran it over there. I'm thinking about finding another source of vacuum and running that line to it and see what happens, if nothing else i can always switch back. Kinda waiting for these damn thunderstorms to stop though, been going on all night =(
 
Trace it to its location and look for a breakage. Not all of the vacuum sources work the same. But if you could get it on a different one you could see if the actuator opens the valve. You could hook it to the boost gauge nipple and see what happens. It wouldnt function that way but it should test it
 
mmathews said:
Trace it to its location and look for a breakage. Not all of the vacuum sources work the same. But if you could get it on a different one you could see if the actuator opens the valve. You could hook it to the boost gauge nipple and see what happens. It wouldnt function that way but it should test it
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I was wrong, it does move when the car is running vs not running, i unplugged the vac line to it and saw it move, same when i put the line back on, i went around the block with the vac line off, i have boost like i am supposed to, runs fine that way, whats next?

Edit: I want to delete the electronic boost solenoid seeing how no other year s/c had them. Whats the best way to do this?
 
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I went outside and disconnected the boost solenoid's electrical connector and pulled the vac line off the boost bypass then put it back on to insure it still worked (and it does!) so then with the electrical connection off i test drove the car, i have boost! it peaks about 10psi near redline, I'm not stuck at 3psi anymore. Any draw backs to the wiring harness on the boost solenoid being disconnected even though the bypass valve still functions properly? If not I'm gonna leave the harness disconnected but keep all the vac lines the same.
 
I dont have a harness on mine. Can you hook it all upand work the throttle from the engine bay? See if its sticking. If its stuck shut your car will boost but the sc uses more hp during vacuum and generates heat. Its pretty much charging the whole time.
 
mmathews said:
I dont have a harness on mine. Can you hook it all upand work the throttle from the engine bay? See if its sticking. If its stuck shut your car will boost but the sc uses more hp during vacuum and generates heat. Its pretty much charging the whole time.
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Its not sticking. When i'm at idle its open, as soon as i remove the vacuum line it shuts, put it on it opens. I rev the engine with the vac line on no boost, just goes up to almost zero vac, take the line off it boosts somewhere around 2psi when i rev in neutral. With the electrical connector on or off it makes no difference until i am driving, then it boosts as it should at idle its the same as if i had the connector plugged in. (Hope that makes sense, if not i will try and get a video tomorrow when its light outside).

Long story short it still gets the required vacuum to work the bypass valve, its just the computer is no longer intermittently interrupting it like it was before.
 
The valve moves a little when i rev it, doesn't completely close, just moves a little, when i drive depending on how much throttle i give it its not always in boost, it still registers on the vac side (if i give it more throttle then it will go to the boost side) so it looks as though everything is working properly now with the electrical connector unplugged. I drove it more today, i missed having rwd! I'm gonna call this case closed (for now). Might look into a full delete of that pointless 1989 only solenoid later on.
 
Helltime said:
I went outside and disconnected the boost solenoid's electrical connector and pulled the vac line off the boost bypass then put it back on to insure it still worked (and it does!) so then with the electrical connection off i test drove the car, i have boost! it peaks about 10psi near redline, I'm not stuck at 3psi anymore. Any draw backs to the wiring harness on the boost solenoid being disconnected even though the bypass valve still functions properly? If not I'm gonna leave the harness disconnected but keep all the vac lines the same.
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I would leave the boost solenoid plugged in and just disconnect the vacuum hoses from either side of it, and couple the hoses together to bypass the solenoid.

David
 
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