No boost?

35th

Registered User
Changed my oil in my 90 sc 5sp yesterday took it to the store (ran normal) then all the sudden very low power so I started messing with it and noticed I can't get boos over 0. Ran codes nothing relative other than default spark error which has been there on and off since 2018 can't figure out why... SC bypass is opening and closing I did notice MAF doesn't seem right if I unplug with car idling no change
 
At idle -22 ish vacuum I do see the gauge climb up to about -4 at wot but no higher than that basically its like driving a non SC 3.8...swaped out the dis module and front harness (already had spares) checked for as many vacumm leaks as possible found a small one fixed it and no change. Unplugged cam sensor only notable difference was no tach(didn't drive it unplugged) but rpm climb was about the same maybe a little slower hard to tell for sure anyone got any suggestions?
 
Stupid question, but did one of the supercharger belts break? I had that happen once, and it would fall on its face with any significant throttle input, and obviously wouldn’t make boost. Another thing to check is for any restriction in the air intake. If you sucked in a piece of plastic or something into the air intake, it could block off enough airflow to let it run under light load but not make any boost. Otherwise if there is no restriction on the inlet side, the blower is spinning, and the bypass valve is working, it should make boost.
 
With motor idling, close the bypass valve manually and see if it makes any change to the engine sound (should load it a little). Also hold bypass shut and manually goose the throttle to about 3000 rpms. Should hear a whine noise from supercharger. If it sounds the same with bypass opened or closed, there is probably something wrong with the bypass valve like flap isn't sealing or the supercharger rotors aren't spinning. Low fuel pressure can also make the car run very sluggish.
 
Sounds the same belt is good and I don't get the whine I even took the vacuum line off the bypass put a screw in the line to close it off drove it that way then zip tied the bypass open with vacuum plugged off and no change either way. If the belt is spinning the rotors would have to be turning (unless the broke off which I would've herd I would think)
 
When was the supercharger oil level checked last? I know you said you had just changed engine oil.
If you are getting 22"s of vacuum at idle, I wouldn't think you could have a large enough leak in piping
or cooler to lose all the boost.
 
Sounds the same belt is good and I don't get the whine I even took the vacuum line off the bypass put a screw in the line to close it off drove it that way then zip tied the bypass open with vacuum plugged off and no change either way. If the belt is spinning the rotors would have to be turning (unless the broke off which I would've herd I would think)
The bypass valve could be broken internally so it's not actually closing the flapper valve that forces air into the motor. Or the input shaft on the supercharger could be broken and the rotors in supercharger aren't moving. Easiest way to check that is to remove the supercharger top & upper IC tube (as a unit) and remove supercharger belt so you can spin the blower by hand to verify the rotors are moving. To inspect the bypass valve you will need to pull the supercharger to see inside the valve (part of inlet plenum).
 
I’m thinking maybe the supercharger drive coupler failed. Pull the supercharger belt off and spin the blower pulley and see how much resistance it has. If it is turning the rotors, you should feel the weight of the rotors when you turn the pulley. If the pulley spins freely like an idler pulley, then I would guess the coupler broke.
 
Sounds the same belt is good and I don't get the whine I even took the vacuum line off the bypass put a screw in the line to close it off drove it that way then zip tied the bypass open with vacuum plugged off and no change either way. If the belt is spinning the rotors would have to be turning (unless the broke off which I would've herd I would think)
I had a supercharger seize (low oil) & the coupler broke-I didn't hear a thing.

I would try what David & MadMikeyL are suggesting.
 

Found the problem just my luck both my tbirds are basically down my 97 has a cracked intake and now my SC needs a super charger.... Guess it's gonna sit for awhile
 
Unless someone on here has either a super charger or just the snout with the input shaft they'd be willing to sell for cheap??
 
I have a stock early model I’ll sell for cheap. Let me know your address and I’ll get it boxed up and get you a shipping quote this weekend.
 
random question...is there a way to tell if your SC has been ported ? the reason I ask is because supposedly mine was but I highly question that as i've found most of the things that where "done" when I got the car where not.
 
Easiest way to tell would be looking for areas where the casting flash is gone and there are sand scratches in its place. Or post some close up pics of it.
 
not meaning to sound ignorant but this is my first super charged car. where would the porting be? im assuming on the rear of the SC where it goes into the upper intake portion I mean the portion that the throttle body also mounts to. explaining things is often difficult i guess is it the piece thats oval that is the identifier for the early model super chargers vs the late model square? is that where I'd be looking?
 
Porting will most likely be on the inlet side, but could also be on the outlet. The inlet porting is far more effective, especially on an early model blower, so if they knew what they were doing, that is where they would have spent most of their time. Generally when porting the inlet, you want to smooth out the hard edge where it transitions to the rotors, and push the corners up to make it look more like a smiley face when viewed from the rotor side.
 
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