dude ~~~? loss of boost at 3300 rpm

22lbsofboost

Registered User
Ok so i bought my 12th sc. 94 5 speed. Car makes 20hg vacuum at idle. Car makes 12-13 psi and it slowly drops to 10 or so until 3300. Then in falls clean into 3 hg of vacuum. All the way to 5 grand. Belts not slipping u can hear the blower
Spinning at same speed. No dust from belt slip.the belts are not slipping. Every vaccum line i check is perfect. I even bungied the bypass valve shut lol. So im at a loss. I pulled the ic, the top. Cleaned and resealed and it didnt change anything. Ive had a LOT of these cars since i.was 17 im 38 now this is my first time experiencing boost loss like this and i been cussing up a storm.lol. i wont be giving up on the car so i need help from my SC brethren. Wheres mike puckett!!!
 
Since it's building boost and then loosing it my guess would be gasket issue. Happen to spray them and see if the idle changed?

If possible swap out the pulley and belt just to double check.
 
Man i sealed all the intercooler tubes and the top beautifully and can hear the rpms of the blowet changing very consistently with engune speed i.promise belt slip.isnt my issue and dudes imma tell ya its like somebody hits a switch it tapers from 13 to 10 from 2 to 3300 then it goes to nothing QUICK like theres a damm solenoid in line with thebypass vacuum.hose but there isnt. The hose is nice and tight no cracks all my vacuum boots are flexy and pliable and no oil on any of them the car is really clean and only has 100k on it
 
Cats plugged? Next time, look underneath and see if either side is glowing - you can also use an IR temp gun to check/compare them.

Did you remove the supercharger belt and rotate the pulley by hand...see if it makes any noise outside of coupler slack. Check the fluid level...when cold.
 
Bogging and falling on it's face is usually fuel system issues....Bucking, backfiring and banging out is usually ignition related. From what you are describing it sounds like you are starved for fuel. I would start by testing fuel pressure.

David
 
Ill certainly test fuelpressure but man.the way it performs so damn well up.until.a certain rpm and its the exact same way 40-50 different times its so consistent. Im curious if the damn bypass butterfly is slipping on the actuator arm. Its retarded. Imma try to.post a video for yall and please know i.appreciate the help.the sccoa board has changed a bit over the past twelve years but i see sime names i remember!
 
Fuel pressure should be about 38-40 psi with zero vacuum boost, with normal idle vacuum of about 21 inches pressure should drop to about 32 psi...with boost fuel pressure should increase pound for pound with boost. So if you have 10 pounds of boost you should see 48-50 psi fuel. Bad pressure regulator, broken, cracked or missing reference hose to the regulator, clogged fuel filter or weak fuel pump could all cause the issues you are having.

David
 
Yeah, stole the k&n out the expedition, rigged it up hot air style lol. I flipped the tube backwards. Now I peg my gauge. Fixed thank u guys
 
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