Good vacuum, low boost on '90 sc

R

rev

Registered User
Well, the car went through a lot of things lately. We changed the engine and the tranny (from my other 3.8), cause the old engine was making some strange noise, could be the connecting rod bearing, dont know yet. Thats not the problem now anyway.
So after the engine swap everything was fine, just didnt have enough boost. Like WOT @ 3000 only gave me around 4psi, had around 7 @ 5000. The blower on the engine made some rattling sound, so I desiced to put on the blower from the original engine.
Everything went fine (well, except I had to change the MAF, didnt idle).
I have even more vacuum then before, around 20 inch idling, but on the boost side Im still missing around 4-5 psi. It builds up faster, but still, doesnt go above 7-8. Car feels weak compared to the first drive i had in my red '90 sc. I didnt check the boost on that one, I was just busy with screaming and stopping :D I appriciate any ideas, because Im completetly clueless.
Thanks
 
One thing to keep in mind, that not only can you have vacuum leaks, you can have boost leaks as well.

I'd start checking how the sc top is faring (I know mine leaked and I resealed it and claimed about 2 psi back) as well as the sc inlet. Have you ever had the blower opened?

Another common point for leaks is the IC itself as its only sealed with an epoxy that over time will harden and become brittle.

Just a couple of ideas anyways.
 
I'm having the same problem. I just put a new motor in my '90 5 spd. It idles at 19-20 in. hg, and that's with a cam in it, so I know there aren't any leaks. The SC is ported (early model) with a 10% pulley, GTP rotors and a fresh rebuild, with the right size belt on it. At peak RPMs, I'm only getting 9-10 lbs of boost. I should be close to pegging the boost gauge, shouldn't I? I made sure I sealed everything when I put the motor back together. Is there any way to check for boost leaks?
 
mine stock almost pegs the guage?? But it only vacs at about 17:confused: Ill post a vid. But its all completely stock minus tearing the engine down and cleaning everything.
 
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I was just thinking that the other thing you might want to check on is the bypass valve completely closing or is it getting hung up on something. After doing the head gaskets, the valve wasn't fully closing on mine as it was getting hung up on the clamp for the bypass tube.
 
bigpoppa822 said:
I'm having the same problem. I just put a new motor in my '90 5 spd. It idles at 19-20 in. hg, and that's with a cam in it, so I know there aren't any leaks. The SC is ported (early model) with a 10% pulley, GTP rotors and a fresh rebuild, with the right size belt on it. At peak RPMs, I'm only getting 9-10 lbs of boost. I should be close to pegging the boost gauge, shouldn't I? I made sure I sealed everything when I put the motor back together. Is there any way to check for boost leaks?
Click to expand...

Threw all my mistakes with this issue I found out that you need a BIG cone air filter, not the little 6inch ones for more air. Also you have to suck cold air, not hot air. I've seen so many people have a cone filter in the engine compartment sucking hot air. That makes no sense! Also , if you have open exhaust you will loose boost because its leaving faster than it did with stock exhaust. I have a stock SC and opened exhaust (2 1/2 dual to only two Flowmasters) and a 10% pulley on th SC and my boost gauge is pinned. I found the easiest way to check for leaks is to take a small propane tank and open it and let it spray on every joint and fitting and when the motor revs up, you found your leak. (Don't light the propane and only do it when the car is running)
 
Your method is ok to look for vacuum leaks, but doesn't show you any boost leaks. I had vac around 21 and no problems, but I knew under boost the sc top was lifting and leaking.... the propane method would never detect that.
 
First thing to came to my mind is if you have good vac, but low boost, the bypass valve may not be moving completely. It should snap shut when you go off vacuum. You can watch the arm and see how much it moves.

You may be able to disconnect the vacuum motor and manually move the arm as well. If you can do that you can manually close it and see how the car behaves.
 
Mike8675309 said:
First thing to came to my mind is if you have good vac, but low boost, the bypass valve may not be moving completely. It should snap shut when you go off vacuum. You can watch the arm and see how much it moves.

You may be able to disconnect the vacuum motor and manually move the arm as well. If you can do that you can manually close it and see how the car behaves.
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Checked the bypass valve today, and it moves ok. Also reseald everything that needs sealing there, gonna check it tomorrow, hope it helped :)
 
Jacob_Royer said:
Not really 10.01 in the 1/8th.. It was faster stock. :(
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Somethings not right! have you chipped it? or had it tunned because something is not right
You might need to go up a size on injectors and fuel pump and have the thing calibrated. I bet it's way outa tune and not running even close to right! that thing should be making 370+ftlb/tq
 
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Well, resealing like everything got me around 1 psi, still missing like 3. Also raced with my friend's Seat Leon FR 2.0 TDI (170hp), and he gave me like 10 meters on 0-60 (does 0-60 in 8.2s). I just cant figure out whats wrong, everything seems to be fine, but engine is still weak :confused:

ps: just a few guesses, I'll check the intercooler for leaks, and the exhaust if it really fits. Also, could a worn blower result in loss of boost? Like I dont really have any supercharger whine at all..
 
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