squealing sound under vac?

quicktsi

Registered User
Okay I have tried searching but could not find anything. It sounds like when you let air out of a balloon and pull out the 2 sides and it squeals. It only does it when the car gets up to temp. It only does it under vac from 10-20. Above 10 vac it goes away. The car idles at about 18 vac. The sound appears to be coming from some where around the back side of the supercharger. Any ideas of what to check?

I just got the car back together from doing head gaskets, snout rebuild, ported exhaust manifolds. This is the only thing from getting the job finished.
 
I have the same sound on my 92.....

However, the sound goes away as I give it some throttle.....:confused:

I haven't had a chance to track it down, but I think it might be the IAC valve going bad.....
 
Mine goes away when you give it a little bit of gas and get over 10 in the vac. It did not do this prior to the rebuild.
 
does it sound like this?? If so, mine did it for a while too after i got the headgaskets finished up. Its your PVC vavle. I put a new one in, and fixed it. Its been 3 or 4 months now since the new vavle, and now its starting to make that sound again, i think it has to do something with blow-by. Im not quite sure yet.

<embed width="600" height="361" type="application/x-shockwave-flash" allowFullscreen="true" allowNetworking="all" wmode="transparent" src="http://static.photobucket.com/player.swf?file=http://vid383.photobucket.com/albums/oo279/93tbirdsc/100_3596.flv">
 
Sounds like you have a vacuum leak somewhere. Since it goes away as the vacuum gets less, that is what I would start to look for. Is the CEL on? How does the car idle, rough, smooth? That also is key in this. If the light is on, I would start looking for the leak at the Mass Air Flow and check any and all connections beyond that. I have not heard a PVC valve doing that but anything is possible. Check all your connections. You have to remember we have a lot more places where this can happen. It can be an intake manifold gasket, plenum leak, throttle body. It can be the vacuum lines to. Those are the small black plastic lines and all it takes is a pinhole and the older these car get, the older the lines are and they become brittle. If the CEL is not on, I would still check everything. I don't think it is your intercooler tubes, intercooler, sc. If those leak, then you would have problems under boost. Hope that helps. Good luck.
 
No CEL and the car idles completely fine. I got the pressure tester set up, and after dinner I will start playing with it. I think it may be coming from the diverter/bypass valve area. I will check the PCV valve. Can you block the IAC valve to test it? It does not sound like the one in the video, mine is also a very high pitch buzz.
 
check the throttle bushings,

also check the vacuum acutator for the bypass valve. and then check the pivot point of the acutall bypass valve. the bushing wear out and will cause a very high pitched whistle..
 
also check the vacuum acutator for the bypass valve. and then check the pivot point of the acutall bypass valve. the bushing wear out and will cause a very high pitched whistle..

How do you check/test the vacuum actuator? I did notice the the pivot point seemed to have a lot of play in and out. Any part numbers for the bushings?

I did pressure test the system and found one leak at the lower IC pipe to IC. The Teflon gasket was out of whack and did not seal properly. Prior to fixing the lower IC it would only make 5lbs of boost. After fixing the leak it easily made 10lbs, but it still squealed when the car warmed up.
 
well a vacuum leak and a pressure leak may or may not show up when testing.

if you have a seam or bushing that is leaking under vacuum and you apply pressure to check it may seal it up and you wont find the leak. the best bet for finding this noise is going to be a long piece of vacuum line. start motor and get it to make the noise. you can hold the hose to your hear and then use the other end to searchr around behind the supercharger at the points mentioned. when you pass the hose in the area of the whistling the noise will become very loud in your ear.

I like to use mechanic sthescope for this and remove the metal probe from the end and just use the rubber hose. same princaple.

also the IAC will whistle (but shouldnt be loud enough to hear out of hood)

I have an Isuzu trooper 2.6 that is whistling like madd at the piviot of the throttle. but i suspect its the bypass valve that is giving you fits.
 
How do you check/test the vacuum actuator? I did notice the the pivot point seemed to have a lot of play in and out. Any part numbers for the bushings?

I did pressure test the system and found one leak at the lower IC pipe to IC. The Teflon gasket was out of whack and did not seal properly. Prior to fixing the lower IC it would only make 5lbs of boost. After fixing the leak it easily made 10lbs, but it still squealed when the car warmed up.

To check the bypass valve bushings, just cover the bearing area with your thumb or something. If the sound goes away, there you have it.

(If it turns out to be a leak from that area, the problem would be that the seals on the bearings have worn out. The bearings are replaceable, but it's hard to find the OEM size bearing. There is a substitute available. There is a thread from a couple years ago about it.)

It also could be a leak at your blower top.
 
With my problem, it was the PCV valve. Ill get another video tomorrow and post it.

I replaced it with motorcraft version PCV valve yesterday.

I decided tonight to regroup and clean the garage up and play with it tomorrow.
 
I replaced it with motorcraft version PCV valve yesterday.

I decided tonight to regroup and clean the garage up and play with it tomorrow.

Let me know, i can get my SC to squeal only when its under a load, not at idle. The new PVC valve hasnt gotten that bad again.

Again, my 94 sc did the same thing, squealed only while between 20 to 10 inches of vacuum.
 
I did test the bypass actuator tonight. I pulled the line and the flipped the valve and then held the line and it did not budge until I let my finger of the vacuum line.

I did get all the tools put away and the mess cleaned up.
 
Okay so I tried something. I disconnected the bypass valve vac line and flipped the valve and closed the line, then pressure tested it again. This time, around 10lbs of boost I got the squeal.

I ended up pulling the SC last night. It appears the inter cooler pipe to intake manifold teflon gasket had tweaked just enough. I will put it back together and see what happens.

My other question is, if I a system is sealed properly how much boost should the system be able to hold when testing?
 
I did test the bypass actuator tonight. I pulled the line and the flipped the valve and then held the line and it did not budge until I let my finger of the vacuum line.

I did get all the tools put away and the mess cleaned up.

so the vacuum side of the system seems to be in check. but did you check the acutaly piviot points of the valve itself?

with the door closed you will have vacuum leaking. if you hold it open like you did you area allowing all the boost to pass through it.

just a thought.

you really need to get in there with a piece of rubber hose held up to your ear to pinpoint the noise as its happening.
 
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