2.4 NA, 200,000 miles.

I'm getting a squealing noise that sounds exactly like a slipping fan belt, however a new serp belt didn't cure it. Here are the clues:

1.) It only happens after driving for a while and the engine is good and warm.

2.) It's somehow related to crankcase vacuum. When the squealing is happening, the oil filler cap is very difficult to remove. It's REALLY sucked down tight. Once the cap is lifted up, the squealing stops immediately. Screwing the cap back on, the squealing restarts immediately.

3.) The noise seems to emit from somewhere below the #4-#5 intake manifold runners. I have put the stethoscope on everything I can find in that vicinity and can't hit on the source of the noise.

4.) Turn the A/C off and the squealing SLOWLY diminishes. Turn the A/C back on and the squealing SLOWLY returns. (My theory on this part of the puzzle is that that when the A/C is shut off, the reduced engine load translates to a change in idle control which affects the vacuum.)

I've read that there is a vacuum pump that can act up and cause issues something like this. Also, this car is one of those that will occasionally emit a short (1-2 seconds) "buzzing" noise every few seconds after the engine is shut off. Is this related?

Any ideas what might be going on?

Just an after thought:

When was the last time you cleaned the throttle body? It is possible that there is enough gunk at the butterfly to completely close it off at ilde, which would cause extreme vacuum.
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My back feels better when I sit in a Volvo seat

Good food for thought. Thanks.

BTW, I also posted the situation over at MVS. I got an interesting response about a blown out vacuum regulator on the oil trap box, which sounds quite plausible.

http://www.matthewsvolvosite.com/forums/viewtopic.php?f=9&t=63820

Checking the various online parts catalogs, I see that the oil trap for the NA engine does indeed have some sort of regulator gizmo attached that the turbo version doesn't have.

Anyway, I guess I just need to dive into it and see what's up. And no, I don't recall cleaning the throttle body in the 3+ years (or is it 4+ years?) that I've owned the car. I bought it with 150,000 miles on it and now I'm at about 200,000. So it hasn't been cleaned in at least 50,000 miles and it's absolutely possible that it's NEVER been cleaned. I was going to do that along with the PCV service but may just go ahead and do it alone, first, just to see what happens.

My biggest problem with the whole thing is that the wife is currently 800 miles away with her car so if I tear mine down and then need to run for parts I'll have to pedal my bike.

Using a bike to get parts is progably good for you. Just make sure the ETM is plugged in securely when you finish up. A can of TB cleaner and an old toothbrush works wonders.

Save the PCV system for a time when you have a back up vehicle.
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My back feels better when I sit in a Volvo seat

your engine should be have pressure not vacuum, if you take the oil cap off you should feel air coming out, I would try tightening up the intake bolts.
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2001 v70 T5, 2001v70XC, 2002 XC70, 2004 XC70, 1988 740 turbo wagon

Sorry Mopar, but the combustion process creates vacuum. Having positive pressure in the crankcase is not a good thing.
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My back feels better when I sit in a Volvo seat

As I think I mentioned further up the thread, someone over at MVS suggested the oil trap vacuum diaphragm likely ruptured, exposing the crankcase to full manifold vacuum. Yep, he was right.

Note 200,000 miles of gunk inside that thing.

I'll bet that if you put a vacuum gauge on the engine you would see much more than 20psi. Something is restricting air flow to the intake manifold when the engine warms up. BTW, there is NO vacuum pump for the engine, there is only a vacuum pump for the evap emmissions system and that pump only runs at startup or sometimes at shutdown (which is what you probably hear running for a second or two).

Your noise could be a small leak in the intake manifold gasket, made worse by the increased vacuum.

Because the noise goes away when the AC is turned off, slowly, may be an indication that the idle control system is blocked - whatever that is on the 2003 cars. Your idle rpms should not change with the AC compressor engaged.

Good puzzle...
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My back feels better when I sit in a Volvo seat