Possible AOS failure?
 

I have a stock 2.7 liter 2001 Boxster with just over 35,000 miles on it. Last night when I pulled into my driveway after work, which is only 5 miles away, I suddenly got a loud squealing sound from the motor that I’ve never heard before. It kept on squealing and seemed to stop for a couple seconds if I revved the motor, but then resumed squealing at idle.

Because I had the serpentine belt and water pump replaced 650 miles ago, I suspected it might be the belt. I immediately searched this forum for posts about engine/belt squealing. An hour later, after I read several, I tried to remove the oil fill cap with the motor running. The motor did not squeal (after cooling off for an hour), and I did not run it very long or rev it. However, while it was running, there was too much vacuum for me to get the oil cap off, even wearing gloves and using both hands. The cap came off easily with the motor off, and I saw no evidence of collapse of the oil fill tube. (By the way, I never got a CEL or any smoke out the exhaust during all of this.)

Is the AOS the most likely problem? Any advice is welcome. Thanks.

That's what it sounds like. The squealing sound is air being sucked in past the front crankshaft seal behind the pulley. The squealing goes away when you rev the engine since there is less vacum. Watch for smoke out the exhaust especially at startup. :barf:

Yep, that much vacuum is a sure sign of an AOS failure. I wouldn't drive it.

Excessive vacuum, to the point where you cannot get the oil cap off, is a sure sign of AOS failure.

I am surprised that this has not been accompanied by any (lots of) white smoke.

Regards, Maurice.

post back and let us know what happened and what the repair cost ran you if you go to the dealer. Thx

Randall Neighbour (on this forum & also in Houston) referred me to his indy mechanic.
I bought a new AOS from Zim's Autotechnik ($125), and I hope to get it installed in the next day or two. I'll let you know.

Got the new AOS installed on Friday the 13th. We popped the old one open and saw a 1/2 inch rip in the diaphragm, which produces the high-pitched squeal as air is sucked through it. On the road again with no more noise. Very reasonable labor charge, approx. half what the dealer gets.