Is my AOS going? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

I removed the oil filler cap when the engine was running. There didn't seem to be much of a vacuum holding it on, but putting my ear near the opening it "sounded" like sucking. And the note of the engine went quite low.

Clikman, mine is doing the same. Hopefully we'll get a reply from those who had AOS failure. Rightnow, would be a great time for me to replace mine if it is.

I think what you are experiencing is quite normal. Even with a new AOS, when I removed the oil filler, I could hear a vacuum being pulled and it lowered the idle rpm a little.

When my aos failed it was VERY difficult to pull the cap off. It was far more vacuum than normal. I pulled the cap off the other day and experienced similar results to you, but I could actually get the cap off. When mine had failed, it was blowing huge amounts of smoke on every start up and I had a squealing noise from the motor. I think your good.

With my car being white is was easy to watch the AOS go, near the end the rear cover would get a ever so slight coating of oil, not anymore since a/o replacement.

944boy, that definitly help. I've heard you should not get any air sucking at all to ok to have air sucking. With your AOS replaced and is doing what mine is doing, I feel a lot better so I'll go and stump on my gas peddle today! Here's to you...

In order to reduce oil contamination, foaming and moisture build-up and minimize pressure build-up, the car's crank case is under a slight vacuum by a connection with with intake manifold. The AOS sits in the middle of this connection to collect most of the oil that is pulled from the crank case and returns it to the engine. It is completely normal to hear or feel a vacuum when opening the oil cap.

Thanks blue2000s, that's a great explainaton! Here's to you too.. PS. I was out there in CO (Lafayette visiting my in-laws) a couple years ago and man, did I feel bad visiting them a week earlier and missed the Porsche Parade.

reviving this one.. i'm testing my MAF and some other items on my car currently, and while it was idling, I pulled the Oil filler cap off. The car's idle dropped when I did that, and it wasn't very difficult. My question is; is my AOS failing if the idle drops or is that normal?

Did you even read this thread? Your answer is right here:

Hi everybody, this is my first time to write something in here. Anyways, you're all talking about vacuum on the crankcase. Well, I have pressure on the crankcase. It's so bad, that when I take off the filler cap I can hear the pressure exiting and see a blue oil cloud coming out. Furthermore, when I pull out the dipstick, the oil is blown out out of the dipstick hole!!! The car overheated recently, could it be that the head gasket is blown??? I hope not. Any ideas??? Thanks, cvisi

intermixing with the fluids? might wanna get that one checked out asap!

You obviously have some sort of blow by. Maybe caused by leaking rings or blown head gasket. Have the cooling system pressure checked and go from there.

hi, thanks for the reply. Anyways, i found the problem. So here it is. Everybody tells me that when the AAOS faila, the vacuum is too big. Well in this case, the problem was that the diaphram inside was working perfectly well when there was only little vacuum. Tested it with a vacuum pump. Up to 15Hg all was ok. diaphram closed with the vacuum, then opened when pressure built up on the other side and so forth. Now, when higher vacuum was applied (30Hg and above) the diaphram closed once and then never opened again, til the vacuum was shut off. I ordered a new AOS, but I'll open the old one for sure when I get the replacement, to see how that thing looks from the inside. I'd guess that there's some kind of breather channel for the diaphram to open again when pressure builds up on the crankcase side.