What can pressurize crankcase?
 

I recently had the AOS replaced on my '04 Boxster S; once that was done the RMS started leaking. Today I saw oil fumes coming out from the oil filler cap with the engine idling and fully warmed up. I opened the cap and there appeared to be a fair amount of pressure in the crankcase. What could cause this? The engine has 95k miles and runs great. I don't think it is a ring issue because it runs so well and has had consistent oil changes.

Is there something that could be missed or reversed during the AOS replacement that could cause this?

You bet, the AOS is the only source of crankcase vacuum, normally about 5 inches of water in level. I would revisit the AOS install before larger problems start.

JFP I'm a Boxster newbie and about to replace my AOS. I don't want to make a mistake, so can you explain how the AOS works so I know what I am doing? Thanks.

JFP can provide more insight. In the meantime, you might check this article out: Pelican Technical Article: Boxster Air / Oil Separator Replacement - 986 / 987

The AOS basically functions much like a PCV system that separates the oil vapor and liquid from the vacuum stream entering the engine to be burnt. It is mounted on the passenger side of the engine near the bell housing:

http://www.pelicanparts.com/techarti...small/Pic1.jpg

It has multiple vacuum lines running off it, and there have been updated designs released over the years. Some of the later M96/97 engines had AOS units that had water passages in them as well.

As these are a common maintenance replacement item, there are several DIY write ups on how to do it, which is not hard, but it can be time consuming, and requires using a cable operated hose clamp tool to save your sanity. The following site will provide a useful introduction to the sport, but do a search as several write ups also include some hints on how to streamline the replacement if you are working on the ground:

https://sites.google.com/site/mikefocke2/airoilseparatorreplacement