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From the 996 tech articles:
"One poor-man's way to check for vacuum leaks is to artificially create a leak and see how the engine reacts. There are two primary vacuum systems on the Carrera engine--crankcase vacuum and intake manifold vacuum.
You can test for proper crankcase vacuum by removing the oil cap while the car is idling. The engine should change idle and begin to run a bit rougher with the oil cap off. If there is no change in the running of the engine, then you might have a crankcase vacuum leak somewhere (sometimes caused by a failing air-oil separator--see Pelican Technical Article: Porsche 911 Air Oil Separator).
To induce a vacuum leak into the intake manifold, you can disconnect one or more of the hoses that connect to the intake. One example would be the hose that connects to the air-oil separator. If you crack one of these hoses open just a bit and the engine rpm doesn't change, then you might have an intake vacuum leak somewhere."
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