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Porsche Engine Oil / Air Separators 101

The Engine oil air separator directly affects the engine crankcase atmosphere because basically, all Porsche’s from 1980 on, including the current 986, 987, 996, 997, Cayman & Cayenne engines, all are positive crankcase type systems. This means that the engine crankcase has vacuum supplied from the intake manifold to collect and burn the residual combustion by-products or blow-by in the crankcase by running it through the engine again, all in an effort to lower the vehicle emissions.

On most occasions when the oil separator fails, the low pressure (vacuum) in the engine crankcase rises to an unusually or abnormally high figure and, more likely than not, it ends up drawing engine oil into the intake system.

This usually fouls spark plugs and, in rare cases, causes damage to the oxygen (O2) sensors, catalytic converters and possibly various other sensitive fuel injection components. In extreme cases an oil separator failure with excessively high vacuum can actually cause the rear main seal to make noise, i.e. “honk”.

Interestingly enough, the only way to test the oil separator is to periodically test the engine crankcase vacuum with a water filled Manometer (aka: a Slack Tube tester), which is an ultra sensitive vacuum gauge. On BMW’s and Porsche’s the engine crankcase vacuum is normally around 5 inches) of water. When oil separators fail the crankcase vacuum can rise to the 8-12 range and even higher, depending on the extent of the failure.

When the oil separator starts to fail on Boxster’s (986 & 987) you generally start to see a substantial amount of smoke from the exhaust tailpipe. When the oil separator completely fails, the engine exhaust tail pipe smoke can reach dangerous proportions. You’ll think your Boxster has turned into an anti-mosquito smoke machine.

When the oil separator fails on 996-997s, it usually does NOT create the same exhaust tailpipe smoke as a Boxster. It is almost like being a silent killer (and a costly surprise).

Sometimes when an oil separator fails, it can cause the Check Engine Light (CEL) to come on. you may not be experiencing any other symptom, but the CEL should get your attention. This condition should be checked out utilizing a Porsche factory PIWIS tester. It is important to see exactly what is going on, otherwise, you’re just guessing.

Tony Callas & Tom Prine

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