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Well, it seems my 2006 Cayman S has developed a "Rough Idle" only at low RPM's (700-1100 RPM) it's been doing this for about 500 miles and no CEL. The RPM's will drop 100-200 RPM at regular intervals 1-2 seconds, if driving at low speed (5-9 mph) in 1st gear (i.e. traffic jam) it get's pretty jerky. Now with my foot totally off the accelerator it's smooth as silk, but with just "a touch" of pedal it starts getting irregular.
I'm thinking a bad coil would throw a CEL and would mis-fire through most of the RPM band, a bad Air Oil Separator would throw smoke and CEL (but there is a good amount of vacuum/sucking when I remove the oil cap while the car idles and RPM's go up quite a bit)

I've removed & cleaned the MAF sensor, Plenum (there was a little bit of oil in the plenum) & cleaned the TB, changed the Air Filter, reset the E-GAS...

Any Ideas, if need be I can post a video of it.


Regards

Gary
 

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It is normal for there to be some vacuum (low pressure) when the oil filler tube cap is removed. In cases where the AOS is quite gone the pressure difference can be severe enough one can't remove the cap.

Unless you had to strain to remove the cap I do not think there was enough pressure difference to condemn the AOS on this alone, provided you can tell me you left the cap off for few moments the engine idle smoothed out and the idle speed dropped to near normal. This would indicate the air through the cap was the only leak. Of course, when you replaced the cap the DME should then take only a few moments to adapt and to undo what it did to accommodate the air leak when you removed the cap.

However, an early symptom of an AOS failure can be a variable idle speed. Nothing gruesome but what you describe is in the ballpark.

A bit of oil in the intake is normal. Was the throttle body the butter fly valve oily? If yes this can be a sign the AOS is going as it routes too much oil vapor to the intake which fouls the throttle body with oil.

Last but not least have you checked for any pending error codes?
 
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