Idle speed when oil cap removed...
 

Ive read on a previous post that the engine should start idling unevenly whenever the oil cap is removed.

The reason I ask is that yesterday while checking the oil level on '88 coupe, I didnt notice any particular difference with the cap or off...it seemed to idle exactly the same !

I'm fairly new to Porsche ownership, so can someone please tell me what should really happen when I remove the oil cap as I am concerned that I may some problem.

The idle on my 83 drops about 200 rpm when I remove oil tank cap.

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Robert Stoll
83 SC
83 944

Your engine computer (DME) will compensate for the changes. Mine only drops about 50 rpm. It has to do with the crankcase ventilation system, but I can?t explain it. Maybe someone else can explain the exact cause.


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Ted Stringer
nuke3@juno.com
'84 911 Targa aka pocketrocket

Sometime between 76 and 93 Porsche changed the point at which the crankcase ventilation joined the intake tract. Early cars had the vent before the idle circuit so that removal of the oil cap did not affect idle operation.

At some point they moved the vent to a point after the idle metering circuit. On these cars, there is an additional restrictor in the vent line(s). If this restrictor is functioning properly there will be a minor change in idle (usually an increased and steadier idle). If this restrictor is missing the engine will die at idle when the cap is removed. Additionaly the restrictor can become clogged with oil and crud, if this happens then the cc is not venting properly and no change in idle will be observed.