Oil Cap and Idle
06-27-2008, 06:57 PM
#17
Race Car
Pat - the idle difference in mine was extremely noticeable and it would die within about 2 seconds when removing the oil cap. After I installed the new one, the idle barely changed with the oil cap off. There was a slight, I mean very slight vacuum when you cupped your hand over the filler neck. Before, with the ruptured one, it felt like it would suck your hand into the filler neck.
Slight vauum draw with little idle change (maybe ~50-100 RPM) is normal, drastic idle change with large draw, AOS is more than likely is dead
. GL 
Slight vauum draw with little idle change (maybe ~50-100 RPM) is normal, drastic idle change with large draw, AOS is more than likely is dead
. GL You are correct. With a properly working system, there really should be no measurable vacuum in the crankcase. The idle may or may not change very slightly and will eventually balance out (usually higher) when you remove the oil cap. The cooler the ambient temperature is, the more idle change you may experience. For those who have a vacuum level that can actuallly be felt when removing the oil cap....you have a problem and are probably using alot of oil. There is also a problem if your car stalls when removing the cap.
Last edited by 1999Porsche911; 06-27-2008 at 07:32 PM.
06-28-2008, 12:13 AM
#18
Drifting
Join Date: May 2006
Location: Another Ex pat Brit in SoCal
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I had the local stealership tell me that the vacuum from a dead AOS would lead to RMS weepage, so they 'recommended' me doing the RMS at the same time..
No **** for the extra $1000 - what a croc.
No **** for the extra $1000 - what a croc.
09-06-2022, 04:10 AM
#20
Advanced
Help/Info
You are correct. With a properly working system, there really should be no measurable vacuum in the crankcase. The idle may or may not change very slightly and will eventually balance out (usually higher) when you remove the oil cap. The cooler the ambient temperature is, the more idle change you may experience. For those who have a vacuum level that can actuallly be felt when removing the oil cap....you have a problem and are probably using alot of oil. There is also a problem if your car stalls when removing the cap.
Last edited by Garage19builds; 09-06-2022 at 04:12 AM.
