tripeedee

Thought this experience may help someone else.
I checked my oil this morning and added a little. I quickly put my oil cap back on and started her up. She idled rough at 1st, then all was well until I came to a stop light. I noticed she was idling around 1500 rpms. It did this every time I came to a stop but ran fine.
I stopped and check my oil cap only to find it wasn't reinstalled properly . I put it back on, making sure it was snug and presto! The idle problem solved! I was surprised that just not having the oil cap properly snugged could cause this much of and issue, but it did.
Maybe this will help someone else diagnose an idle problem.

IamSMC

Shouldn't oil level be checked after engine is warmed up, at idle, engine running and car on level ground?

Are things different in a 996 than before?

Palting

Manual states engine hot, off for several minutes, level ground. No ral difference between engine cold or hot, in my experience, so long as the oil is fully drained back.

pat056

I always check mine in the morning. I just make sure that the level guage says that the oil in over the mid-point. I've corrolated this with the dipstick and feel it gives a more accurate reading than when I fill up as that always a little on the low side. i believe that's because the oil hasn't draind back in the engine. If I stop the car for 5 minutes and don't fill up with gas, the oil guage reader will say I need to wait anywhere from 5-10 minutes before checking. I don't see why filling with gas changes that time. Plus I know my garage is level. I don't know about the gas station.

BluesCast

I always check my oil cold. and it's always full, always. Weird.

Tippy

I hate to be the bearer of bad news but your Air Oil Separator is busted. Search for threads talking about the AOS, runs about $125 or so for the part alone.

BluesCast

Also, search Renntech.org here is one of their links with photo.

http://www.renntech.org/forums/index...howtopic=20211

1999Porsche911

If your oil seperator system is leaking, you would have no vacuum from the oil filler tube amd therefore, no idle change when cap is removed. Therefore, you can safely assume that you seperator is fine.

996superfly

So to clarify on AOS, if you have a bad AOS you would not experience any idle change if the oil filler cap is remove while the engine is running.

Big buff of blue/white smoke when first start engine after hard driving point to bad AOS!

Marc Gelefsky

Yup, you do not check the oil with the engine running as we do with the Air cooled cars.

UCrazyKid

glad you caught that the oil cap was loose and put it back on. I let mine go long enough to get a check engine light on for idle out of range. Saved you a couple of bucks.

Tippy

If the bellow is leaking, yes, the oil cap removal will not affect idle. If the vacuum lines that run to it are leaking, yes the oil cap removal will not affect idle.

If the diaphram is ruptured inside the AOS, removing the oil cap WILL affect idle.

Tippy

If the diaphram is ruptured, the idle will change.

pat056

Tippy
I hope you're wrong. Since it's been running fine and it seems to be a cause/effect type thing maybe I'll be lucky.
It ran fine last night. I put oil cap on wrong today and it idled high. I reinstalled oil cap with engine running, and it immediately corrected itself.
Anyway, reading about replacing an AOS on a 996, it doesn't sound like a fun DYI

Tippy

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