Oil Cap Air Sucking Noise

[dja1980]

About a year ago I had a CEL, and my local Porsche dealership recommend trying a new oil fill cap because they suspected an air leak. They reset the code, and everything was fine until last week when I got another CEL.

My first thought was to check the oil fill cap since that had been the culprit previously. When I checked it out with the car running, I noticed that the cap was not completely clicked into place. However, when I completely tightened it, I noticed a sucking air noise coming from the cap. Does this mean that I have another bad oil fill cap?

I noticed that when I put the cap back to the “half tightened” spot, it seemed to make a tighter seal, and no air was getting sucked through the cap. Am I correct in assuming that the cap should be fully tightened, and that NO air should be getting through?

[Burg Boxster]

Cap should be firmly tightened.

Get the CEL code read (AutoZone and most places like will pull it for free w/ their OBDII). Most likely it'll be code P1128.

Sounds like you might have a bad cap again. More than likely though it's a cracked oil filler tube which is spreading open when you firmly tighten the cap... It is DIY-able w/ plenty of instructions here. Search on oil filler tube if that is indeed the case.

Start w/ getting the CEL code read first. Post it here for further diagnosis.

Good luck

[root_werks]

I agree, there are a few common codes for oil fill tube etc. Grab the code(s) and post them here.

[schoir]

It's possible to replace just the rubber gasket in the oil fill cap.

Part number for 986 is 996.107.336.00.

Regards, Maurice.

[dja1980]

Thanks guys!

So when you guys have your car running and you "listen" to your oil cap, yours is silent, correct? I only ask because I just read a few other threads that say the hissing noise is supposed to be there... so now I'm really confused.

I'm now wondering if I got a CEL because the oil cap was in the half-closed position and NOT allowing air in. I left it in the fully-closed position when I discovered it half-closed last week, and the CEL since turned off.

I'll still try to pull the codes and post them here. I think we have a dry week coming up next week, so hopefully I'll have the car out again then.

[schoir]

Originally Posted by dja1980

Thanks guys!

So when you guys have your car running and you "listen" to your oil cap, yours is silent, correct? I only ask because I just read a few other threads that say the hissing noise is supposed to be there... so now I'm really confused.

I'm now wondering if I got a CEL because the oil cap was in the half-closed position and NOT allowing air in. I left it in the fully-closed position when I discovered it half-closed last week, and the CEL since turned off.

I'll still try to pull the codes and post them here. I think we have a dry week coming up next week, so hopefully I'll have the car out again then.

There is supposed to be some slight suction (and an accompanying hissing noise) there. If there is extreme suction (this is starting to sound funny ) then that is an indication that your AOS is failing. In that case, it will be very difficult to pull the oil cap off the tube.

If the oil cap was in the half-closed position, that would allow unmetered air into the system and thereby throw a CEL. If you tightened it correctly and the seal is good, then the DME will turn off the CEL after a few drive cycles because there is no longer an air leak.

Regards, Maurice.

[dja1980]

Ah, ok... that makes. Thanks for the clarification. Maybe I'll let it go for now and see what happens. As stated in my first post, when the cap was in the half-closed position, it seemed to be overly tight and there was zero noise.

Turning it fully-closed allows the cap to pop back up slightly, which I'm now thinking is the way the way Porsche designed it. Since it sounds that there IS supposed to be some slight suction noise, is it safe to assume that NO airflow through the cap could have caused the CEL?

[70Sixter]

Originally Posted by schoir

There is supposed to be some slight suction (and an accompanying hissing noise) there. If there is extreme suction (this is starting to sound funny ) then that is an indication that your AOS is failing. In that case, it will be very difficult to pull the oil cap off the tube.

Regards, Maurice.

And that could also indicate worn rings are causing blow-by.

[Boxtaboy]

Huh?! There's supposed to be a suction noise? I thought it was supposed to be a complete seal? Ok, now you guys are gonna make me open my trunk while the car is running to check if there's any noise near the oil cap. LOL.