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Old 03-10-2010, 11:55 AM   #1
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Question Oil Cap Air Sucking Noise

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?

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Old 03-10-2010, 12:42 PM   #2
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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
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Old 03-11-2010, 07:16 AM   #3
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I agree, there are a few common codes for oil fill tube etc. Grab the code(s) and post them here.
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Old 03-11-2010, 09:08 AM   #4
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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.
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Old 03-12-2010, 10:26 AM   #5
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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.
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Old 03-12-2010, 11:45 AM   #6
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Quote:
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.
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Old 03-13-2010, 05:26 AM   #7
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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?
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Old 03-16-2010, 11:17 AM   #8
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Quote:
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.
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Old 03-16-2010, 11:30 AM   #9
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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.
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Old 03-16-2010, 12:22 PM   #10
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Originally Posted 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.
Actually, please do... lol

I would really like to get a concrete answer on this!
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Old 03-16-2010, 12:45 PM   #11
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Quote:
Originally Posted by dja1980
Actually, please do... lol

I would really like to get a concrete answer on this!
Haha...ok, no prob. When I go home later today after work, I will listen for any noise, and report back!
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Old 03-16-2010, 01:49 PM   #12
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I don't believe there should be air getting into your oil reservoir through the oil cap. The idea of the AOS is to separate gases in your crankcase from the oil and vent them back to your engine to be burned. This reduces emissions and increases power by maintaining your crankcase at a negative pressure. Excessive air coming through the cap, into the case, and into the intake manifold through the AOS, after the MAF, will cause your engine to run with an improper fuel/air mix. This will result in the CEL caused by error codes from the O2 sensors and/or MAF.

This is fairly trivial, and the DME is a little over sensitive to emissions issues.

If the AOS starts to fail then the gas from the crank will entrain oil into the engine causing the engine to smoke. (And ruin O2 sensors, which of course leads to CEL)

If it's air/gas coming out of the oil cap that's bad. Either the AOS is clogged (not real bad), which will produce a slight positive pressure in the crankcase, or you have some serious ring blow by. (bad)
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Old 03-16-2010, 01:58 PM   #13
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Ok, just drove home, and then opened my trunk while engine still on, and took a listen. Absolutely no hissing noise from the oil cap area that I could detect. My oil cap is on tight. I did hear a little gurgling noise from the coolant tank though.
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Old 04-11-2010, 05:52 PM   #14
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Ok, just drove home, and then opened my trunk while engine still on, and took a listen. Absolutely no hissing noise from the oil cap area that I could detect. My oil cap is on tight. I did hear a little gurgling noise from the coolant tank though.

-------------------------------------------

With the oil cap off or on? Mine is sucking air only with cap off; has no CEL or smoke and drives fine. I am a bit confused myself. I hear should not be sucking air and I also hear sucking air is OK with cap off.
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Old 04-12-2010, 02:34 AM   #15
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Quote:
Originally Posted by S3adrag0n
Ok, just drove home, and then opened my trunk while engine still on, and took a listen. Absolutely no hissing noise from the oil cap area that I could detect. My oil cap is on tight. I did hear a little gurgling noise from the coolant tank though.

-------------------------------------------

With the oil cap off or on? Mine is sucking air only with cap off; has no CEL or smoke and drives fine. I am a bit confused myself. I hear should not be sucking air and I also hear sucking air is OK with cap off.
As my post stated, "my oil cap is on tight."
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Old 04-13-2010, 09:10 AM   #16
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Mine doesn't hiss either with the cap on. Sounds like it's normal to have some air sucking with cap off: http://986forum.com/forums/showthread.php?p=211155#post211155
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Old 04-14-2010, 05:56 AM   #17
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Well, I guess since a few of you have stated that your caps are silent in the fully-closed position, this confirms that I have a problem.

If/when I get the CEL again, I will get the code... I'm planning to get more driving in now that the weather is REALLY getting nice!

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