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Rough Warm Idle; Shudder - Aos?


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Howdy,

I have a problem with my car that has me stumped. It's a 2001 986S, 105K on chassis, 35K on engine.

At warm idle, the car feels very rough. It's been doing this for a couple of months, and it seems to be getting a little bit worse. I can feel the engine shaking through the seat back. Idle fluctuation at warm idle is from 740-790 rpm, measured through the OBD2. At cold idle it is approx. 800-810, noticeably steadier. I don't have the vibration problems at cold idle.

No fault codes, no other driveability problems. No smoke. O2 sensors seem to be switching fine, judging by the voltages graphed by my OBD2 scanner.

I removed the TB and intake manifold to clean it. TB was clean, intake tubes and manifold had small oil puddles. I removed the y-pipe between the intake tube and the AOS, and plugged the intake tube opening. The idle did not improve, but there was a lot of smoke coming out of the AOS opening. Is that normal? I assumed that at idle blowby would be minimal?

Does anybody have an idea what's going on here, or what else I could check?

I also just replaced the motor mount, so it's definitely an engine problem...

Thanks!!

Gary

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Hmm, usually a rough idle will show up immediately with codes. Perhaps your standard OBD-II reader is not picking up all of the codes? I would try the Durametric tool just to be sure.

That said, the next step I would try would be to check all of the vacuum lines and connections (see here: http://www.pelicanparts.com/techarticles/Boxster_Tech/21-FUEL-Vacuum_Leaks/21-FUEL-Vacuum_Leaks.htm). Although I would think that a big vacuum leak would indeed cause codes to kick. Another thought is that your dual-mass flywheel may be old and failing, although that supposedly kicks out misfire codes when it doesn't quite respond. Again, odd that there are no codes...

Also check the transmission mounts, they may be worn out too and transferring vibration to your chassis (long shot, I don't think that's it): http://www.pelicanparts.com/techarticles/Boxster_Tech/36-TRANS-Tranny_Mounts/36-TRANS-Tranny_Mounts.htm

Hope this helps,

-Wayne

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Hmm, usually a rough idle will show up immediately with codes. Perhaps your standard OBD-II reader is not picking up all of the codes? I would try the Durametric tool just to be sure.

That said, the next step I would try would be to check all of the vacuum lines and connections (see here: http://www.pelicanpa...uum_Leaks.htm). Although I would think that a big vacuum leak would indeed cause codes to kick. Another thought is that your dual-mass flywheel may be old and failing, although that supposedly kicks out misfire codes when it doesn't quite respond. Again, odd that there are no codes...

Also check the transmission mounts, they may be worn out too and transferring vibration to your chassis (long shot, I don't think that's it): http://www.pelicanpa...anny_Mounts.htm

Hope this helps,

-Wayne

Wayne, I appreciate the swift and expert reply. It's driving me nuts that I can't diagnose this. My OBD2 reader is a BR-3 unit, it uses the proper ISO protocol. I doubt (but have no proof) that it's not finding all codes. Wouldn't also codes set a CEL?

I'll try my best to check the vac lines. Funny that you should mention the DMF, I have been thinking that too. The PO claimed that the clutch had been replaced when engine was replaced, but I have no receipts for that. I'll check the trans mounts too. Thanks for the idea.

Another thing that may or may not be related: I have some oil puddle on top of the driver side cylinder head-to-block valley area. The passenger side is perfectly dry. Could be a head gasket, but then again the power steering overflow line is just above that location, and also the breather line from the AOS. I know I have overfilled the power steering fluid in the past, and the AOS breather outlet looks very oily and gunky. Perhaps the AOS has failed badly and is pressuring oil out of the breather tube?

Gary

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Update:

I checked for vacuum leaks. Removing or releasing tension on the oil cap changes idle RPM significantly. I then tee'd a vacuum gauge into one of the intake manifold vac lines, and I get a solid 19 inHg of vacuum at idle. So, I think I can rule out vacuum leaks at this point.

I also tried unplugging the MAF. The idle / shuddering did not get better. The appropriate code P0102 was set.

Any thoughts on how to check the DMF or the transmission mounts? Is there a measurement that can be taken on the transmission mounts to see if they have sagged?

Thanks again!

Gary

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A couple of things. I'm not sure what you mean by breather hose on the air-oil separator, there isn't a breather attached to this unit? Are you talking about the vacuum hose attached to the manifold?

To check for leaks at the manifold, what you do is attach your vacuum gauge and the introduce a leak and measure the change. If there's not much change after introducing a leak, then you probably have one already.

Removing and replacing the oil cap only checks crankcase vacuum, which doesn't directly affect the fuel injection system (except through perhaps a fault air-oil separator).

There's a factory procedure for checking the dual mass flywheel, I discuss it here: http://www.pelicanparts.com/techarticles/Boxster_Tech/44-TRANS-Clutch/44-TRANS-Clutch.htm

-Wayne

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Wayne, please see the attached picture (not mine, taken from the web). The yellow indicates the line that goes from AOS to a breather valve on top of the driver side valve cover.

I'll check for vac leaks again with your method, thanks!

After thinking more about the issue, it seems unlikely that it is the trans mounts or the DMF. I say this because the shudder / rough running does not happen right away, but only after maybe 1-2 minutes after startup, at warm idle. There doesn't seem to be a reason why the warming up of the engine should affect the DMF or the trans mounts.

Thanks again!

post-9524-127449731696_thumb.jpg

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  • 3 years later...

I realize I'm bringing a 4 year old thread back here, you still around Gary? I have the exact same symptoms and have already been through all that's mentioned above plus some. Have a new AOS in hand but hoped to get some confirmation before burning a new part maybe unnecessarily.

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  • 3 months later...
  • 2 weeks later...
  • 6 years later...
  • Admin
1 hour ago, Arcane Inferno Gaming said:

 I am about to trade my BMW 328i for a 99 Boxster that has this same issue after a new transmission was installed. The owner says he cannot see anything wrong with vacuum lines and has cleaned his MAF and the other steps listed here funnily. Any resolutions yet??? Specifics please.

 

:welcomeani:

 

Do you have any fault codes?

Fault codes are critical for diagnosing these sorts of problems.

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