A dirty MAF is not the problem. When my 02 Boxster manifested this error code the first...

Our 2001 986 ran perfectly all the way down and back to BRBS this year, but just threw a P1128 today on an ice cream run. The engine feels like it is running fine. There is lots of info on the web about how to diagnose this, but I had a specific question regarding removing the oil filter cap while running to check the AOS. When I removed it there was definitely some vacuum there. The idle held steady but I could hear the engine running rougher when the cap was off and it restored to normal when it was replaced. Does this indicate a normal AOS? I will also check out the other unusual suspects: dirty MAF, leaking vacuum lines or intake boot. I have also cleared the code to see if was just something spurious.

Chris - '01 Boxster - Seal Grey / Black / Black ; '87 944 Turbo (track beast) ; '86 944 - needs a new home!

More info - I removed the top hose on the AOS and there was just a film of oil there. I also sprayed some quick start around the intake hoses and there was no increase in RPMs.

... a vacuum leak.
Check to make sure that the O-rings in the AOS hose (both sides) is in place and sealing properly.
Also check the O-ring in the Oil Cap's underside and that it is sealing properly as well.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

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Thanks Pedro. The seals look ok and spraying starter fluid on them did not change the idle. I will drive it some more and see if the code comes back. If it does I think I will try cleaning the MAF next.

time is was from a leaking oil filler tube cap. One could hear the cap leaking by moving the cap on the tube even with the cap secure.

Another time the AOS proved to be the problem.

My money is on the AOS in spite of the fact you were able to remove the oil filler tube cap without any difficulty. The big vacuum is a later failure stage symptom along with excess oil in the intake along with engine exhaust smoking, etc.

Thanks for the input. The light came back on today after a few days driving around, P1128 again. I will try cleaning the MAF first since its cheap and easy just to eliminate the possibility and then dive in further if that doesn't help.

and if it is the AOS this can deteriorate rather quickly and put the engine at some risk of damage. After you clean the MAF and are driving around if at any time the engine starts to act up, whether the CEL is lit or not -- shut the engine off and make arrangements to get the car flat bedded to a shop you already have lined up to diagnose the true cause of the P1128 and any other symptoms and fix the source of the error code and symptoms.

I had recently removed the oil cap while my car was idling, it almost sounded like it was going to stall, is this normal?

So I can eliminate a dirty MAF. I was hoping to find a smoke machine to use to check for vacuum leaks but nobody seems to rent these locally. I guess I will have to take it to the local P-car mechanic after all.

Reviving my old thread - Well after cleaning the MAF more thoroughly last June, the CEL stayed away until yesterday and now P1128 / P1130 are back. Now when I try and remove the oil fill cap I'm getting significant pressure and I can just barely remove the cap. I don't recall it being so difficult last year when I tried this test. I'm guessing this is pretty good evidence my AOS is on its way out. Seem reasonable? I'm somewhat thankful this occurred now, rather than during the trip down to BRBS!

Chris - '01 Boxster - Seal Grey / Black / Black ; '87 944 Turbo (track beast) ; '86 944 - needs a new home!

on my Boxster was signaled by a CEL and error codes. I believe P1128/P1130 were the codes, but I can't be sure. Whatever they were they pointed to a mixture problem and thus I initially attributed the problem to the MAF but it turned out to be the AOS. A few days later, thankfully at my destination (2K miles from home) the billowing oil smoke upon cold start made it clear to me the MAF was ok; it was the AOS that was the problem.

A hard to remove -- very hard in some cases impossible to remove (unless you wear a blue suit with a red cape and fear Kryptonite) -- while the engine is idling oil filler cap is in my humble opinion the clincher.