Re: Whenever I read a post of idle variations like you describe I always think "AOS"....

Hi,

I have an '01 S and I am starting to have idling issues. When I start the car it idles between 1200 - 1400, then after the car warms up it goes down to 750, but after some stop and go traffic it will then fluctuate from 1000 to 1400. Could this be a throttle body issue? or can I get some type of cleaner I can put in the gas?

TIA

Michael

If you want to check the TB pull it. If you find it dirty and oily... that's likely from a failing AOS. The AOS hose connects just behind the TB and when the AOS starts to go bad (at least in one failure mode) it dumps a lot of oil vapor into the intake system and this can make its way the very short distance to the TB and foul the TB and its butterfly valve.

Unless you've been running with a bad air filter or have an aftermarket air filter (one that uses an oiled element) the MAF is not likely in need of cleaning.

But it is easy to access and you can eliminate a dirty MAF. (Though, how it got dirty is the real problem...)

What you can do is disconnect the MAF from the engine harness and see if the idle symptom persists. Ideally after you disconnect the MAF you want to clear error codes (even if there aren't any) to erase any learned adaptation by the engine controller arising from a suspected bad MAF. If you fail to do this after disconnecting the MAF the engine may run worse or exhibit other symptoms due to a it having attempted to adapt to a bad MAF.

If the idle symptoms return then the MAF is probably not the cause of the symptoms.

Also, check the oil filler tube cap for an air leak.

Until you eliminate or confirm as the case may be the AOS keep running the engine to a bare minimum. There's a risk that if the AOS failure progresses it can put the engine in danger of hydraulic lock up which can ruin the engine.

If you are running the engine and the symptoms get worse, or the engine gets noisy or noisier shut off the engine now. You've just run out of diagnostics time. You'll probably have to take the car to a qualified tech/shop for diagnosis.


Sincerely,

MarcW.

an AOS gone bad before it went bad enough to smoke.

The next time after a couple of check engine light events and just a minor stumble (hardly noticable at that) concurrent with the check engine light coming on and with no other symptoms for nearly 2K miles until I started the car after arriving at my destination only then did the engine smoke like crazy.

If the AOS is suspect, and I think it is, it should be id'd as the culprit *before* the smoking starts. When the smoking starts engine oil is being fed to the engine and this ain't good.

Sincerely,

MarcW.

thanks Marc. The idle goes from good to bad to good again. I did just got a new readiator cap after having a bad aftermarket cap. The cap does not tighten any more, is there something else I can do? It does not seem to make any noise and it is fine when I drive, but the idling is getting worse. Is the AOS hose hard to replace? I would imagine it is only a little harder then removing the TB.



Edited 1 time(s). Last edit at 12/07/2010 10:58AM by Michael SYS Driver. (view changes)

I just read Pedro's wiite up on this and it seems like a DIY I can do, but of course I would still need to the TB cleaning as well, correct? Thanks all and thank you Pedro, it is like having PPBB again.

Quote
Michael SYS Driver
thanks Marc. The idle goes from good to bad to good again. I did just got a new readiator cap after having a bad aftermarket cap. The cap does not tighten any more, is there something else I can do? It does not seem to make any noise and it is fine when I drive, but the idling is getting worse. Is the AOS hose hard to replace? I would imagine it is only a little harder then removing the TB.

There's a call out on the idle speed and how much it can vary up and down and the variation is just 10 maybe 20 rpms either plus or minus.

Radiator cap has no effect on idle speed. The radiator cap can develop a leak though and if coolant level dropping then suspect the cap.

What I was referring to, at least what I thought I was referring too, is the oil filler tube cap can leak -- at least the original one on my 02 Boxster developed a leak. The leak at idle is signficant but at higher engine speeds not so much.

The AOS needs to be replaced, if a bad AOS is the proper diagnosis. The AOS is a DIY project for some. Search out writeups on this project and decide if you're up to it.

But until the AOS is id'd or cleared as the source the less you run the engine the better.

Sincerely,

MarcW.

sorry, I meant that I got an aftermarket radiator cap and that one leaked but the oil filler cap is tight. Can you hear something from the oil filler cap when at idle? I will check the MAF and go from there. Thanks Marc.

but a co-worker heard a hissing sound and reached over and wiggled the oil filler tube cap and could make the hissing noise change. When I leaned over and got my good ear closer I could hear it too then and played with the cap and confirmed there was a vacuum leak around the cap.

A new cap -- $13 or so -- fixed the vacuum leak that had me convinced the MAF was bad to the point I had replaced the MAF, the MAF that proved years later to be good.

With the engine idling give the oil filler tube cap some wiggling. Try to shove it to one side or the other, lift it up, try to tilt it, etc. There can be some play -- IIRC -- but under no conditions should the cap leak, should you hear a hissing noise that is a vacuum leak around the cap.

Another oil filler tube cap characteristic is that under some failing AOS scenarios the AOS subjects the engine to so much vacuum that with the engine idling one can unscrew the cap he (well, I couldn't) remove the cap against the pressure difference.

Sincerely,

MarcW.