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Idle Fluctuates

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1999 Boxster Tiptronic

Had AOS replaced a week ago, yesterday, after accelerating hard to 80 mph, the car, when idleing, fluctuates between 750 and 1,400 continuosly until the transmission shifts to 2nd and rpm exceeds 1,400. I also have a pending code P0506.

Any suggestions? Thanks in advance.

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P0506 Idle Air Control - Idle Speed Too Low

Possible fault cause:

- IAC valve is sticking.

- Air cleaner clogged.

Perhaps the oil was not cleaned out of the throttle body after the AOS replacement?

Thanks Loren. Can you comment on why this happened a week later and all of a sudden?

Thanks again.

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OBD II is kind of funny - it is always trying to compensate or wait before it throws a fault code. In that sense it is good - if you get a fault code with OBD II then you very likely have a problem.

Thanks for the response.... What I meant to ask was not necessarily the code, but the way the idle is fluctuating all of a sudden. After the hard acceleration, the idle problem started - when you are in park, the engine revs as if you were doing it with the gas pedal?

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The newer-style air-oil separators appear to be slightly different than the old ones with respect to the hose that connects to the throttle body. If you upgrade / replace your air-oil separator, then you should also get a new hose to connect it to the throttle body, otherwise you will have air / vacuum leaks that may cause the problems you're experiencing. I would try this first, and see if it makes a difference. If not, then check your entire car for vacuum leaks, I have an indepth article on doing that here: http://www.pelicanparts.com/techarticles/Boxster_Tech/21-FUEL-Vacuum_Leaks/21-FUEL-Vacuum_Leaks.htm

The engine revving problem you're talking about is called "hunting", which typically means huge vacuum leaks. Also, try blocking off both the air-oil separator port and the throttle body port to see if the problem goes away.

Hope this helps,


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If the problem started after the AOS replacement, recheck the proper fitment and the condition of the bellows between AOS and engine, the O-rings on the tubes, inclusive those on the AOS which go in to the engine bloc, the O-ring behind the throttle body, the seal between the idle valve and throttle body, the O-ring in the oil filler cap and the intake tube between air filter and throttle body, to be short, everything you have removed and replaced.

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Thanks for all the info thus far.

This past weekend, I cleaned both the throttle body and the idle control valve - the problem continues....

Question: Are there any moving parts apparent inside the ICV cavity? While cleaning the assembly, I noticed that nothing moved inside although there appeared to be a "flap" inside one of the orifices. I recently saw a picture on one the forums showing a "valve" that opened / closed.

Any advice?

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