w6grace

Ok, I had a rough idle that is now much better after replacing oil filler tube and seals on the J tube. When I start the car now it idles at about 700 for a few seconds, then slowly builds up to about 790, and sits there for the rest of the drive. It seems like it tries to bring it back down to 700 occasionally but then builds back up to 790-ish.

I may need to clean out the TB and ISV again now the vacuum leaks are fixed... But it seems the high RPM was unrelated to the roughness... Any suggestions, maybe a new MAF or ISV. I don’t really want to start replacing things at random (any way to test these items)?

Macster

Do you have an e-gas equipped car?

If so you might consider performing a recalibration of the e-gas system.

The steps should be in your car owners manual.

IIRC they are: Turn the ignition key on but not to start. Avoid touching the gas pedal during this procedure until it is necessary to do so.

Leave the ignition on for 60 seconds.

Turn the ignition off.

Wait 10 seconds.

Now the next time you start the car the e-Gas system should be ok.

Whether this is the cause of the symptoms you report I can't say. But if you have an e-Gas equipped car it is the first step I'd try.

If your car doesn't have e-Gas then the ICV comes info for suspicion. I have no direct experience with these but some who have have reported good results by cleaning this valve.

If the e-Gas recal or ICV clean doesn't help then you can clean the throttle body, though if it is not dirty, oily (as it was in the case of my Boxster's 1st AOS failure) then the odds are the throttle body's condition is not the reason for the idle symptom.

BTW, IIRC idle speed is 740 +/- 40 RPMs. 790 might not be that far off. Also, the tach is not that precise an instrument. Can you view the idle speed with an OBD2 code reader/data viewer?

I will add though that it is counter to my experience that the idle speed would start off and then slowly climb and remain elevated. The usual flow is the engine starts, the idle speed zooms up then quickly settles down to what is termed a fast idle then as the engine warms the idle slows to its normal speed. Depending upon the engine's temp the initial zoom up may be less and the engine may quickly settle into its normal hot idle.

Also, last but not least there still could be an air leak. The AOS is always at the top of the suspect list.

But if you have had the throttle body out then where this connects to the intake can be an intake leak. My experience is the throttle body is a bit of a chore to get installed again.

w6grace

The 99 Boxster is not e-gas, it is good old cable.

I have taken the ISV out twice now for cleaning. I suppose I could try that again now the vacuum leak is fixed, but not sure if that will help.

I cleaned the TB thoroughly last month; it is possible that I did not re-seat it fully. I will check that next. It was slightly dirty but not to the point I would expect it to interfere with the RPM.

I will re-describe how the RPM works. It does do the zoom up after starting and then the high idle. Once it warms a little it drops and pauses around 740, then after about 4 seconds it slowly moves up to the point there the needle is just touching the bottom of the 800rpm line, and that is where it sits for the rest of the drive.

justin-in-athens

You might just try pulling the battery for a bit. The car should idle higher upon first start as the ECU is trying to get the car up to operating temp and get the cats warmed up. Pulling the battery should force the ECU to run the idle adaptation program upon next start. This is how i understand it anyway OBDII runs drive cycles which may affect how quickly it will readjust the idle.