Hi everyone!

I have a '97 Boxster 2.5L and when driving and letting off the throttle (especially between shifts and 40+ mph) the RPMs stay hung up at whatever RPM I was cruising at and sometime would climb to around 3K. Its alot more noticeable with the AC is on. I've tried searching on all Porsche related forums but can't find anything.

Throttle body was cleaned as well as the idle control valve. AOS is file as I did not find any oil in the throttle body. I was thinking that this might be a vacuum leak kind of doubt it as RPM drops back to normal in 3-5 seconds after the clutch pedal is pressed. Throttle cable is fine too - I've logged via OBD2 throttle position as I drive and I see that when the RPM get stuck throttle is closed but MAF continues to report some air flow.

Next I have installed a new MAF, changes the engine air filter, and fixed a minor air leak in AOS tube. Did not help at all. Lastly I have replaced the ICV too (this was my best bet) - again no luck.

At this point I'm out of ideas... Hope somebody would have any ideas about this issue. Thanks!!!

 

I'm not that familiar with the throttle set up of the pre e-Gas cars but IIRC the throttle is cable operated. Thus it could be the cable is hanging up. In the motorcycle world we used to replace the throttle cable (all cables) periodically and lube then in between replacement intervals.

Be sure the pedal hardware is not binding and all return springs are present and accounted for.

Last but not least you need to be sure that any vacuum line to the fuel pressure regulator is intact from the intake manifold is intact and mounted tension free. Doing the AOS and addressing a leaking AOS can have this line disturbed.

 

Thanks for the quick reply! I'm pretty sure that the cable moves freely. Otherwise I would be able to reproduce the issue when I'm parked. But this happens only when I'm driving the car and at speeds > 30-40 mph. Another observation, when I'm driving downhill and letting the engine to decelerate the car by letting off the gas pedal, this problem can occur randomly. I.e. I would feel that the can decelerates and accelerates while I'm not touching the pedal e.g. not touching T/B.

I'll look up the fuel pressure regulator line today, thank you for pointing at it! Will let you know tomorrow. Thanks!

 

How does cruise control work on non-E-gas?

Mike

 

One thought is the cable binds from the engine twisting when under load. I admit a bit of a reach, but that's all I got.

 

The "cruise control" on my Harley-Davidson motorcycle was a thumb screw in the throttle that locked the throttle grip from moving (closing) from the return spring.

But how cruise control works with the pre e-Gas systems beats me.

 

No cruise control option on my car . But normally that would be a separate servo that would pull the cable for you on the throttle body side. My guess.

 

Not to put too fine a point on it but you have to be more than pretty sure. If you're wrong you've gone away from the real cause of the symptoms to another area where you might do more harm than good.

Check the engine mounts and tranny mounts to be sure they are all intact and supporting the engine/tranny as they should.

I think my Boxster has (another) bad engine mount. The shifter moves a bit when I come off the throttle in the higher gears. I don't normally let my hand rest on the shifter when I'm not shifting so I only notice it rarely but then when I purposely leave my hand on the shifter I can make it happen at will.

IIRC -- it has been a long time ago, back in 2004 -- this was a symptom that was present way back when the front engine mount was found to be bad.

The throttle can't close completely. It would jam. Mechanically/physically it is prevented from closing all the way. I would expect the MAF to recognize some, though small, amount of air flow.

And I'll end how I started. You have to be sure the AOS isn't bad. Oil in the throttle body is one sign the AOS is possibly bad but the absence of oil doesn't give the AOS a clean bill of health either. There are other places it can develop a problem a leak and cause the engine to run weird. In short you must be 100% sure there are no intake leaks and this includes from the AOS.