Rpms not dropping

[san rensho]

Vacuum leak?

[gikser]

Im gonna smoke it out and see. I steered away from a leak as when i shut the ac off the rpms drop

[uladzislau]

I know it's 2011 post but I've got the same problem recently and can not find the issue as well. 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. This points to a bad MAF or possible bad idle control valve.

So did anybody figure out this issue at the end? I'm gonna start with MAF and if it does not help will try idle control valve.

[BYprodriver]

Originally Posted by uladzislau

I know it's 2011 post but I've got the same problem recently and can not find the issue as well. 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. This points to a bad MAF or possible bad idle control valve.

So did anybody figure out this issue at the end? I'm gonna start with MAF and if it does not help will try idle control valve.

Thanks for searching 1st, always good to post your year model & mileage also.
I agree with your plan & also recommend you have someone work the gas pedal while you watch the T/B fully open & close just to be sure it's adjusted properly.

[uladzislau]

Originally Posted by BYprodriver

Thanks for searching 1st, always good to post your year model & mileage also.
I agree with your plan & also recommend you have someone work the gas pedal while you watch the T/B fully open & close just to be sure it's adjusted properly.

Thank you for the reply!

It's an early Boxster '97 2.5L engine, no e-gas, around 70K milage. I'm pretty sure the T/B is fine, when I was cleaning it the cable was long enough to allow TB to be closed completely. Plus I did not have the issue before ever and I don't think the cable would "shorten" suddenly. 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.

The problem only occurs when the car is in motion, idle RPMs are stable and no problems whatsoever. Seems like the higher speed - the higher probability of RPM to get stuck.

[snakebite]

Isn't there a sensor/potentiometer for the accelerator pedal that could be faulty?

[uladzislau]

Originally Posted by snakebite

Isn't there a sensor/potentiometer for the accelerator pedal that could be faulty?

I have early non-egas model so there is no sensor at the pedal. It's mechanically connected to the throttle plate and I've confirmed that the link does not get stuck and moves freely.

I have just installed MAF and a new air filter. Plus I have found a small air leak in AOS where it gets connected to the throttle body. Somebody before me have damaged the sealing ring on the AOS tube at the side of the throttle body and some air was leaking through it bypassing the throttle plate. I'll change the sealing ring and will try it out.

Big thanks for suggestions!