RPMs not dropping / sticky RPM

[uladzislau]

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!!!

[JFP in PA]

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!!!

First of all, get the car scanned with a Porsche specific tool, we have learned to not always trust data generated by third party scan tools, they sometimes feed you erroneous data and send you on an unnecessary wild goose chase. Verify the throttle position data with the Porsche tool, if it is in the fully closed position when the RPM's seem to hang up, check the fuel trim data to see if the car is momentarily either rich or lean. If rich, you may have an injector issue which the Porsche tool will help to isolate to one bank or the other. If it goes lean, it is an air leak somewhere in the system, which is not necessarily a simple loose or disconnected hose, it could be somewhere internal to the system like the EVAP purge or vent valves momentarily opening.

Problems like this are difficult to diagnose, as you are finding out; you need to collect as much verifiable data to try and isolate the source.

[uladzislau]

Thanks! I'll try to get my hands on Durametric to check the values once more. Do you know if something special happens in Boxster engine around 40 mph speed? I never get this issue while driving at city speed limits and RPMs would always drop back to normal once I slow down below approx. 35 mph. Again the issue is erratic, it may occur or may not on high speeds but never on slow speeds. That's why I guess ECU opens up some valve after 40 mph and then fails to close... wild guess.

Edited February 7, 2014 by uladzislau

[JFP in PA]

Thanks! I'll try to get my hands on Durametric to check the values once more. Do you know if something special happens in Boxster engine around 40 mph speed? I never get this issue while driving at city speed limits and RPMs would always drop back to normal once I slow down below approx. 35 mph. Again the issue is erratic, it may occur or may not on high speeds but never on slow speeds. That's why I guess ECU opens up some valve after 40 mph and then fails to close... wild guess.

I can't think of anything that operates at a specific speed for your model year, the EVAP system opens to vent fuel tank fumes stored in the activated charcoal under certain conditions, as does the SAI system, but they are not speed related in any way to my knowledge.

I would suggest that once you get your hands on the Durametric, utilize its excellent data acquision abilities to log the car's variables over a lower speed session, and then above 40 MPH; then compare them for differences. Something has to be changing.