Idle not dropping when I clutch

[JeTexas]

I've searched, but I can't quite find anything to match this situation.

I have a 99 C2 Cab with a 6-speed. In slow, stop-and-go traffic, when I clutch, the I can watch the engine RPMs drop to 800 and when I'm stopped, my idle is steady at 800 RPM. However, if I spend some time on the freeway and get over say 60 mph, when I get into traffic again and come to a stop, when I clutch, my idle hangs at 2100 RPM and stays there until I come to a complete stop at which time it then suddenly drops to 800 RPM.

I thought maybe I had an air leak, so I did a visual inspection, but everything looks good, and I'm also thinking that if it was an air leak the idle would stay elevated once I come to a stop.

Any ideas what could be causing this? I'm not getting a CEL, and it doesn't really affect the driveability of the car at all, but it's annoying.

[RFM]

I would start with a clean up of the idle valve, which is mounted on the left side of the throttle body. You can find here the DIY procedure in the relevant section.

[violaGT3]

I would start with a clean up of the idle valve, which is mounted on the left side of the throttle body. You can find here the DIY procedure in the relevant section.

+1

[JeTexas]

Well, I cleaned the throttle body, the IAC valve, and the MAF sensor (which is less than a year old) ... my throttle is much more lively, but when I clutch, my idle is still sticking at 2000 rpm until I come to a complete stop when it suddenly drops to 800 rpm. Any idea what I should check next?

[RFM]

Check the throttle cable, perhaps the cable tension is a little too tight.

[1999Porsche911]

A bad coolant temperature sensor will also cause your idle to stay high. Remove the cable and clean the contacts. Inexpensive item to replace.

[JeTexas]

A bad coolant temperature sensor will also cause your idle to stay high. Remove the cable and clean the contacts. Inexpensive item to replace.

I'll check it, but you'd think that would also affect the gauge on my dash, which seems to be working properly.

The incredibly frustrating part is that it's intermittent, and I can't replicate it while sitting in the driveway, only on the road.

[JeTexas]

I rented a scanner and checked the car this morning, but it's not throwing any codes. The only other electronic component I haven't checked is the Throttle Positions Sensor. After reading this Wiki on TPS failure, it sounds like my symptoms.

http://wiki.seloc.org/a/Throttle_Position_Sensor

Of course, the only way to check it is to order a new one and swap it, so that means another week of waiting. Unfortunately, it looks like I'm going to have to dremel/drill off the old one because I cannot get the screws holding it on to budge at all.

[Loren]

The TPS is pretty easy to test with Durametric, PST2 or PIWIS.

In most cases if there is a problem with the TPS it will generate a fault code.

It could still be that the ICV is sticking and needs to be replaced.

[JeTexas]

The TPS is pretty easy to test with Durametric, PSt2 or PIWIS.

In most cases if there is a problem with the TPS it will generate a fault code.

It could still be that the ICV is sticking and needs to be replaced.

I wish I had one of those testers available. I guess I need to bite the bullet and order a Durametric. Both the TPS and the IAC have over 100k miles on them, so I figure it doesn't really hurt anything to change them out, and since the TPS in only $80 whereas the IAC is $300, I'm going try that first. If the IAC is bad, it must be that the solenoid is just completely worn out because it's clean and not binding at all when off the car. Supposedly the TPS I ordered from Autozone should be ready for pickup today.

Helpful hint, if you lose the little paperclip-looking retaining bracket off the IAC plug, you can hold it on with a twist tie until your local Porsche dealership finally gets one ordered for you.

I'm in Houston and have dealerships near both my house and my office, yet neither one seems to keep ANYTHING in stock.

[JeTexas]

I swapped the TPS last night. The car ran fine on the way to work this morning, but it did yesterday too. We'll see what happens at lunch and on the commute home. Intermittent problems are such a pain.

I swapped out the 4 x .17 x 12mm phillips head screw on the TPS with hex heads, so that if I have to replace this thing in another 100k miles it's not such a pain in the *** to get it off again.

Also note the twist tie Porsche recommended as a temporary fix until I can get a new retaining clip for the IAC plug.

<a href=" title="DSCF2030 by ffacker, on Flickr"><img src="http://farm4.static.flickr.com/3053/3080406428_125581107c.jpg" width="500" height="375" alt="DSCF2030" /></a>

[JeTexas]

The high idle is back. Should have listened to you guys. Ordering the IACV now.

[JeTexas]

I replaced the IACV -- no change. If I go over 3000 rpm, when I clutch the idle hangs at 2000 rpm until I come to a complete stop, then it abruptly drops to 800 rpm. I'm off to return the IACV and the TPS.

[whall]

Je,

Many years ago, I had a '77 280Z that had a similar problem....the engine RPM would hang at 2000RPM until I had slowed to under 10MPH and then drop to the usual idle RPM. If I was to slow, while in gear, to less than 2000RPM and then de-clutch, the engine RPM would increase back to 2000 until I slowed to less than 10MPH. As I recall, there was a speed signal sent to the fuel injection computer while decellerating and that signal was incorrect.

Perhaps one of the electrical/electronic whizes can find a similar electrical situation with the 996/986 fuel injection system.

Bill

Edited December 6, 2008 by whall

[JeTexas]

Je,

Many years ago, I had a '77 280Z that had a similar problem....the engine RPM would hang at 2000RPM until I had slowed to under 10MPH and then drop to the usual idle RPM. If I was to slow, while in gear, to less than 2000RPM and then de-clutch, the engine RPM would increase back to 2000 until I slowed to less than 10MPH. As I recall, there was a speed signal sent to the fuel injection computer while decellerating and that signal was incorrect.

Perhaps one of the electrical/electronic whizes can find a similar electrical situation with the 996/986 fuel injection system.

Bill

That sounds exactly like my problem. I left it with my mechanic this morning. He couldn't get any codes or get the problem to happen while test driving it, so they only charged me $50 instead of the $99 diagnostic service charge, but I still have no solution.

[JeTexas]

After a Sunday of discussion with more car friends, the last suggestion was to start pulling every electrical plug and clean each one with contact cleaner. It's not very scientific, but I guess it can't hurt anything, and maybe it will help.

[JeTexas]

Wish I had a better update, but despite all my plug cleanings, research, etc. I'm still at a loss. It seemed to be doing it less though until yesterday when it did it almost the entire commute home from work. Then it was gone again this morning.

Ran into a guy who said he had a 99 Chevy S-10 that had the exact same intermittent symptom, and he never could figure it out -- ended up selling the truck after driving it that way for two years.

I'm determined to solve the mystery.

[Foster]

Are you sure it's not the accelerator pedal? Mine is getting a little sticky, and it does something similar once in awhile. Mine only sticks for 1-2 seconds though.

I'm going to try to rebuild mine soon, but I've been told you really should just replace it.

[JeTexas]

Are you sure it's not the accelerator pedal? Mine is getting a little sticky, and it does something similar once in awhile. Mine only sticks for 1-2 seconds though.

I'm going to try to rebuild mine soon, but I've been told you really should just replace it.

I wish it was that simple, but no. I've checked the pedal and the throttle body, but neither is sticking. Also, when this happens, I can rev the motor and let off the pedal and the rpms will go to whatever I've revved it to and drop right back to 2000 rpm. Then, once I get under 10 mph, it drops to 800 rpm, and it will not hang when I'm at a stop.

[JeTexas]

I finally got a Durametric. It didn't find a thing as far as codes. Guess I'll have to see if it's possible to test while I'm driving since that's the only time the problem happens.

[Foster]

Some boxsters had a fluctuating idle problem that ended up being software related.

[JeTexas]

Some boxsters had a fluctuating idle problem that ended up being software related.

I'm wondering if it is a software problem because when I come to a stop, it's like something resets and the idle drops to normal. If it was a vacuum leak or sticking throttle or pedal, it would stay at the higher RPM. Unfortunately, it's so intermittent, I can't seem to ever make it happen when I have someone checking it.

It seems to be happening less and less and the symptoms are changing a bit. Now I can go three or four days without it happening and then suddenly I'll clutch and the engine will stay at whatever RPM it was at when I hit the clutch and remain there until I put it back in gear and continue to behave that way until I come to a stop. Then once I've come to a stop, it usually resets and behaves normally.

I've been avoiding taking it to Porsche since the holidays had my budget so tight, but I may finally give in if I can't figure it out within another week or two.

[1999Porsche911]

Some boxsters had a fluctuating idle problem that ended up being software related.

I'm wondering if it is a software problem because when I come to a stop, it's like something resets and the idle drops to normal. If it was a vacuum leak or sticking throttle or pedal, it would stay at the higher RPM. Unfortunately, it's so intermittent, I can't seem to ever make it happen when I have someone checking it.

It seems to be happening less and less and the symptoms are changing a bit. Now I can go three or four days without it happening and then suddenly I'll clutch and the engine will stay at whatever RPM it was at when I hit the clutch and remain there until I put it back in gear and continue to behave that way until I come to a stop. Then once I've come to a stop, it usually resets and behaves normally.

I've been avoiding taking it to Porsche since the holidays had my budget so tight, but I may finally give in if I can't figure it out within another week or two.

So, when are you going to try seeing if the coolant temperature sensor is causing the problem?

[JeTexas]

Some boxsters had a fluctuating idle problem that ended up being software related.

I'm wondering if it is a software problem because when I come to a stop, it's like something resets and the idle drops to normal. If it was a vacuum leak or sticking throttle or pedal, it would stay at the higher RPM. Unfortunately, it's so intermittent, I can't seem to ever make it happen when I have someone checking it.

It seems to be happening less and less and the symptoms are changing a bit. Now I can go three or four days without it happening and then suddenly I'll clutch and the engine will stay at whatever RPM it was at when I hit the clutch and remain there until I put it back in gear and continue to behave that way until I come to a stop. Then once I've come to a stop, it usually resets and behaves normally.

I've been avoiding taking it to Porsche since the holidays had my budget so tight, but I may finally give in if I can't figure it out within another week or two.

So, when are you going to try seeing if the coolant temperature sensor is causing the problem?

I already pulled that connection and cleaned it as you suggested. I haven't replaced the actual sensor, but I'm assuming it's ok since the temp gauge on my dash is working correctly.

[1999Porsche911]

Some boxsters had a fluctuating idle problem that ended up being software related.

I'm wondering if it is a software problem because when I come to a stop, it's like something resets and the idle drops to normal. If it was a vacuum leak or sticking throttle or pedal, it would stay at the higher RPM. Unfortunately, it's so intermittent, I can't seem to ever make it happen when I have someone checking it.

It seems to be happening less and less and the symptoms are changing a bit. Now I can go three or four days without it happening and then suddenly I'll clutch and the engine will stay at whatever RPM it was at when I hit the clutch and remain there until I put it back in gear and continue to behave that way until I come to a stop. Then once I've come to a stop, it usually resets and behaves normally.

I've been avoiding taking it to Porsche since the holidays had my budget so tight, but I may finally give in if I can't figure it out within another week or two.

So, when are you going to try seeing if the coolant temperature sensor is causing the problem?

I already pulled that connection and cleaned it as you suggested. I haven't replaced the actual sensor, but I'm assuming it's ok since the temp gauge on my dash is working correctly.

I am not saying the sensor is deifinately the cause, although it is a common one, but a bad sensor will still povide accurate readings to the dampened down display.

Edited January 9, 2009 by 1999Porsche911