High idle "fixed", but why?

Following the advice of Rennlister "jumpy-chunky," I turned off the traction control of my 1999 C2 and the "sticky idle" problem of engine revving at 2500 rpm immediately went away. Of course I am thrilled, it has been a very frustrating problem, and I am going to have quite a few spare parts lying around. But now I want to know why this worked? What is the correlation between engine idle speed and the TC? Obviously I would like to have both working at the same time.

The second bullet point is intriguing. I know you have an inverse issue of PSM seemingly causing high RPM, but it is interesting to me that Engine RPM and PSM do have a relationship. I wonder if it could be possible that PSM is taking in data from the other sensors that makes it think it needs to increase engine RPM? I'm just spitballing with you till the pros show up

Sounds like more evidence your TPS is inaccurate. When you gonna get the sensor value? Even the boys at Autozone can get that for you..

Disabling traction control fixed this issue on my car as well. I also have a '99 C2 with a the 220, 222, and 224 option codes and was experiencing the same issue. I guess I'll call my local shop on Monday and see if they can pull sensor values off my TPS. It just seems wild that the car is perfectly normal with TCS disabled and has a sticky idle with it enabled. I would have thought that an out of spec TPS would cause issues in both situations.

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Better hope its the TPS and not this yaw sensor. This yaw sensor lists for $5K and fits 99 only..

I don't have TPS. I do have TC and LSD. "Jumpy Chunky" says the fact that I have a mis-matched set of tires (should be 265/35/18 & 225/40/18 and I errantly used 265/40/18 on the back) threw off whatever sensors there are feeding info to the TC (I presume ABS brakes?) into believing the wheels were rotating at a different speed and fouled things up. I can buy that. I don't understand the relationship between the TC and raising the idle, however. I have read in desperate times the TC will cut fuel to the motor; but raise it? Two indies with a million years between them were completely baffled by the TC + idle situation, though they confirmed the tires were out of whack. They scanned the car and it was completely clean. One of them looked at me, shrugged his shoulders, and said "you learn something new every day" LOL

Opps - Throttle position sensor, not tire pressure sensor (just did those on my truck) I replaced the throttle position sensor as part of my plan to spend as much money as I could on this problem. If it is that yaw sensor, I hope it would show on the scan, 'cause I scanned clean today! I will be driving without TC until I can afford (another) set of rear tires.

Can you send me the part number off that Yaw sensor?

996 606 110 01 Thank god for Bone yards......$250.

What doesn't make sense to me is how the ECU gets the engine to run at a higher rpm. The throttle plate is not motorized. The amount of air allowed into the engine will be limited by this. This will limit the speed at which the engine can run. -The ECU can command the plugs to fire at an earlier time or faster rate, but that will in effect retard the timing. It won't make the engine run faster -The ECU can command the injectors to add gas or change the timing of injection along with the spark. But this will result in rich mixtures -If the TPS is reading incorrectly(or any other sensor), this might lead the ECU to bad decisions, but I don't see a way to realize those decisions Where does the extra volume of air come from that allows the higher engine speed?

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ISC ( idle speed controller) basically its a controlled air leak bypassing the throttle plates. The ISC controls idle speed and also let's the rpm down slowly to keep the engine from stalling or dipping, it also keeps the engine from crackling and piping on deceleration by adding extra air and avoids stalling.The DME uses TPS and other info for decisions on how much and when to open the ISC ( extra air).

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the ecu is commanding the idle air control valve to open. This is why u will need to monitor the sensors real time while driving to see what is causing the ecu to send out faulty commands.

Porschetech3 and DeJeeper: Thanks! This was really bugging me. I started searching to figure it out and found the part. Now I get it. Something in TC is opening this valve. The interesting thing is that the ECU reset eliminates the problem for a short bit. A learned setting. OK Craigmakes321, when do you get this car instrumented so we can see what the problem really is?

I have another car (an automatic) which raises the rpms when breaking. It does that because the ABS/brake/ESC system needs more RPMs to function optimally. Could be that the TC system detects a traction issue and 'primes' the systems etc thru higher revs? Maybe the switch that tells the ECU it is in neutral/de-clutched is broken and the car thinks it's in gear but stationary...just a guess.

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That's a really good thought... I confirmed that my rear tire sizes are ALSO off. That means that both me and OP have '99s with mismatched rear tires (265/40R18 rather than 265/35R18), 6MT, and TC/LSD. I wouldn't have thought it possible. I'm just going to drive with traction control disabled until I need to replace my rear tires as they are brand new MPSS.

The man who solved the problem for me, a Rennlister who seems a bit shy perhaps, has PM'd me an explanation that makes sense to me for the relationship between the TC and the idle problem. "Kinda makes sense that the Porsche engineers/ racing Dept. would want the DME to intervene when rear wheel speed was either too fast ( initiate brakes ), or too slow ( open up ISC valve). Coming into or out of a tight corner at a track" The wheel sensors are assuming the rear wheels are going to slow ('cause i have the wrong tires on them) so it is opening up the idle valve. Case closed for me!

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AHH !! I thought your issue of "idle sticking" was persistent even after you had clutched and stopped..So what you were experiencing was JUST on "deceleration".....makes a big difference to me, and to the DME.. !! Kudos to the anonymous Tech !!

It was whenever the car was moving, more dramatic at higher speeds and warmer engine. Was not just on deceleration, though quite noticeable when foot was off the gas and the engine just stayed at 2500. Car never had idle problem when the wheels weren't turning. For example, could rev like mad in the driveway, it always returned to 850 or so when i came off the gas pedal. In any case, ultimate proof will be when i get the correct tires on it, which may be a while since i have new ones on it now, and I blew the budget replacing sensors..... I appreciate all your suggestions in the course of this saga!

Nice. I like it when a story comes together... Please update when the new tires are on. That will be the final datapoint!

I experienced the same behaviour when having said 265/40R18 at the back (it's an Audi R8 size I believe) fitted by the previous owner. Went away as soon as I replaced it with the correct 265/35R18. Running the 265/40R18 I noticed the lack of engine braking when releasing the throttle but as soon as I dabbed the brake pedal engine braking returned. BOSCH calls this function MSR in German: MotorSchleppmomentRegelung.or engine drag torque control in English. https://www.seat.com/car-terms/e/engine-drag-torque-control.html