Idle Speed "Hanging" At About 2200 RPM Between Shifts - SOLVED!!

I am posting this as a new thread simply because I spent a couple of hours chasing other threads that had similar symptoms, and I failed to find a solution using the 996 boards. I actually found the solution on a different site for a 986. Anyway, I purchased a 1999 996 6 speed coupe with about 145k miles, and it ran great with the sole exception that when I was traveling at 30 MPH+ and I pushed in the clutch, the engine would sometimes hold about 2000 to 2200 RPM instead of returning to the normal 700 to 800 RPM. What was even more concerning is that during engine braking the car would sometimes even hold the higher speed and cause the car to surge forward in order to maintain 2200 RPM. Given the symptoms, I strongly suspected that the issue was computer-driven rather being something mechanical, simply because a mechanical issue would be unlikely to consistently lock in on one engine speed regardless of gear, load, outside conditions, etc. I searched multiple forums that discussed the usual suspects like the idle control system, the AOS, the throttle body, the air flow sensor, even the coolant sensor; none of which solved the issue. I finally found the solution buried deeply in a 986 thread. The elusive solution was actually to: CHECK THE TIRE SIZES . As it turns out, the previous owner had put the incorrect tire diameters on the car, and the resulting tire speed differential was making the computer think that the car was losing traction, and it was messing up the engine speed. It sounds nuts, but simply disabling the traction control instantly solved the issue (at least until I get replacement tires that are actually correct). I really have no idea how the previous owner drove the car without going crazy. I hope that this helps someone else in the future, and props to the poor technician that logged over 20 hours with some lady's 986 before finally figuring this out. Follow-Up: I installed the correct outside diameter tires, and the problem is now completely resolved. No more need to disable the traction control system.

Wow, that's quite interesting, but I have a newbie question then to this: My '99 has 17" wheels/tires and am reading that others may have the 17" or 18" wheel options, Yes? How does this not create the issue you ran into?

Interesting, I had a similar thing happen with my 97 M3, but worse. It just cut power all of the time. I had a wheel emergency, and swapped different wheels on one side only. Good thing for me, the traction control light was blinking, turning it off solved the problem, so it was a quick fix. Good job figuring it out, and thanks for posting.

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Because your tire sidewall is the correct height. For a 17" wheel the side wall would increase and for an 18" wheel the sidewall would decrease. As long as it's in spec you shouldn't experience what the OP did.

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Bull3t is exactly correct. I should have been more descriptive in the OP. The 17" and 18" original tires specified by Porsche had essentially the same outside tire diameters because, as he said, Porsche had specified a lower profile tire for the 18" wheels than that 17" wheels. The issue with mine is that the previous owner used tires that did not match the width and aspect ratio that Porsche had specified.

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What was the tire size that you removed?

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Aspect ratio? Are you saying they don't fit on a 16:9 television screen? Fit better on the old 4:3 tube TVs? Just messsin with you! lol Glad you found a solution. I would love to know the exact wheel and tire sizes are causing the problem. Actually, many people put non-OEM sizes of wheels/tires on their 996 cars with no problems other than physical scrubbing around the front fender wheel lining and rear finder edge. Makes me wonder if you're not experiences something with the TC system. I was about to say there could be a problem with the manual transmission. I had a car one time that had a hanging clutch that wouldn't completely pull off the flywheel. Interested and similar symptoms to what you described.

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Good info, thanks guys!

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I was hesitant to go down the road of the specifics for the car that I bought simply because it was so jacked up. The wheels that are on the car are 19" (I am not a fan of wheels that large on a 1999 car) with 235/35 R19 tires on the front and 285/35 R19 tires on the rear. The front tire diameter is about 25.5", and the rear is about 26.9", so that is a pretty large differential. I was actually surprised that they don't rub, but they have not so far. The 986 that was on the other website where the Porsche Tech figured out the problem had a more subtle differential. It had 265/45 R18 rear tires instead of the correct 265/35 R18 tires. I ordered some 275/30 R19 tires that should be here next week. They have a 25.5" outside diameter, which exactly matches the front. I am 99% sure this will solve my issue, but I will post a follow-up soon to verify the solution. My preference will be to get some 17" or 18" factory wheels/tires, but I will have to wait for some to become available locally. I would have guessed that it was anything else other than the tire diameter differential other than the fact that simply disabling the traction control completely solves the problem. I guess it could still be something with the system itself, but it would almost certainly be throwing a code if that were the case. I will know for sure in a week.

As a follow-up to the original post, I ordered some rear tires that had the same outside diameter as the fronts (there are multiple tire diameter calculators out there via a simple Google search). I had them installed today, and the issue is completely resolved!! The rear rim is a little wide for the tire that I had to use, but it is much better to have use of my traction control again. Also, the car has a better looking profile without the incorrect sidewalls.

Are you saying that tire width was also a contributing factor?

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Correct. At the same aspect ratio, a wider tire will have a larger outside diameter than a narrower tire. For example, a 285/35 R18 tire has a larger outside diameter (25.9") than a 275/35 R18 (25.6"). I didn't know those off the top of my head. I cheated with the calculator.