The revs climb and fall by themselves

Hi, I'm hoping someone can give me some insights, before I go and talk to the dealer. I have a '99 C2 tiptronic. Driving the car under a constant amount of throttle the revs go up and down slightly by themselves, it's not much, say <100 rpm. It stops doing it when the engine has warmed up. Any thoughts? Thanks in advance.

Vacuum leak�

Does your car have traction control? What are your tire sizes?

Quotes source post: 18041495

Does it do this at idle too?

Yes it does have traction control, tyres are 265/35/18 and 225/40/18. Thanks

Hi, no it's fine at idle, it only does it when driving. Thanks

Hope you get to the bottom of it! The reason I asked about tires is that I also have TC and have experienced some strange behavior with my car. It turns out that my 265/40R18 (a little too tall) rear tires cause issues for me. With TC off everything is normal. Since your tires are the correct sizes I'm going to guess it's a vacuum leak as well.

Test the DME for fault codes, any cheap OBDII tester will do this, even AutoZone will do it free. Look for code P0740 ( Torque Converter Lock-up fault. ) If P0740 is present, next take it to a Porsche shop who is familiar with the ZF5HP transmission fill procedure and have the trans checked to make sure it is full of fluid. Then get back with us.......

Thanks, I've ordered an OBDII tester, waiting for the courier now.

Hi, the OBDII arrived, plugged in, no faults, so I guess I'll look into the vacuum leak. Aside from a smoke machine are there any other suggestions on how to track down the vacuum leak?

Could just be your 02 Sensors are a bit sluggish when cold.(and your very observant).. Set you OBDII tesert to monitor the "front/Sensor 1 02Sensors voltage on both banks 1&2" while cold, see if the switches from "high voltage to low voltage" corresponds to your RPM fluctuations...

+1 on vacuum leak

Thanks for your help everyone. If it is a vacuum leak, aside from a smoke machine (I'm not sure I could get access to one) are there any other methods to track down the leak?

The old school method is to let the engine idle and spray some carb cleaner or starting fluid around your vacuum lines/connections (also intake runners, boots, throttle body etc.). If there�s a leak it�ll suck in the spray and the idle will race/increase�

Guys make a smoke machine with a rubber glove and a cigar or $99 of amazon.

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I have used this method for years on carbureted cars. It works great, and I don't see any reason why it wouldn't work just as well on EFI engines. Biggest down side is I don't think you can easily hook up an auxiliary tac to a 996, you will be able to hear the revs though.

I recently bought this unit from eBay and it worked perfectly. https://www.ebay.com/itm/282339070632

I thought that vacuum leaks were most noticable at idle, when the leak is a significant percentage of the intake? Vacuum leaks are less noticeable at throttle, when more air is coming in through the throttle body, so the leak is a very small contribution. OP says his car is fine at throttle, but worse when driving. Which to me, seems unlikely to be a vacuum leak. But I'm not a mechanic, and I didn't even sleep at a Holiday Inn last night.

Quotes source posts: 18075623, 18075623

Ding !! Ding ! Ding !! Winner !! Winner !! That's the reason I didn't mention vacuum leak in my opinion either, but of course our internet diagnosis is only as accurate as the text symptoms of the OP... Yea, but a big dose of common since and some Quantum Mechanics and Classical Mechanics helps too, even if it came from a Holiday Inn..