Rpm's fluctuating while driving

I have a 1999 996 carrera tiptronic and it has a surge in engine speed between 1500 and 2200 rpms. The tac and engine speed goes up and down, in a steady rhytm. It does not do this while in park at the same rpms. I know this is hard to explain, but there are no check engine lights. It does this in all gears in the same range. It is smooth below and above the range I have listed. First thought was transmission, now I am not so sure.

I have gone to 3 mechanics. The Porsche mechanic did the PIWIS with the following results.

1. He did a 89757 internal malfunction test with piwis tester - none registered.

2. fluid level - good.

3. tested MAF - good

4. tested for power and ground on TCM module - good

5. removed torque converter lock up wire and test drove- good

6. checked fuel pressure - good

7. no codes in vehicle

8. solenoid circuits are intact and complete

HIS GUESS: He said the main line pressure was checked with a piwis tester and it it showed the main line for transmission fluid control loses pressure on and off which is a sign of a weak solenoid.

What I have done:

1. Cleaned the TCV ( very dirty, also looking inside of it the flap that opens and closes is not fully opened or closed. Is

this normal?)

2. Unplugged the MAF and drove, no changes

3. Cleaned the throttle body.

NOTE: What is happening is the car rpm’s are jumping up and down as if I am pressing the gas pedal and letting off, but it is doing it on it’s own. It does get worse on a grade. It does it while crusing between 1500 and 2500 rpms, when the car has no need to shift, which leads me to believe it may not be a transmission issue as was though early on. I can use the manual mode of the tiptronic in all gears and it does it in that range. Note: If the car is in Park, there is no problem in that same range when I rev the engine, it’s only when moving. Also, as a reminder I have no codes with the PIWIS or Durametric tester.

I would appreciate any help. Thanks

I have exactly the same problem, although my car is a 2007 911 with Tiptronic. We have changed engine oil/filter, transmission fluid/filter, both MAFs, and RPMs still fluctuate on the uphill section, especially when the car is cold (within the first two miles or so).

Have you ever resolved this problem?

Thanks much -


I am having the same problems with a 2012 GMC Acadia. Would be great if someone out there could help with a diagnosis.