I thought I'd share this one with everyone as it has made a very big difference to my car.

For the last several months, my car ('05 997S) has not been running correctly. It just didn't 'feel' right, wasn't producing the power I'd expected (or remembered) and occasionally the ECU would adapt wrongly - it would pull a load of timing for no apparent reason and stay that way until the ECU was reset by disconnecting the battery.

Several visits to my stealer and a local independent (both of whom have PIWIS testers) unearthed nothing. No fault codes, no warnings about failed sensors - according to Porsche's official test mechanisms, the car was 'perfect'!

I finally decided to spend some money and put the car on the dyno to see what was going on. The first thing we found was the fact that the car was only running 21-22 degrees of timing at full throttle and peak power when 24-26 degrees should be the norm (according to Todd at AWE - thanks for the info). Secondly, the shop found that the MAF was trying to run the car madly rich at part throttle - it was overreading the amount of air going into the engine. This was being compensated for by the lambda sensors and was showing up as a very high percentage long-term fuel trim correction. Additionally the car was running too rich at full throttle.

The tuning shop explained that my car's MAF was out of calibration but not severely enough to throw a code or a CEL. Apparently the ECU will allow for a near 20% misread before throwing a code!! It was out far enough, however, for the ECU to fail to adapt properly and it was pulling timing as a result. (Incidentally, even running too rich and with 3-4 degrees of timing pulled, the car was still running 320-325 wheel HP on that dyno on that day - not too bad)!

Replacing the car's MAF sensor has completely changed the car. It's now adapted properly, is flying compared to before, sings where before it was rough and uses a lot less gas!

The point of the story is that your car could be running far below par and if you rely solely on the Porsche tester, you'd never know there was anything wrong. I can only recommend that if for any reason you feel your car isn't as it should be, delve deeper. You're probably right that something's wrong - you know your car - you just need more expertise than a Porsche PIWIS check to find the problem.