[ischmitz]

Let's get some things clear: As far as I know 1990 964 cars did not have the check engine light hooked up at all. So you won't get a light. If you connect a hammer to the diagnostics port you can monitor pinging. Now if you don't get a warning on the hammer and can hear pinging at the same time you can be sure your knock sensors are faulty.

The knock sensors are essentially very unsensitive microphones. They are piezo cristals that generate a voltage under deformation. If they are broken or not connected there is simly no voltage. Hence the DME would never notice a broken knock sensor that never sends a signal even if you have heavy detonation.

If you can hear the pinging you can be sure you are on your way of permanently damaging your engine: broken rings, hammered rod bearings just to name a few. And it ain't cheap to fix those puppies, ask me how I know.....

My recomendation is to have a qualified Porsche shop to verify that you knock-sensors are working and that your DME is able to retard timing.

One comon reason for pinging is that older 964 engine have carbon build-up on the head domes and pistons. Those act like a catalyst and lead to premature detonation under high compression. High ambient temperatures and poor gas quality compound this effect.

Ingo