I have been driving this car for three years, and have put about
30,000 miles on it in that time. it was a bit carboned up when I got it, and I think the previous owner drive it gently- too gently. It uses very little oil and is a dream to drive.
I recently did two jobs on my 1990 245 with just over 200,000 miles. First I replaced the timing belt and cam seal. The leak was barely a dribble, but it was nice to get it done. The idler pulley was also replaced.
The other job was to stop the annoying exhaust header pipe rattle. At about 2,000 RPM the inner and outer layers of the exhaust header pipes would rattle annoyingly, very much noticeable inside the car. I drilled two holes in each pipe, about 25-30% from each end, and inserted short sheet metal, self-tapping screws. Once the holes were tapped, I removed them and cut them off so that they would protrude as little as possible into the exhaust flow. Rattle completely gone.
Since that time the car is running noticeably better- smoother, with more power, and better low-end torque. This is not just a matter of some psychosomatic reaction to the work I did
Here's my theory: I do not believe that it was the timing belt. The rattling of the pipes, even at times when inaudible, created some frequencies that were picked up by the anti-knock sensor which was retarding the timing a bit. Is that possible?