I test drove a 92 300D 2.5 turbo today. The car seemed to have a knock at idle. I drove about ten miles, when I parked the car I opened the hood to locate the knock and the knock stopped. When the knock stopped it started to smoke(white) more but ran smooth and sounded normal. When I was driving the car I thought it felt under powered like the turbo was not spooling up. I am now thinking it was a stuck injector that freed itself.
Any ideas? 262,000 miles oil pressure seemed strong, temp was good. Any thoughts on the smoke?

 

What kind of injection system do they have? Electronically-controlled rotary?

Any codes?

It could be a sensor or mechanical problem causing timing to be off, if it has variable timing. Could be a contaminated fuel issue. Could be an injector issue, but a "stuck" injector would be a miss and more noticeable; if it's an injector, it's most likely a pop-pressure problem.

I think that they all knock though.