This is a very common complaint for the 1986-1987 603 diesel engine. They smoke light bluish-gray on startup and idle rough. You take it down the road and a half-mile later everything is fine.

I think its a great problem. It helped me get my car CHEAP! Previous owner thought it was major engine damage.

It doesn't seem to be a problem with glow plugs on my car. I replaced them all with verified good plugs, with nearly the same results. When you preheat with the glow plugs, you might notice that the car starts smoothly with no smoke - then about the time the glow plugs cool down (like two seconds) the smoke starts pouring out and the idle roughens.

Later diesels with the aluminum heads solved this problem by extending the time the glow plugs stay heated after engine startup. On some models they can run up to a minute after the engine starts. No problem on these cars.

The idea of a poorly conducting glow plug relay is worthy of testing - and I have not done that. It should be easy to verify though. You just get a voltage reading at the glow plug when the relay is engaged. The system should be providing a reasonable voltage to the plug, say 11 volts or more. However, I can tell my glow plugs are working - and you can to if you ever try to start the car instantly without waiting for a glow plug preheat. They really, really do not like to start without the glow plugs.

Hopefully there are enough people not reading here that will quickly discard their 1987 300Ds at "destroyed engine" prices!

Ken300D