I've had this problem ever since I bought the car: the engine won't start when I attempt to crank it shortly after it's been shut off. I came up with a fundamentally wrong theory that this problem is caused by the high engine temperature which affects glow plugs timing. Even though the theory was wrong, the workaround -- turning the key from position #0 to position #2 several times before attempting to start the car -- actually helped. So I haven't really tried very hard to fix it. I did some searches on the symptoms, which mainly turns up broken IP shutoff valve at the culprit, manifested by oil in the vacuum lines, which is something I don't have. I also don't have any problems with shutoff.

Recently I was working on fixing my ACC, which was mangled by PO, apparently in a misguided attempt to get 100% recirculated air. I got ACC fixed and verified that it holds vacuum, but when I started the car, ACC was just as dead as before. Turns out there's no vacuum getting to ACC. Why? Long story short, the two vacuum lines at the kill switch at the ignition lock housing were switched. So the IP shutoff valve was getting residual vacuum when cranking, and the rest of the vacuum system was getting a nice leak. I can see how one make this mistake and not realize the lines are hooked up wrong: shutoff still works, and for other problems it may not be immediately clear what caused them. One more thing to check when diagnosing "won't start when hot".

The other thing to check is whatever is listed in the MB tech bulletin titled " failure to start when hot' or similar wording...

The TSB titled "Engine is difficult to start at operating temperature" just warns about oil in vacuum lines, something that has been covered at legth at this forum. Curiously, the reason given is "leaking diaphragm of modulator pressure vacuum unit on automatic transmission". Odd. Oil would have to travel a really, really long way to get from the tranmission modulator to the IP shutoff valve.

All I had seen was the title... glad you filled that in...
Somehow it seems that oil in vacuum lines can be the cause of burning precombustion chambers... but I don't really understand that one yet... just read it in the FSM....