The crankshaft position sensor may fail. Symptoms of this are: The engine will crank—but not start—especially when the engine is warm. The car may start again if it is left to cool off, but it may run roughly or have poor performance. Communicating these specific symptoms to your technician can save diagnostic time.Google+
Car Problem Reports
Mercedes-Benz CLK350 Engine Performance Problems Due to Crankshaft Position Sensor Failure
Mercedes-Benz CLK350 Problem
Visitor, 2006 Mercedes-Benz CLK350
I can't believe that MB hasn't correced this problem. We experienced it on a 2000 S430, which had to be towed to the shop. Dealer said it is a common problem, and he gets 1-2 every day. THIS WAS 4 YEARS AGO!!!