Originally Posted by Idle

The first place to check is the cold start valve. It is that round thing with an electrical plug attached to it and a gas line from the injection pump that is bolted to the side of the intake manifold.

If it is stuck in the open position then it is just blasting gasoline into the intake manifold and fouling your plugs. The best way to test it is to disconnect the electrical line going to it, removing the gas line going to it (which is under pressure or should be so careful here!) and then hooking up a length of rubber hose to it and seeing if you can blow through it. If you can then it is stuck open. If you can't then it is closed.

Now hook up the gas line, leave the electrical plug unconnected, shoot some starter fluid into the intake through the air filter hose (to simulate the extra gas the cold start valve would provide if it were hooked up) and see if the engine runs like it should. If so you have discovered a leaking cold start valve and the source of your extra gas.

Now you will need to determine if it is the cold start valve that is sticking in the open position or if the sensor is telling it to stay open longer than it should.

You can pick up a nice online manual, which is really a download, on Ebay for about $25 or perhaps find someone selling a DVD of the manual on Ebay. This manual was meant for the Mercedes techs so it can be somewhat difficult to understand when you are trying to test sensors but if you own one of these cars you MUST have the factory manual to keep it running.