I have a 1990 300 SEL. I think we have the same engine and we definietly have the same problem. You didn't mention if you had trouble with cold start, hot start, or both. I am assuming you have a CIS system on your car. There are numerous fuel injection checks you can make but you will need a pressure guage and troubleshooting procedure. Items that can effect starting include the fuel pump & check valve, accumulator, cold start valve, electro-hydraulic accuator (EHA), and fuel pressure regulator. If these items check out, I read recently in Mercedes Enthusiat that cold solder joints on the controller circuit boards or failing electrical connections can also cause misleading problems. Keep after it and post back if you solve your problem.