Re: 300E misfiring



The O-V-P relay is a mystery to me. It has a fuse on its top. It's known to cause strange engine behavior although I don't know why that occurs. Look for it and, for now, give it a light tap on its side.

To review: the engine runs well once it has warmed up. You've replaced most of the ignition parts. Have you checked the cold start system? If it's not working the engine will start cold only with difficulty and indeed need to be "warmed up" before the car can be driven for the air / fuel ratio will be too lean while the engine is cold and it will have poor throttle response - cough and sputter when you step on the accelerator.

We've speculated on the MAS (mass air sensor) and the O2 sensor. I'd set those aside from consideration if the car passed your state's emission test AND drives normally once the engine is warm.

I recall last year another owner had a similar problem - terrible engine performance when the engine was cold and warming up and suddenly, with a click, the engine ran perfectly. The over voltage relay was the cause.

So the suspicion is on the engine's cold start system and the over voltage protection relay.
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