not 400e but w126
I had a 1986 420sel. It had problems stumbling, surging, and then stalling out. My friend who now owns the car replaced the fuel pump relay, and it runs alot, ALOT better. It now idles a bit rough, and he thinks there is a vacuum leak. But the problem is now gone. But that is on a different engine, so I do not know if that could be the problem. Good Luck.
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