I asked this question a couple of years ago. The mystery has never been solved so I'm reviving the question.

We own a '91 300E which has been a wonderful car. Has about 130K. It's the sixth E class we've owned.

Well, these cars have DECELERATION FUEL CUTOFF. It cuts off the fuel when you're coasting. Decel Cutoff works fine, but ONLY after this procedure: you must press the accelerator to the floor one time. After you do this, the Decel Cutoff works perfectly for the rest of the day. You must do it at the start of each day and it's fine for the rest of the day. The engine must be running and the pedal need be on the floor for only an instant.

If you don't do this, the Decel Cutoff does NOT work and the fuel mileage drops a lot. I wonder if the car is in limp mode and the brief WOT kicks it out.

Also, the (high-speed) cooling fans come on just any old time. Often, in the morning they come on as soon as you turn the key. However, this problem is only a few months old....the decel cutoff has been wacky for a couple of years now.

Decel cutoff used to work normally. It stopped working and the fuel mileage dropped from ~24 to ~16. Only by luck did I figure out I could make it work normally with WOT one time at the beginning of the day. Actually, I was aggravated with it, floored it and it started working! So that's how I learned the trick.

Crazy, huh?

We also have a '90 300E (which does not share these problems. It's bulletproof) so I've done some parts swapping. The problem is NOT FI Computer, OVP, idle microswitch & wiring, throttle position switch & wiring, cruise control, 02 Sensor, fuel pump relay or the fan relay. I've cleaned all the ground connections by the battery and behind the instrument cluster. As routine maintenance it received new plugs, wires, distributor cap and rotor.

One other thing. The temp gauge is just a bit unstable. Sometimes it's in the normal position one needle-width above 80; most times it's one needle-width below. It often bounces around between those two positions while you watch it. All the other temp gauges I've seen have been very stable.

Surely something here rings a bell with someone???

Thanks