My 300E is surely haunted!
 

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

A little clarification please.
 

Exactly what does the deceleration switch not working cause the engine to do?

Re: A little clarification please.
 

When you take your foot off the gas at speed, the computer sends a negative signal to the EHA to shut off the flow of fuel to the engine entirely until the RPM drops to ~1300. On the 300E it activates about one second after the switch closes; on the 190E, for example, it is immediate with no delay.

So it's still dumping fuel into the injectors even though it doesn't need it? And people wonder why I hate Bosch!:rolleyes:
But back to your problem, does the engine slow down like it should or continue to maintain it's rpm?

Try spraying some carburetor cleaner around the intake and push the plate down and spray some inside there too. It sounds like your air flow plate is sticking, or it could also be the air flow position sensor -- but I'd put money on the former.

If you're seeing temp needle "bounce", check the single pole temp sensor just forward (on head) of the two pole ACC temp sensor. If connection is good, try a part swap out and see what happens, if you haven't already.

G'luck :D

Hmmm....Interesting Idea. I'll check that. Thanks.

Something is sticking, and you're "unsticking it" when you fully depress the throttle pedal.

I would start at the pedal and follow it right to the throttle plate, and make sure every moving part is working properly.

It is a pretty freaky problem!!

No luck here. Plate moving up and down properly. Everything throughout the linkage is also free and moving properly.