We own a '91 300E which has been a wonderful car. Has about 120K. Used to routinely get around 22 MPG average. It's our fifth E class we've owned.
Anyway here's the deal: When you take your foot off the gas at highway speeds, you're supposed to feel the car decelerate. But our car usually gives this strange sensation that your foot is still on the gas or the cruise is set; it decelerates ever so slightly. Except sometimes, when you take your foot off the gas and it decelerates normally. Or other times when it seems to surge; It's a subtle feeling like someone is turning the engine on and off. And yes, the throttle is in fact closing all the way; it's not hanging open a bit. None of our other 300Es have ever done this. Also, the MPG averages about 17-18 now.
The fuel is supposed to shut off completely under these deceleration conditions, until the engine slows to 1300 RPM, at which time fuel flow resumes. Our car seems to be confused, usually. It works properly maybe 20% of the time.
I have checked the switch that under the air cleaner that signals idle position of the throttle. It's fine. I replaced the speed sensor in the back of the speedometer with another I had: no change.
BTW, the car usually fails the decel shutoff test as outlined in the factory manual. But sometimes, it performs the decel shutoff just fine.
I have ruled out the following potential problems: linkage/cables, computer, idle microswitch & wiring, throttle position switch & wiring, cruise control, over-voltage relay and anything in the fuel pump relay. Maybe the electrohydraulic actuator on the back of the fuel distributor?
Ideas? This is driving me nuts! I don't like problems I can't figure out.