Also, as for the microswitch testing, what I originally did was engage/disengage the arm while holding the throttle linkage open. Nothing happened. I was expecting a fuel flow cutoff.

Thanks Duke,
I agree the throttle switch on my car does not switch clearly either going to idle or going to full gas.
I cleaned already the area around the throttle butterlfly and improved a bit the idle (the picture was taken with everything removed).

Now I need to check also the adjustement of the idle stop. Does your manual indicate how to adjust the idle stop? I did move that screw some times ago and would like to be sure that I brought it back to nominal position.

I've never touched my idle stop adjustment. I believe there is a procedure to adjust it while monitoring the duty cycle of the idle speed controller. I'll have to do a little research and get back.

Duke

It appears the thrust cutoff switch was indeed the culprit. I will know for sure after my afternoon and evening drives.

Since installing the new one last night, I have maintained a constant 550 rpm in drive and in neutral, and engine thrust is definitely cutting off when I lift off the accelerator at higher speeds.

I have also noticed that, once warmed up, my acceleration is a lot peppier than it has been for the past year or two. Normally, on moderate to hard acceleration, I rev to 4000-4500 rpm before the car shifts. Now I am seeing 5500, and the engine pitch is also higher – like it was when I got the car. I assume the engine is breathing easier now, and am surprised that the thrust cutoff switch has such a significant impact.

I am also still running lean (and have been for about two years), which I assume is the reason why, on soft acceleration, there is a slight lag before the engine spins up – almost like a turbo lag. I suppose that will clear up once my fuel/air mixture is adjusted.

What is the "thrust cutoff switch"?

The two throttle linkage switches I am familiar with are the "microswitch" on the linkage, and the "throttle valve switch" at the end of the throttle shaft.

It's been awhile since I looked at the schematics, but as I recall, the microswitch is the trigger for deceleration fuel cut-off, and the TVS is the trigger for idle speed control. (I incorrectly referred to this as "throttle position switch", or TPS in a previous post) In this context the microswitch could be referred to as "deceleration fuel cutoff switch", but the MB literature just refers to it as "microswitch", which is the generic name for this type of device.

Note that when you push on the throttle pedal and the linkage is properly adjusted the linkage flips the microswtich before it moves the throttle shaft lever, which flips the TVS idle circuit. Likewise, on closing, the TVS idle circuit will not flip until after the microswitch flips.

The TVS also has a wide open throttle circuit to signal the ECU to go into open loop enrichment at WOT.

Duke

The Thrust Cutoff Switch is the microswitch. At least that is what the dealers' inventory system calls it.

I want to spend some time examining the other switches and linkage compoenents since I am pleased with the improvement in performace that resulted from replacing the microswitch.

Resolved.
 

The Cutoff/Microswitch was definitely the source of the problem. Car has behaved normally (550 RPM in gear and in neutral) for the past two days. Thanks for all the good feedback.