Lots of good advice there, but the single most common cause of this behaviour is the fuel pump relay.

A friend of mine recently had an identical problem that was eventually traced to the fuel pump - it's poor performance was the cause of a weak mixture and bad O2 reading.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)

Check the warm up regulator

Next time it stalls try and open the gas cap. i had a 190e that had a pinched off breather line to the charcoal canister. After opening the gas cap the car started right away. then would stall later down the road. Driving with the gas cap off it didn't stall.

First of all, let me thank everyone for their advice.

Second, let me apologize for omitting what turns out to have been a key bit of information: My 300E is equipped with the ASR traction control system. I had not realized that would make a difference. But if my MB dealer tech is correct, and I think he is, the ASR was critical to understanding the cause of the stalling.

Unlike other 1991-1992 300Es with the M103 engine, a 300E with ASR has an electronic accelerator system. Instead of a conventional throttle control, there is something called the electronic accelerator pedal actuator. The “gas pedal” moves a potentiometer that sends a voltage signal to the electronic actuator pedal control unit. The signal is proportional to pedal position. The control unit determines how far to open the throttle, based upon the driver’s positioning the gas pedal but also taking into account the traction control system input (which can retard the throttle) and the cruise control function (which is integrated into the electronic accelerator control unit). The control unit sends a signal to the actuator, which in turn opens or closes the throttle. The control unit has other functions as well, including control of the fuel pumps (via the Engine Systems Control Unit, or MAS).
The actuator is essentially a (very special) reversible DC motor, which can open and close the throttle with both great precision and high speed (full open to full close in 120 milliseconds, or 1/8 of a second). The actuator also includes another potentiometer (to tell the control unit what the actual throttle position is) and a safety switch.
The safety switch is the apparent culprit here. The safety switch is part of a circuit that shuts off the fuel pumps if the throttle is open more than a small amount if the accelerator pedal is in the idle position. In other words, if the driver takes his foot off the gas and the throttle position does not follow immediately, there must be an error and the circuit shuts off the fuel supply.
According to the tech, there is a history of the safety switch “hanging up” or sticking, thus stalling the car. In my case, the sticking only occurs until the engine reaches a certain temperature. Since the actuator is mounted on the engine, it is sensitive to the engine heat.
I did a lot of reading about the systems involved here, and read lots of posts on this web site and others, before I became convinced that this was the correct diagnosis, but I did. (And by the way, the AllData site was very helpful in this regard, with good system descriptions that I did not find in the MB service manual.) One of the things that convinces me the tech is correct is that I can see the effect of the pedal movement very directly when the engine stalls. If I am going fast enough when it stalls (20 mph or so), the tachometer drops to zero but the car keeps moving without any indication from the instruments that there has been any failure. The tachometer signal passes through the MAS on its way from the EZL circuit to the tachometer. So when the MAS loses power, the tach fails. If I am moving fast enough (at least about 15 mph), and I push the accelerator pedal down again, the switch is again closed, the engine will re-start (from the momentum of the car turning the engine over) and I see the tach pop back up. None of the other instruments dies, no idiot lights come on, so the tach clearly has not lost power – it lost its signal, which can only mean that the EZL or MAS has lost power.
I can keep moving the pedal back and forth, opening and closing that faulty switch, and thus starting and stopping the engine, until I either warm up past the point at which the switch hangs up, or lose momentum so that a rolling start no longer works.
The parts are on order and the “surgery” is scheduled for later this week. Needless to say, the very special and sophisticated design of this actuator implies a high price, nearly a thousand bucks. And because so few 300Es have ASR, the part is rare enough it is not stocked in the US. So it is enroute from Germany. I will let you all know how it turns out.

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DavidB29
1992 300E with ASR
35 years of Diesels until now!

Did this problem turn out to be related to the ASR as described?
Or was the crankshaft angle sensor the cause?

Check for bad spark plug wires.

Remove air filter housing and check the idle control valve hoses -- your idle control valve is sticking closed. The key symptom is stalling when coming to a stop only -- if it quits while running normally, it's the fuel pump relay or fuel pump(s).

If the idle control valves are rock hard, they are leaking. Check all the rubber vacuum line connectors, chances are they are bad too, and all your symptoms sound like a vacuum leak. Make sure the brake booster line is tight and the booster doesn't leak.

The rubber boot between the throttle body and the fuel distributor is probably bad, and check for leaks at the injectors with a bottle of carb spray -- while idling, spray a bit on each injector. If the idle changes, tis leaking air. Get new o-rings for the plastic sleeves the injectors fit in and change those, too. Best to do the injector seals, throttle body boot, and ICV lines all at once, as the injector lines have to come off anyway and the ICV line to the manifold is a real bear to change with the throttle body on.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Im guessing he hasnt checked this thread in 4 years

It was the ASR. Replacement of the motorized throttle actuator did the trick.

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DavidB29
1992 300E with ASR
35 years of Diesels until now!

He still lives!

Thanks David!