I have a 1989 260E that was on excellent behavior when I bought it this winter, but as soon as it got warm (DC-Metro area) it regularly stalls on startup and at idle, when in gear. (It never stalls when in neutral or park.)
These are my observations:
1. The stalling only occurs when the temperature is above 70 degrees according to LCD thermometer in dash.
2. On initial startup, the engine will rev until I let go of the key. Then it will drop to zero and stop. I have to restart about three or four times before the car will idle at about 400-500 rpm (I did not think it could idle at such low speed). Usually, the ABS light will remain on.
3. If I depress the accelerator while starting, the engine will not lose power and stop, but the ABS light will remain lit. On one or two occasions, the light went off when I was idling after startup and the engine raced to about 1000-1500 rpm, then dropped to zero and shut off.
4. After driving for a while, the ABS light will go off.
The problem is obviously electrical in nature. The fuel pump is not getting enough power/voltage to maintain an adequate flow of fuel, and the engine is stalling.
The fact that the car did not behave like this in the winter indicates that the problem is heat related. But it is not the outside temperature that is the culprit, but rather the INSIDE temperature of the car. I know this because recently I ran the air conditioner for an extended time and after it got nice and cool in the car, the ABS light went off and the car did not stall again. Not surprisingly, the one or two times the car stalled in cold weather was when I had been driving for some time and the heater was on at full power.
I am no mechanic, but my educated guess is that an electronic component, i.e. over-voltage relay, fuel pump relay or other devices, is being affected by the interior cabin temperature. Perhaps an old or defective resistor (which becomes more resistant when heated and less resistant when cooled) in some electronic circuit is causing this problem, and choking the voltage to the fuel pump.
I have not taken the car to my mechanic yet, but before I do I'd be happy to know if anyone else has made these same observations.