1992 300ce with 104 engine. While driving at operating temp, car will start running rough and it feels weak. after about 2 minutes or so it stalls out and won't restart. tow to mechanic he inspects ignition system(plugs look new). Installs new coil. Car runs great. Weeks later same stall out. And yet a third time several weeks later. Mechanic checks it out - can't find anything. I pick up the car - starts ok and runs for about 10 minutes then same thing. About two hours later it starts ok and I drive it for a few minutes around the neighbor hood OK.
The issue is that stall outs can't be predicted, but after car sits following a stall out it starts ok.
I feel like its fuel - filter, pump, whatever.
Any hints on how to diagnose the problem -
Any help greatly appreciated - help keep car from the crusher

Is it a fuel problem? Connect a fuel pressure gauge to the fuel test port and monitor it while driving. If the pressure stays it probably isn't a fuel problem.

Diagnosis...attach a test wire to the fuel pump with a test light bulb in sight while driving. If the light goes out, power is being lost to the fuel pump. Probably due to a bad fuel pump relay. If the light stays lit but the pressure drops it's the pump. If the pressure drops and the light goes out it's probably the fuel pump relay.

My guess without diagnosing is a bad fuel pump relay, they are inexpensive and easy to try; however, remember stalling can be a real safety hazard.

Has the wire harness been replaced?

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

You say that it is an M104 engine. But which one? In Europe, the 300 always had the M104.980 engine with separate ignition and fuel system. It was the 320 that had the integrated system. They are very different.

RayH

It's quite expensive if it's the M104.98x engine - 150€ + on eBay. Possibly NLA at MB but around 500€ when last seen.

R

If you have a crank sensor in bell housing. A stick knocked mine loose, even dealer could not find problem

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

try to verify first , is it a fuel , or ignition fault, run a meter off the fuel pump see if its losing voltage , that will help to rule out its not the fuel pump relay,, run a meter on both pumps positive lead, next if the overvoltage relay is old just replace it there very common to fail, it powers up a lot of componants, next check ignition if its cis has and ignition module they fail when hot, your going to need to monitor the coil and see if when it stalls there is or is not spark, on an older car 560 sl i found my ezl was shutting down signal from fuel relay to ezl ie the signal voltage signal was lost when hot and if i remember i replace the ezl

^^^^ I vote for this - bad crankshaft position sensors don't like heat when they're on their way out, so they stop working when they get hot, then they just stop working completely.

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Robert Fini

'12 ML350 BlueTec, 87k
'06 E320CDI, 270k
'05 T1N Sprinter 2500/Pleasure-Way Plateau TS, 69k
'97 C36AMG, 313k (son's)
'94 E320T, 249k
'93 190E Sportline LE, 168k (daughter's)
'84 190E-2.3/5spd (Stage Rally Racer)
'66 230 W110 Sedan (Barn Find, Vintage Racer build in progress)

Good idea about the crank sensor... I didn't think of it. Try lightly tapping on it with the handle of a long screwdriver to see if it acts up.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer