Strange happenings with 1987 300E. Recently had tune-up, mechanic said I needed new distributor cap. Drove car 500 miles or so. Now car quits running and won't start after about 2-3 minutes. Cranks OK just won't fire. Seems to run fine while it is running, but will just quit unexpectedly. Would this be a cap or should we look for something else???

search for OVP there are many posts on this little device. something in one of the posts might explain your problem exactly. do a search and look for the part numbers of the older and newer part. If you have the older one, look into replacing it if you suspect it is the problem.

Others have had stalling problems etc, and the ovp was the culprit in a lot of situations, but not all.

ALon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon

Definitely think this is fuel related. Prime the injectors and car runs for about 15-30 seconds then quits. Perhaps the fuel pump or relay??? Don't know how reliable the fuel pump is, don't see many posts on this. plus is worked then didn't work. Won't start at all now. How do I check for problem??

Sounds like you really have isolated it to a fuel problem.

Both relay and pump failures are common.

Both can be intermittent.

While it is not starting, try simply whacking the fuel pump with something hard but blunt (handle end of big screwdriver, for example). If it starts, you know the pump is the problem. This sounds silly, but I have 'nursed' several cars with sick pumps home in this way, including my 300E once.

If that does not work:
Have an assistant turn the car to the On position while you grasp the pump. You should feel (and hear) it come on for a couple of seconds. I believe the key needs to go all the way back to lock each time to repeat this test.

If it is not coming on, check the voltage at the pump during this (2-3 second) window. My digital voltmeter will not establish a reading that quickly. This is one time where an el cheapo 'lightbulb" type voltage detector is better. If it is getting juice but not running, the the pump has seized up.

If it is not getting voltage, odds are the relay is bad.

Good luck.

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1986 300E 5-Speed 240k mi.