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Car runs fine for 15 minutes, then runs rough and stalls

1992 Mercedes 300E with 160,000 miles that I’ve owned for 15 years. Car runs fine for approximately 10-20 minutes and then starts to sputter and cough and if I don’t increase the RPM’s it will stall. If I increase the RPM’s it will smooth out for a little while, but after a couple of minutes with higher RPM’s it will sputter again and finally stall. After it sits for awhile I can start it up again and it runs fine for another 10-20 minutes. No check engine light is on and the car is OBD 1. This problem preceded the weather recently turning cold. Thus far I replaced the spark plug wires as the other ones were old, but this didn’t help. The spark plugs themselves were replaced about 2 years ago. At this point I’m suspecting it needs a new distributor cap and rotor (when I replaced the wires some of the cap’s posts had corrosion, which I cleaned off with a brush), but before throwing more parts at the car I would like some suggestions. Thanks in advance.

How long do you have to let it sit before it will restart? Presumably before it sits “awhile” it will just crank and crank without firing up? Any smell other than a normal exhaust smell? Like gas smell for instance?

There are quite a few possibilities - loss of spark, loss of fuel, or too much fuel. My money is on fuel given the fact that you can keep it running longer with some throttle. But try to find out which. Pick up a spark tester and can of starter fluid at an auto parts store. When it stalls and won’t start check for spark. If you have spark then forget the ignition side for now. Floor the accelerator while you try to start it to see if that gets anything out of it (this is a flood clear). If that helps you are flooding from something like leaky injectors, bad fuel pressure regulator or even faulty engine coolant temp sensor. If the flood clear doesn’t help, blow some starter fluid into the intake and if that at least gets it going even for a bit then you are losing fuel. Check the fuel pressure and watch it as the car runs.

How old are the O2 sensors in there?

Thanks Cigroller! I will have a chance to try this in several days. And as far as I know the O2 sensors are original.

Hey, did you ever resolve this issue? My '86 Firebird is doing the exact same thing and I’ve tried everything.

Long shot…the next time it stumbles, pull over and remove the gas cap. See if a “woosh” of air goes in or out. Then try to start the car.

You won’t get a response after 3 years.

Your 86 Firebird injection system is way, way different than the Benz which has CIS so even though the complaint may be the same the cause can be something entirely different.

The first thing you need to determine is what is missing when it stalls; fuel pressure, or ignition spark.