1995 S500 problem
 

My 1995 S500, 130K miles, has developed a problem. It starts and runs perfectly cold, but after 10 or 15 minutes developes a slight miss which over a few minutes escalates into stalling. The previous owner said that he had the engine wire harness replaced and it shows no sign of degraded insulation. I tried unplugging the mass air flow sensor and it momentarily effects the idle (briefly smoothing out) but then settles right down into the same problem. It did however make the SRS light come on. I searched the forum but could not find whether crank position sensor failure is often related to engine temperature. The plug wires and distributor caps are about 6 months old and I see no arching anywhere. Any ideas? Thanks for any help.

Fuel pressure?

Possibly the pump is overheating & dropping out.
Fuel pressure needs to be 52-55 lbs.

Thanks mbdoc. I'm going to borrow a fuel pressure gage today and see what happens when it starts to miss.

Fuel pressure holds steady at 52 lbs. for about 15 minutes, even after it started missing intermittently. When I shut it off, the pressure dropped to 44 and then actually rose in the first minute back up to about 55 lbs. I suppose from the fuel heating up in the rail? Regardless, it held pressure well after shutoff, lost about a pound and a half in the first 30 minutes. While it was idling, I noticed that it seems to be running rich, judging by the exhaust smell and sooty exhaust tips. I left out a huge detail, the car had been sitting for about 3 weeks. It might be my imagination, but it seems to be taking longer to act up each time I drive it.

Well the car seems fine; I've driven it daily with no symptoms at all. It's scary trusting it as I have no idea what caused it but given the excellent reliability it had given before, it's pretty much a wash.

Could have been moisture in the fuel, sometimes fresh fuel can do wonders.

That's an excellent point. The garage isn't heated and the temps during the snow were single digts. I might have tried a bottle of Heat. Next winter I'll probably drive her during the snow. For a rear wheel drive, the car is heavy enough and the traction control good enough that it does very well. My worries are other drivers and the effects of all the salt they dump here.