This is a friend's car, '87 560 SEL, stalled and won't restart - happened Friday about two blocks from his house on his morning commute. I'm going over to look at it today.

Dave reports that occasionally on a cold start it will just barely idle (500 or less revs) and runs very rough, but then cleans up. Friday he got about two blocks from his house, and then it stalled and would crank but not start.

There seems to be a number of KE -equipped cars that show these symptoms. I'm going over to Dave's this afternoon and plan to check for spark. If okay, then it's probably fuel system related, and I'm going to look at the OVP relay and fuel pump relay.

Any other suggested troubleshooting?

I believe 560s have two fuel pumps. If one fails, will the other allow the car to operate at low load?

Duke

Dead fuel pump may or may not allow car to run, depends on whether it locked up or not. If so, no fuel.

I'd suspect a bad OVP or fuel pump relay, definitely fuel pump relay if he also has a history of intermittant stalling followed by immediate restart -- this is symptomatic of a bad fuel pump relay. A diagnostic clue, if he noticed, is that the tach will indicate proper rpm.

If the tach drops to zero with the engine turning, it's the OVP dropping the engine computer off line.

A bad accumulator will do this, too, as will a bad fuel pressure regulator (check for fuel leaks on the "bleed line" of the regulator -- if there is fuel in there, replace).

Proper diagnostics for those require a fuel pressure gauge.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Bad coil will do that too. And just for fun, a bad coil can show spark if you hold plug wire with test plug to ground. But it won't fire under compression. Squirt some ether down the throat and see if it will fire off. If it will start with ether, but not on its own, it's probably fuel. If it won't start with ether, it's probably ignition. Unless of course something horrible has happened like chain break. Take a can of ether with ya and start there.
Usual DIY mantra: "don't ask me how I know".

Thanks, Peter!

We checked it out this afternoon - didn't take long!! Turned on the ignition and heard the fuel pump(s) whir for a second or two; then pulled a plug wire and used a spare spark plug grounded with a test lead to check for spark. Spark was there!

The OVP relay fuse was okay, so I left the OVP alone. Pulled the fuel pump relay and the pins looked okay - no corrosion or anything obvious. Put the relay back in and sniffed the exhaust while Dave cranked it. I detected a fuel smell, but the engine didn't even so much as cough or hickup. It acted like there was no fuel or way too little to form a combustible mixture.

It's getting towed to his indy tomorrow morning! Dave's been using him for at least a decade and he seems pretty good. He can check fuel pressure to see if the accumulator or pressure regulator are okay. I don't have a proper gage setup.

The car was parked about half a block from Dave's house, just across an intersection. As we were wrapping up and scratching our heads I noticed another W126 parked on the intersecting street. Part of the grille was broken out. I suggested to Dave that we might be able to get the hood open and "borrow" the OVP and FP relays. Then we just laughed - guess we didn't want to end up in the Manhattan Beach police blotter - "local resident and Redondo Beach man arrested for breaking into a Mercedes before sundown on Halloween."

I'll follow up with the Rx when Dave gets it back from his indy. The car needs to be running by next Saturday as I have tickets to the UCLA-WSU game and Dave's supposed to drive!

Duke

FOLLOWUP:

BobK - YOU GET A GOLD STAR! Indy said it took a while, but he finally isolated it to a weak coil! Certainly not the first old coil to get flakey after over 15+ years and 180K miles, regardless of make.

The indy also changed the cap, rotor, plug wires, and fuel pump relay - all with OE/Bosch parts as the installed parts appeared to be original. Wasn't cheap, but it should be good for at least another 150K miles.

The engine starts in an instant and runs smooth as silk, now! It's a sweet drivin' car.

Duke

Easiest way to test/isolate is to swap in a spare coil. At the time, I had two 560 engines and just "borrowed" the coil from my wife's car. Boy does the light go on when you do that. Drives ya crazy since ya see a spark outside the engine. Dynamics of creating spark inside a cylinder are quite different. The 560 motor is really sweet. They are so nice and smooth. Last a long time if ya take care of them. Mine had 350,499 miles on it when the girl hit me. Don't think the heads had ever been off. Wife's has 140k and runs well but needs valve seals. Gotta do that this winter.