Folks,
The engine in my1991 560SEC will occasionally die for a few seconds, and then start running again. It will do this at any speed, various external air temperatures, engine cold or at operating temperature, under cruise control or not, any fuel tank level. It runs just fine, suddenly stops with no sputter, all dash lights and gauges look like I had just turned the key on and not yet started the engine. I can turn the key off and back on and start it right up every time. Sometimes it will do this every few minutes, or go for a week and not do it at all. It does not sputter, just shuts off. No unusual gauge readings, oil and water are fine.
It's at 140,000 and running terrifically otherwise. I had never changed the fuel accumulator, so did. Fuel filter is not very old. Changed distributor cap, rotor, plugs and wires about 10,000 miles ago.
I can't replicate the problem when I have tools around. Any suggestions on approach to sorting this out?
John

 

I'd go for interuption to the main electrical supply.
Sure your battery terminals are tight.
Main lead to fuse box OK? Maybe there is an intermediate connection point somewhere down this lead that is loose under the toeboard carpet or similar.
Any corrosion on fuse box?
Good mechanical grip on fuses?
Relays seated OK with no corrosion etc?
Try a new ignition relay. It clicks when you turn on the ignition (or at least should)
If the engine just hiccuped I'd look at the ignition/distributor circuit but since you get all warning lights up I believe this fault is closer to the battery.
Ignition switch contacts?
ECU multi connector tight / corrosion?

Shouldn't be too difficult to find. Easy restart rules out fuel, plug leads etc etc.

Could the cut out be associated with a road bump which might suggest the relay I referred to?

Stuart.

 

Stuart,
Thanks for quick reply, good check list.
I don't think it's a main power supply problem, if you mean ALL electrical power. Nothing else electrical in the car is interrupted (radio, etc.).
I've tried hitting bumps on purpose, but haven't reproduced the problem that way yet.
No corrison on fuse box, fuses are all firmly seated (and reseated, just in case). No looseness, tightly gripped. Have interchanged fuses between locations. No change.
I've checked all relays in the fuse box and the two large ones in center up near windshieldfor corrision. None apparent.
I'll try a new ignition relay.
I actually wonder about theignition/distributor circuitBECAUSE Iget all warning lights up, just as if I've turned on the key and not started the engine.
I've tried "wiggling" the ignition key to make the problem occur, no luck yet.
I've not looked at the ECU multi connector, I'll take a look.
John

 

Got it! I replaced the Fuel Pump Relay ($143), that appears to have taken care of the problem. There are several references to this relay andsimilar problemson the http://www.mbcoupes.com/web site. Being so damned intermittent and brief in duration, I couldn't simply trace the problem down with a multi-meter. I was thinking about running some wires to some lamps so that I could see what might be turning "off" while I was driving down the road. But the relay did the trick, another learning to share.

Thanks again, Stuart, for your response.

John