Engine Shuts Off While Driving...

[jriewer]

I have an 87 420 SEL in which the engine has been shutting off while I am either sitting at a light idling or when I am coasting down a hill and not pressing on the accelerator. I suspected my fuel pump relay and jumped the pins that keep the relay running when the ignition is turned on but the engine continues to shut down intermittently. I also suspect maybe a low idle was causing the problem so I disconnected the idle control valve which increased the idle about 750 more rpm but the engine again died while coasting down a hill yesterday. I replaced the fuel filter about three years ago so I don’t suspect it’s the filter and the fact the engine simply dies instead of sputtering before it dies makes me think its not fuel delivery that is source of the problem. Usually the engine starts up again as if nothing is wrong, yet a few days ago, for the first time, I did have to crank the engine several times to get it restarted. I live in San Francisco and driving around this town with the possibility of loosing my power brakes while going downhill adds a whole new element to city driving. Any suggestion on where to troubleshoot would be appreciated.

[raymr]

Crank position sensor?

[jriewer]

I will look at that sensor but one of the sympoms that I am experiencing, the fact the car stalls when the accelorator is not being presses, either when coasting or at a stop light, doenst seem to match with others experiences when a bad crankk position sensor is the culprit. Am I missing something here?

[Educaid]

Just an uneducated guess but it may be the voltage regulator. A $35 part.

[saumil]

Faulty Idle/Full load throttle valve switch ? There is also a decceleration fuel cut-off switch that may be faulty.

[jriewer]

Thanks Saumil for the suggestion. The first post I read about a throttle swith going bad seemed to highlight my problem. When I disconnected my idle control valve to keep my rpms up in an attempt to prevent stalling, I felt a distinct engine braking as soon as I removed my foot from the accelerator when i was doing 60 mph down the highway. Does anyone know if this is expected behavior if Im driving without the idle contorl valve connected or does this point to a bad throttle switch?

[just-n-time]

You may also need to check the EHA value,be sure you are good there,it controls the fuel threw the fuel distributor, you should check the potentiometer too.this little Jewel keeps the fuel from dipping to low on deceleration.

[Mike Murrell]

And let's not overlook the infamous OVP - over voltage protection relay. I had a somewhat similar situation a few yrs. back that was cured by a new OVP.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=24S1DLNMJ25711GAH0&year=1987&make=MB&model=420-SEL-001&category=P&part=Overload+Relay

[just-n-time]

my wife has a 420 sel 90 mod.she two has had the same trouble,though hers is rare,in-fact she just had it happen after a fill up at a Phillips station with half a tank left. Her crankshaft position sensor is new, and as I said it is rare.

[JBoggs]

Probably not the same, but my 560ls did the same thing, replaced the fuel pumps, still did it, replaced the fuel pump relays, fixed the problem. Heat related I think, only did it when it was good and warm, after cooling for a few minutes would start fine.

Ya never know.

John