I have a mercedes 190E 2.0L Auto (1990 model) which was runnning
beautifully until it refused to start 4 weeks ago with absolutely no
previous symptoms.  the car did not even try to start i.e not even a
cough or splutter. At first I cheked the usual suspects:

Ignition system - nice fat blue spark found at ech cylinder, new
distributor cap fitted just in case, new rotor arm fitted, new spark
plugs fitted, new air filter fitted - all functioning fine - all fuses
cheked, all fine

Fuel System - blockage cleared in fuel distributor by dismanteling and
rebuilding it, new fuel filter fitted, fuel pump and fuel pump relay
checked and working fine - fuel at inlet to distributor was fine at
high pressure, and a nice fine spray was emmited at each injector

However, none of these replaced parts solved the problem.  Today
however I narrowed down the problem to that of overfueling.  The spark
plugs are being saturated with fuel and are therefore not sparking
atall.  I came to this conclusion after I dried out the cylinders (by
leaving plugs out for a week), and dried out the plugs too. I span the
engine with no plugs in to clear any fuel in the cylinders and then
replced plugs and tried to start the car with NO fuel pump relay.  The
car fired and ran rough for a few seconds and then cut out (probably
due to lack of fuel as pump was not running), but at least it proved
the car still runs.  when the pump was replaced the car would try to
start for about 10 seconds but not quite manage it, and then the plugs
would become saturated and the car would not start atall until
everything dried out again.

Another test I carried out was to crimp the fuel inlet line to restrice
the flow of fuel to the engine, as well as un-attatching the cold start
electrical connector.  doing this I managed to get the engine running
for a couple of minutes, but as soon as the crimp was released the
engine cut out again, thus proving the problem to be fuel over-supply.

I think have narrowed the problem down to three possible causes:

1.  Faulty fuel distributor
2.  Faulty fuel pressure regulator (the part that sends unused fuel
back to the tank - if not working correctly all fuel goes into engine
and none back to the tank)
3.  Faulty cold start valve (and/or temperature sensor) causing too
much fuel to be permanently injected into the engine.

If anyone has had this problem before I would be grateful of some help
in pointing me towards which of the three (or indeed any other possible
causes) is causing the problem.

Thanks