Hi all, need some help here...

1993 W124 320CE M104 with distributorless ignition (in the UK), car has been idling like a pig recently - shooting up from 600 - 1500 rpm.

Today I cleaned out the air filter (not particularly dirty), sprayed some isopropyl alcohol on the air mass sensor and the other sensor in the inlet pipe.

The car started then shut-off after 10 seconds or so.

I worked out the fuel pumps weren't switching on and there was no fuel pressure so I took out the fuel pump relay and bridged pins 30 and 87 and could hear fuel gushing into the fuel rail, fuel pumps were on and working well.

I duly changed the fuel pump relay but this has had no effect, there is no pressure at all in the fuel rail with the ignition on.

Can anyone shed any light? There seems to be no fuel activity with the ignition on.

w:

Have you run the engine by bridging the pins as you indicated above? Have you driven the car with the pump bridge in place? Does the engine run properly and idle OK?

If the engine/car run OK with the pump bridge, then either the relay is defective, or the trigger signal from the key switch has gone on holiday.

as posted or the ignition signal to the relay is missing. good luck, chuck.

No the engine would not start with the bridge in place but I wasn't sure if it would cause an issue so I didn't try for that long. If the bridge is kept in place so the pump runs all the time does the excess fuel simply return to the tank?

I notice when the ignition is on there is total silence, I seem to remember some electrical component(s) under the hood would make a slight noise.

I changed the OVP around 15 years ago, I wonder if it's gone again...

Excess fuel is returned to tank continuously when the pump/engine are running.

It would be in order to now check the ignition system for operation. Sparks being produced, and at the right time. As porkface noted, there is a "running" signal produced by the ignition system that tells the pump relay to remain "on".

Check the fuse on the top of the OVP.

When the pump was running I heard it fire a couple of times, there is certainly a spark.

The fuse on top of the OVP looks ok, when the unit failed some years ago the car would cut-out when hot then not restart for some time.

Scratch that, tried today and seems there is no spark.

The OVP relay was changed today which hasn't cured the problem.

check spark at the coil itself. then look for the trigger to fire to coil on 1 or 16, not 15, on the coil. trigger-yes, probably the coil w/no spark. trigger-no, probably the crank sensor below the oil filter. spark at coil but nowhere else, cap rotor and wires. bosch cap and rotor, karlyn or beru wires, nothing else will live long enough. and if it has spark, gotta be blue and fat, not yellow and spindly. hard to see under your hood, the light here sucks. good luck, chuck.

This is the model with no distributor - 3 coils fed by the ECU.

Weis you did say you had bridge the pins on the fuel relay . And did you plug the relay back in to the socket it came from? with bridge still in place on the relay and the wire still across no 30+87 ?. . Check the fuses in the fusebox also, for fuel system

The bridge is no longer there but the relay was replaced - genuine Mercedes new part.

I had a good look at the fuses - none seem to be gone however there don't seem to be any related to the fuel system.

A so called Auto Electrician came today and pointed the finger at the crankshaft position sensor - I changed that this afternoon but no joy.

He then said it had to be the ECU, I had one lying around so changed it but still no luck.

So far changed:
Fuel pump relay
OVP relay
Crankshaft position sensor
ECU

w:

In addition to 12V+ and ground (earth?) both the injection system and the DIS require a crankshaft position signal (which also provides an engine speed signal), and a synchronization signal, which typically is supplied by a camshaft position sensor. No sync signal, no injection, no sparks.

With key on, there should be 12V+ at one terminal of each of the coils.

Ok thanks I will check that out.

Is the camshaft sensor the one near the fuel pressure regulator?

What is the DIS?