W123 230E more fuel supply woes

[grahamf505]

This is unbelievable....

After having problems last week with my fuel supply and having fixed it the weekend and drove 250 miles with no problems, the same thing has happened again.

A quick re-cap.....

Last week my fuel pump relay was tripping out and (I assumed) was causing the fuel pump to go on intermittent operation. The result was the car would die on me, pick up, sometimes cut out and after a few attempts to re-start, I could hear the pump energise and then she would run for a while and then start tripping out again.

I changed the fuel filter and put a new relay on the weekend and drove 250 miles up and down the motorway and around the suburbs in London - so good mix of driving. Absolutely no problems at all.

Imagine my disappointment and frustration then, when I try to use it for the first time since Sunday and half a mile down the road, all the symptoms I had last week re-appeared.

So now what.....??

I'd assumed that my problem was the relay, but surely after only about 6 or 7 hours operation it wouldn't fail again - unless of course there was something else causing it to fail.

What about the fuel pump? It was fine once I'd changed the relay last weekend and was energising and then running whilst the engine was running. Could it be my fuel pump failing intermittently?

The strange thing was last weekend, as soon as I put the new relay in, it was running fine. Maybe just moving the pump around dislodged something and I solved another problem last weekend when I actually thought it was my realy giving problems. However, I did try replacing the new relay with the old one last weekend once I'd done the filter etc, to check if the old relay was giving problems and as soon as I put the old one in, the fuel pump started dropping out again.

Are there any other components I need to check that might be causing this problem? Any help/advice greatly appreciated as an unreliable Mercedes is a very frustrating animal!

Graham

 

[Dieselman]

Check the earth of the pump onto the car body. People often forget the return side of electrical circuits when diagnosing faults.

 

[grahamf505]

Dieselman,

Many thanks.

I'll check this out the weekend. Got to be something daft like this. I'll also put a new dizzy cap, lead set and plugs in just to make sure its not an ignition problem. Given it was wet (damp) yesterday morning I could have a problem in this area.

Took the car out for a drive last night and after some initial hesitancy it then seemed OK, but then started "missing" again after 10 mins or so. When it gets really bad I can hear the fuel pump relay chattering away in sympathy with the engine stuttering. There is no discernable difference between either my new or old fuel pump relay by the way as I swopped these over again last night. hence I think the relay is OK.

Not sure if it might be any of the engine/fuel sensors causing the relay to drop in and out. I know that a couple of the sensors feed inputs to the relay. It could be that if the high tension circuit is shorting causing the engine to cut, maybe there is a feedback circuit to then trip the fuel relay - not sure. Or maybe one of the sensors is giving an input to the fuel relay to trip it out when it shouldn't be.

Understand that you can take the relay out and bridge across the two centre contacts to run the fuel pump constantly. Might be worth me trying this just to make sure I'm not getting any other inputs to the relay that could be cutting the fuel supply. At least then I'll know that I've got a permanent circuit to power the relay

Do you know which two contacts it is I need to bridge across if I do this?

When its running, it runs like a dream. Plenty of power and dead smooth, so I'm pretty sure its not something like a knackered fuel injection system.