W123 Cold start and then cutting out

[grahamf505]

Hi all,

Thought I'd solved this one a few months back, but now seems to have returned with the first cold and damp morning for the last few months.....

Got into the car about 6:30am the other morning, started up no problems then after about half a mile she just died on me and would'nt re start. Motor was spinning over fine but dead as a doornail. I didn't have time to try any diagnostics so left it and came back later in the day. Guess what..srtarted fine and no problems.

Last time I had this problem I thought I fixed it by replacing HT leads, dist cap, rotor arm, etc. I also put a new fuel pump relay in, plus fuel filter and made sure there were no fuel leaks anywhere and all the electrical connections to the fuel pump were good. I'm now thinking that after replacing the bits back in April the weather has been fairly mild and maybe I didn't fix the problem after all. Since the last time it failed and this time, I've driven about 2000 miles with no problems. The only thing thats changed is the weather!

It sounds to me that perhaps something electronic/electrical is failing as its going through a warm up cycle. I'm pretty sure its not a fuel related problem other than the fuel pump possibly failing from an electrical fault. I'm suspecting possibly the ignition control unit for the electronic ignition (sits adjacent to the coil) or possibly the coil itself... My long shot is perhaps a dry joint or broken track in the ignition control unit affected by temperature, but I'm not sure. Any ideas anyone?

Graham

 

[grahamf505]

More W123 230E cold running problem analysis

I think I'm getting closer to resolving this strange fault I have thats spoiling an otherwise fantastic car! From what I've managed to deduce and diagnose, power to the fuel pump is controlled by the fuel pump relay which itself takes input from power sources and also a signalling input from the ignition control unit in the form of pulses. If the engine speed is too high(i.e. more pulses than allowed), the relay logic trips in and cuts power to the fuel pump, hence no fuel and the engine cuts. Fairly simple, but effective logic to act as a rev limiter.

Although I can't find it written down anywhere, I believe the relay will only energise the contacts that supply power to the pump whilst its getting pulses from the ignition.. This makes sense as when you turn the ignition on, the pump runs for a second or two and then switches off. Of course, once the motor starts running, the relay detects the pulses and re-energises to switch the power circuit back in to allow the pump to run. This would make sense as a safety feature as you don't want the pump to continue running if the engine is not turning (particularly in an accident situation).

So given that I've now changed the igntion control unit and the fuel pump relay, it must be a problem with either the magnetic pick up inside the distributor or the pump itself. I'm pretty sure the wiring, etc to the pump is good as I can short across the relay pins (with the relay removed) and the pump runs. I've also disconnected all the engine sensors with the engine running and it still carries on, so if there is anything faulty there it is not stopping the engine and therefore not contributing to my problems.

A while ago, I had a similar problem on a Moto Guzzi motorcycle where I would ride it to work and at exactly the same point about 5 miles from home it would cut out and die and refuse to re-start for 10 mins or so, after which it would start and run OK. It turned out to be the T.D.C. sensor breaking down at a specific temperature during warm up. Once I found and replaced this, I had no more problems.

On the W123, the nearest component similar to this will be the magnetic pick-up coil in the distributor that sends the pulses to the ICU. This would make some sense rather than it being the fuel pump breaking down under temperature as the change in teperature under the bonnet is more extreme than near the fuel pump - although of course you can never be absolutely sure.

 

[Rose Chap]

Hi Graham,

I had a problem similar to this in my first Mercedes, a 1985 190E. It would start fine on the first turn of the key but then splutter and cough and die for a while (several minutes) before then running fine afterwards.

I had all sorts of things swapped out and replaced, but eventually conceded defeat and took it the main agent, who immediately diagnosed and replaced the temperature sensor which governed the auto-choke. This instantly fixed the problem, and only cost £65 ish.

Hope this proves useful. Best of luck getting everything sorted.

Andy

 

[fozzy2926]

More W123 230E cold running problem analysis

I think I'm getting closer to resolving this strange fault I have thats spoiling an otherwise fantastic car! From what I've managed to deduce and diagnose, power to the fuel pump is controlled by the fuel pump relay which itself takes input from power sources and also a signalling input from the ignition control unit in the form of pulses. If the engine speed is too high(i.e. more pulses than allowed), the relay logic trips in and cuts power to the fuel pump, hence no fuel and the engine cuts. Fairly simple, but effective logic to act as a rev limiter.

Although I can't find it written down anywhere, I believe the relay will only energise the contacts that supply power to the pump whilst its getting pulses from the ignition.. This makes sense as when you turn the ignition on, the pump runs for a second or two and then switches off. Of course, once the motor starts running, the relay detects the pulses and re-energises to switch the power circuit back in to allow the pump to run. This would make sense as a safety feature as you don't want the pump to continue running if the engine is not turning (particularly in an accident situation).

So given that I've now changed the igntion control unit and the fuel pump relay, it must be a problem with either the magnetic pick up inside the distributor or the pump itself. I'm pretty sure the wiring, etc to the pump is good as I can short across the relay pins (with the relay removed) and the pump runs. I've also disconnected all the engine sensors with the engine running and it still carries on, so if there is anything faulty there it is not stopping the engine and therefore not contributing to my problems.

A while ago, I had a similar problem on a Moto Guzzi motorcycle where I would ride it to work and at exactly the same point about 5 miles from home it would cut out and die and refuse to re-start for 10 mins or so, after which it would start and run OK. It turned out to be the T.D.C. sensor breaking down at a specific temperature during warm up. Once I found and replaced this, I had no more problems.

On the W123, the nearest component similar to this will be the magnetic pick-up coil in the distributor that sends the pulses to the ICU. This would make some sense rather than it being the fuel pump breaking down under temperature as the change in teperature under the bonnet is more extreme than near the fuel pump - although of course you can never be absolutely sure.

Hello Graham. I now have this problem, exactly as you described, so wondered if you ever got to the bottom of it... assuming you can remember after all these years! Thanks in advance, Steve