420se w126 rough running on petrol

[velosolex50]

Hi
I have a dual fuel 1989/90 420 with poor/rough idle and intermittent roughness and power loss at higher engine speeds. (Absolutely fine on LPG) Also occasional cutting dead on both petrol and lpg. The problem has crept up gradually over a year or so. After reading many threads (thanks to all you thinkers), I changed the ovp relay, which seems to have helped a bit, i.e. the car is now at least driveable on petrol whereas before it had absolutely no power & kept staggering & stalling. My local independant tested the fuel pressure and pronounced it OK, he thought it might be the fuel distributor, but wasn't sure. (££££to change..) AS the main agent charges £85 + vat per hour (and wants at least 2 hours to look at it), I feel able to spend a bit on replacing parts on spec, but what next? I figure that if the ovp relay has made some difference, then the next step should be to change the fuel pump relay in case it has been damaged in some way. The fuel pumps are a bit noisy, but on test each individually provides enough pressure to run the engine at small throttle openings, and when it does have its moments,it goes well. I reckon the fact it goes perfectly on gas (assuming the ovp replacement has fixed the occasional dead cut) excludes the ignition or an air leak.
Thoughts and polite suggestions anyone?
BTW I can strongly recommend the gas conversion, 15mpg @ 50p/litre beats
20mpg @ 95p!
Thanks in advance
Huw

 

[television]

Hi
I have a dual fuel 1989/90 420 with poor/rough idle and intermittent roughness and power loss at higher engine speeds. (Absolutely fine on LPG) Also occasional cutting dead on both petrol and lpg. The problem has crept up gradually over a year or so. After reading many threads (thanks to all you thinkers), I changed the ovp relay, which seems to have helped a bit, i.e. the car is now at least driveable on petrol whereas before it had absolutely no power & kept staggering & stalling. My local independant tested the fuel pressure and pronounced it OK, he thought it might be the fuel distributor, but wasn't sure. (££££to change..) AS the main agent charges £85 + vat per hour (and wants at least 2 hours to look at it), I feel able to spend a bit on replacing parts on spec, but what next? I figure that if the ovp relay has made some difference, then the next step should be to change the fuel pump relay in case it has been damaged in some way. The fuel pumps are a bit noisy, but on test each individually provides enough pressure to run the engine at small throttle openings, and when it does have its moments,it goes well. I reckon the fact it goes perfectly on gas (assuming the ovp replacement has fixed the occasional dead cut) excludes the ignition or an air leak.
Thoughts and polite suggestions anyone?
BTW I can strongly recommend the gas conversion, 15mpg @ 50p/litre beats
20mpg @ 95p!

Thanks in advance
Huw

If you know someone that can solder electronic component like a tv eng; in 99 cases they only ever need to be resoldered. this is a dual relay as is also your engine rev limiter, If yours was made in Ireland it will be well made, If made in Austria the soldering will be very bad. give it a go, nothing to lose, but do every joint.

Malcolm

 

[velosolex50]

Thanks Malcolm, I will have a look. Interestingly, today it ran perfectly for a while before reverting to misbehaviour, which does sounds like a bad connection somewhere rather than a bad pump.
Huw.

 

[velosolex50]

Hmm, I have had a look inside the fuel pump relay, all the solder joints look nice and shiny. No obviously blown components.I am reluctant to resolder it myself just at the moment, as I will probably cook those ics. Cant get to a professional at the moment. Would it be safe for the rest of the car electronics just to remove the relay and put a jumper lead across the relevant terminals? (for test purposes only)
Huw

 

[velosolex50]

Hmm. Bypassed fuel pump relay-no change. Re soldered all connections in relay, again no change. I wonder if a fault in the idle control could cause the symptoms? Thank goodness for the gas conversion.

 

[shayes]

I've today had a replacement Return Valve put into the Petrol pump and can't tell you if it has solved my problem of poor start up when hot. Will let you know. Parts £20.81 from MB Dealership - labour £12.00 from guy we will lose to Canada I regret to say - p***d off with GB.

I noticed how a similar Car to my own went on ebay for for massive amounts more because it had been converted to LPG, so looks like a good investment if the car has many more miles to go and no rust problems and you use the car for say 10k miles per year. With 104k on the clock could be worthwhile, but does this not cost over a £1K to convert?