Misfire on LPG

[Andrewpridmore]

Hi All

Had an ongoing LPG issue for a few months now. Bongo runs fine on petrol but when switching over to LPG it misfires quite badly, second cylinder only. This happened suddenly one day, was fine in the morning into work then awful that evening.

So long as the revs and speed are high enough it is just down on power. Until after about 5 mins driving when the problem vanishes.

It is dependant on how long the Lpg system has been running, not the temperature. I can drive it all day on petrol but when I switch to Lpg I get 5 minutes of misfiring. The misfiring often comes back once the speed and revs are low for too long - in traffic jams etc.

I have changed ht leads, spark plugs, rotor and distributor cap multiple times with known good parts - the parts all fire other cylinders so it doesn't appear to be the ignition side of things.

I have also inspected and even completely replaced the injectors, the Lpg filter and checked all pipework, no issues. When I plug the cable from cylinder 2 injector into any other injector it fires, but none of the cables from the other 3 cylinders will fire the cylinder 2 injector so the Lpg system up to the the injector seems solid. Today I swapped injector 1 & 2 and the issue stayed on cylinder 2 so I can rule out the injector totally.

What is left? Is there anything on the cylinder itself or elsewhere in the system that could cause this on one cylinder only? My Lpg guy is suggesting valve clearances but I thought they were hydraulic on the 2.0 litre petrol (its a 2003 reg new shape)?

We have also replaced the O2 sensor thinking that could be at fault, checked belts, air filter, etc.

I can consistently get it on the motorway & up to 50 mpg on petrol, then switch over manually & let it clear after 5 minutes (all 4 cylinders are then running) & live with it but I'd rather find the cause of the issue in case it is a symptom of something worse.

[g8dhe]

When was the LPG system last serviced ?

There are adjustments in software that probably need tweaking or maybe a valve on the way out, but unless you have the expertise and diagnostics it can be a very expensive business changing parts on the possibility that they may be on there way out!

[Andrewpridmore]

Last serviced in February and he's been through the whole system over the last few months, including the software which sees the misfire but when testing the system says that all injectors are working correctly. He reinstalled previous known good firmware which did nothing.

No obvious engine sort codes other than cylinder 2 misfire. Fuel system goes open loop & fuels trims go awol Once it switches to Lpg (why the O2 sensor got changed) though assume that is down to the three firing injectors trying to cover the load of the missing cylinder.

Next stop is likely to be a look at the valves...

[Katka]

Our lpg system has it's 'brains' in the engine bay so is subject to heating with the engine, and there is a set of push-fit connectors that connects the CPU to the injection system etc. Might it be something here on the cyl2 line that is loose when cooler but tightens up if hot?

[Andrewpridmore]

I did originally think it could be heat related. I can drive for half an hour on petrol then switch the Lpg on & it still has 5-10 mins of poor running. After the engine has settled on Lpg, if I switch back to petrol or stop for a few mins the Lpg has the same fault when it restarts. Sometimes if in a traffic jam the problem will reappear due to idling. It's as if so long as the revs and speed are kept high it is enough to overcome whatever is causing the problem. Once the revs and speed drop the problem returns after a while. Switch to petrol & there is no problem.
Over the Christmas break I had 2 occasions where it switched over normally without any issues so the problem is intermittent as well as on one cylinder only.

[Katka]

We had a problem where the engine sometimes dropped when it came to a halt (there's a thread about it which I'll update when fully fixed - we're waiting parts from Japan) it turned out to be a combination of things... the idle speed was too low and the air intake pipes to the Idle control valve had been butchered by a previous owner and a bodge fix made. This fault had become apparent over time and was intermittent, however I can't see why just one cylinder would be affected. Maybe two separate faults?

[Andrewpridmore]

Problem fixed at last, it was the Lpg computer - replacement for £60 from Lpg shop & it was all sorted...