wont run on petrol but will on gas

hi all my disco is the 3.9 converted to lpg now i havent had much to do with lpg but to be honest i dont think that is the problem it just runs like crap as soon as its switched to petrol it starts to run lumpy and stall any help would be great

well if it runs well on gas it must be something to do with the petrol delivery system right? Is it bad fuel, bad fuel pump, air getting into the fuel lines, blocked fuel filter, clogged injectors, timing?

I had the opposite, it run great on petrol but as soon as I revved it on LPG it would die, it turned out to be a split pipe in the LPG system just after the MAF sensor.

A bit more info would be helpful.

Hi mate just changed the plugs and leads which has made a slight difference the engine tune it is still missing but will rev up I you blip the throttle I have noticed the vacuum unit on the dizzy is not working if that could cause problems

I wouldnt have thought it would have been the plugs and leads if they were playing up you would have noticed when running on LPG (the electrics have to be tip top for LPG not so much for petrol).

Have you checked the fuel (petrol) delivery system?

I have checked I am getting fuel to the rail but not so sure of how to check injector s plus when I pulled the old leads off I took one complete side of while running and it didn't change tune

If theres an air flow meter its worth having a look at that aswell.

If i unplugged the air flow it would cut out but that. Will also affect LPG running as well. Wouldn't it because it seemed to run fine on gas maybe slightly lumpy

a little bit of an update i have taken off the plenum this morning to find that one of the trumpet things was lying on its side so i put this back in and cleaned out the plenum i also found quite a big air leak around the stepper motor where it bolts to the plenum i fixed this and re fit the plenum with a bit of instant gasket to try and eleminate any more air leaks i started it up and it does sound a bit better the vacum advance is now working on my dizzy as leaky stepper made this fail but i still have the bloody misfire and i am at my wits end i need this sorted before the wife makes me get shot of her

craig fea said:

I have checked I am getting fuel to the rail but not so sure of how to check injector s plus when I pulled the old leads off I took one complete side of while running and it didn't change tune

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so on petrol your only running on 4 cylinders? down one side? but it runs fine and you can drive it when running on gas? it sounds to me that one side of injectors could be playing up maybe? I wouldnt know how to check this though sorry. What LPG system have you got single point or multi point? I have the inferior singlepoint (with my system when running on LPG the system simply shuts off the petrol injectors, adjusts the A&R + spark temp slightly and feeds the LPG/air mixture into the plenum chamber, its very basic)

Also check both the LPG system and everywhere else for air leaks that could be getting in, I had a leak from my LPG stepper motor that resulting in mine running like a bag of poo for a bit, its now been replaced with an old piece of pipe (making her a little thirstier but its nearly impossible for me to source the part).

It may be worth going on an LPG forum just to see if anyone els has been experiencing the same problem.

i take it mine is single point as i just have one line tapped into the opening of the plenum i am not to sure if my injector s are working at all if that is possible iunplugged them all individualy and it didnt seem to make a difference to the tick over at all but not sure if this is correct or not i am a plumber not a mechanic

craig fea said:

i am not to sure if my injector s are working at all if that is possible iunplugged them all individualy and it didnt seem to make a difference to the tick over at all but not sure if this is correct or not i am a plumber not a mechanic

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The fuel injectors are not sequential on the 3.9 AFAIK. The 14 CUX ecu bank fires the injectors on a switched earth taking the trigger from the ignition coil.

If you suspect the problem is a fuel injection misfire then you need to check that all the earth points are clean and making good contact, next you need to work through the sensors on the injection system checking values with a multi-meter. You never know the ecu might be having an issue, it could be a lamda sensor, the throttle potentiometer, AFM or coolant thermistor. Search the V8 section.

Could it have anything to do with alternator this may be a daft question but I have just put a brand new battery on for the second time now one day it starts fine and the next there's nothing but hazzards flashing but it won't kick the starter the alternator is kicking out 14.4 so I am once again stumped