X5 misfiring on petrol but runs fine on LPG

[littleshed]

Hi I have a problem with my 2002 X5 3.0i sport, The car has an LPG system fitted and usually runs fine on LPG but when I switch it over to run on  petrol the engine misfires, it feels as though its only running on five cylinders. The usual sequence of events is that when starting from cold it starts up easily on petrol and runs smoothly and I can pull away and drive no problem, but after half a mile or so the engine will start to misfire. if I stop and turn the ignition off and then restart it it will run and drive properly on petrol for a while and then start to misfire again so stop and restart.   At this point the engine is usually warm enough to automatically switch over to LPG and then the car will run perfectly. if however I manually switch it back over to petrol it will begin to misfire again and I have to stop and restart the car and then select LPG...... Oh and if the car is misfiring when it automatically switches over from petrol to gas then it will continue misfiring on gas until I stop and restart it.    However once its warmed up and running on gas it runs really well for the rest of the day starting and running on gas  as it should... Any ideas?  I've already tried a few things, replaced the maf sensor and air intake hoses and disa valve, but now I'm stumped any body have any ideas? 

[Greydog]

Morning Peter and Welcome

Have you tried checking the plugs and coil packs also check the loom to the coil packs in case it is worn somewhere and shorting. A code reader will indicate which cylinder is misfiring. 

Simple things first do you know when the plugs were last changed? You should have hotter plugs for LPG I would get a set and change the plugs , if the misfire persists I would suspect a coil pack swap the coil from the cylinder with the misfire for one of the others if the misfire moves then you know which is the culprit

Hope this helps

Dave