Rough running on petrol but not on LPG

morning all i have a very odd fault p38 4.6 runs on petrol smooth when cold switches to lpg runs smooth all the time on lpg switch off let it cool down starts on petrol and runs rough until i switch over to gas again any ideas as I am not sure where to start gas system has its own injectors

I would suggest your gas system is a little out of tune. The petrol ecu has trimmed the fueling to allow for this so when you start on petrol from cold the ecu reads the long term trim values and bases it's fueling on them while it is closed loop. You need to get your adaptions reset, run it on petrol for a while and then tune the LPG system

you can obtain the software and cable to tune the lpg system, there are many types but the theory is the same.

with my older gas systems i have noticed that changes in temperatue. have meant me having to tweak the tickover slightly, one theory is that lpg suppliers change the mix for winter.

the newer system like yours seems to cope a little better, they are more advanced.

as rewmer said, the petrol ecu has adapted to the out of tune lpg system, i often check mine as i have the cable and software for it and a cable and software for the engine ecu, i'm not able to reset the adaptions tho(or at least i don't think clearing faults does it)

my lpg system runs off the readings from the lambda sensors, on the software the reading flicks from rich to lean constantly, this is correct and the best way to set up the v8 is to have it flick to lean then to rich then dwell slightly on rich then flick to lean and so on. this means it runs slighly rich but cooler than a lean setup.

the dwelling on rich is hardly noticeable but if you watch it for a while you can see the pattern.

i suggest a good service on the lpg side aswell as a retune and adaptions reset (as said mine have never been reset and the petrol runs at about 12% i think.)

when one of my lambdas went down, the petrol ran at 24%, probably a default setting if theres no lambda reading!

so anything in the region of 24% on petrol could suggest bad lambda together with an inactive reading from the lambda!

setting the gas up is quite easy, be meticulous and you can tune them very well and there is very little difference in performance from petrol to lpg!!

let us know what system it is and what lpg ecu you have. also search google for lpg forum, theres loads of info on there.

Thanks guys will book it in to the bloke who fitted it to be honest the performance on gas has always been in my opinion better except at high speeds over 100 but even then switching between them there is no difference the rough running only lasts for about 10 mins if i leave it on petrol and then is smooth as you like the system is Prins and i have not touched it since installation 3 yrs and 50 k miles ago so your advice on having it serviced and reset is a fine idea