Help running rough on petrol but fine on LPG

ianb4310 · Dec 21, 2011

Hi all my 2001 4.0 hse p38 LPG has got me stumped it will run fine on LPG but not on petrol it's rough and lumpy and hard to start when cold,but when the fuel adaptive settings are reset it will run lovely on petrol until it is switched to gas then when it comes to starting again on petrol it's rough and lumpy again ??
A previous owner removed the cats it's showing code p1034 on fault reader which is lambda sensor,I've changed one lambda but other one was stuck I think I'll need some heat to remove but it's still playing up with one changed could the lambda be responsible for the rough running on petrol but fine on gas ???
Any ideas please

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Saint.V8 · Dec 21, 2011

ianb4310 said:
Hi all my 2001 4.0 hse p38 LPG has got me stumped it will run fine on LPG but not on petrol it's rough and lumpy and hard to start when cold,but when the fuel adaptive settings are reset it will run lovely on petrol until it is switched to gas then when it comes to starting again on petrol it's rough and lumpy again ??
A previous owner removed the cats it's showing code p1034 on fault reader which is lambda sensor,I've changed one lambda but other one was stuck I think I'll need some heat to remove but it's still playing up with one changed could the lambda be responsible for the rough running on petrol but fine on gas ???
Any ideas please

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I am here: Google Maps

It is possible - depending on your LPG system if it reads the lambda readings to adjust fueling...

ianb4310 · Dec 21, 2011

It's a new BRC single point system will the gas system alter petrol fuel settings I thought the gas ecu would just look after the gas ?

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Saint.V8 · Dec 21, 2011

WHat I meant was ....it runs rough on petrol as it uses the lambda readings....

If the LPG system uses the same Lambda readings but runs fine....it is not the Lambda sensor....if it doesn't use the Lambda readings and works fine, it could be the Lambda at fault causing poor running on petrol....

I don't think Single Point systems use Lambda readings...(my much older SP LPG system on my '93 Classic didn't)

You are right the Gas ECU will not interact with the Petrol ECU or Vice-Versa.....

I hope that makes sense.....

j the range · Dec 21, 2011

it could allso be your maf senser it dose not use maf on gas just a thought..

ianb4310 · Dec 21, 2011

Would a faulty maf sensor show as a fault code on reader only one I'm getting is lambda sensor on bank 1 & 2?
How do I check for a faulty maf is it just buy unplugging it and seeing if the running alters ?

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ianb4310 · Dec 21, 2011

I think I get it the single point LPG doesn't use lambda readings that's why its fine on LPG
But petrol uses lambda readings hence the rough running if lambdas shot? I've changed 1 of the lambdas but other was seized solid I think I'll need heat to remove !
So with one new lambda and one old lambda will this confuse the fuel settings and still cause rough running ?
So I really need to change the other lambda ASAP

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j the range · Dec 21, 2011

dont unplug your maf with cold engine or you wont get it started again, the way i did mine was got engine warm freind to keep revs up unpluged the maf the miss went away then went and bought a new onw no problems since..

ianb4310 · Dec 21, 2011

Thanks for the maf advice I'll give it a try

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marklwhittaker · Dec 22, 2011

blocked injector mayb?
if your not using petrol much they gum up a little

zen · Dec 22, 2011

what do you mean by rough?

davef2901 · Dec 22, 2011

I know it's daft, but how often do you use petrol? My 4.6 on single point (186000 up ) is fab on gas but bad tempered on petrol for the first few miles then ok, I use super unleaded by the way, cos LPG is higher octane than anything thru the pumps. Hope this is of some help. MERRY CHRISTMAS EVERYONE. By the way, out shopping today with my lovely wife, dumb girl in 09 5 series BMW ran into the back of us. Have you any idea how much damage a stopped p38 can do to a BMW? She's scatched the paint on the back bumper!!

mozz smith · Dec 22, 2011

hi , i had same problem for 3 weeks changed maf and it was perfect ,,, kept old maf stripped it down and cleaned all the sensor strips with nail polish remover then used bit of petrol to wipe down reassembled refitted and worked perfect so now i have a spare just in case .. methinks it was all clogged up with carbon n???? it does work so give it a try .cheers mozz

elsie · Dec 23, 2011

plug's and lead's , always a good starting point . no pun intended

ianb4310 · Dec 23, 2011

Thanks everyone I'm going to try and clean the maf on boxing day I'll let you know if it works

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ianb4310 · Dec 23, 2011

It runs lumpy a bit like its missing

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ianb4310 · Dec 27, 2011

2 brand new lambdas fitted now running beautifully thanks for all the replies guys

Help running rough on petrol but fine on LPG

ianb4310

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Saint.V8

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j the range

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zen

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