Poor Idle on Petrol but LPG Fine?

Hi

My discovery 3.9efi has developed a poor idle problem but only when running on petrol? It seems to almost cut out and then pick up to about 1500rpm and then die again, sometimes even cutting out completly! Any thoughts or ideas would be greatly recieved!! Its getting a bit annoying now as I have to keep my foot slightly on the accelorator to keep it running at junctions! My left foot braking is improving though!

Has it back fired recently, if so then your MAF sensor will be missing a few vital parts also check the rubber hose between the MAF and the throttle body. That's just for starters really good luck and keep us posted regards Steve

Thanks Steve, Excuse my ignorance but where abouts would I find the MAF sensor? I am not very mechanically minded and this is my first disco!!

I`m guessing that it is the box after the airfilter? What should I look out for when checking to see if all the bits are present?

Thanks

Lee

there's nothing obviously visible.. the maf doesn't have any obvious parts that will affect its operation - but the mesh screen on the way in past the air filter being gone will be a sure sign (although not a guarantee) that the backfire was strong enough to damage the maf.. the only way to know for sure is to hook up to diagnostics or temporarily exchange for a known good maf.. would offer mine but I have rough running of my own at the moment and not convinced my maf is good..

could be gummed injectors

mines been stood for a few weeks and now hates starting, very very lumpy on petrol, dies if you leave it but if you switch it straight to gas its fine? Similar problem?

can't be gummed injectors cos single point doesn't use them and multipoint uses different ones..

Right, i`m going to have a look in my lunch hour! I`ve been told that if I disconect the terminal block and drive down the road, if the symptoms are gone then it will definatly be the MAF sensor! Is it ok to drive with the MAF sensor disconected?

Having no Mass Airflow sensor will force the ECU onto an emergency operating mode, so although the MAF suppliers tell you that, in practice you may find it difficult to judge.

I tried it and couldn't really tell as the revs were not consistent enough.

Peter

Ok, Ive just disconected and driven a few miles and the car is running so much better with no stalling or eratic iddling at junctions

I`m still bemused as to why it would run fine on LPG but not Petrol

Is this because the LPG system has it`s own ECU?

By disconnecting the MAF sensor will it harm the engine? The fuel economy can`t be made much worse!!

123xlee said:

Ok, Ive just disconected and driven a few miles and the car is running so much better with no stalling or eratic iddling at junctions

I`m still bemused as to why it would run fine on LPG but not Petrol

Is this because the LPG system has it`s own ECU?

By disconnecting the MAF sensor will it harm the engine? The fuel economy can`t be made much worse!!

Click to expand...

Sounds like you have a hot wire injection system in which case you can run without the MAF but it will be running very rich. My original ecu stopped reading the MAF and I could only get the thing to run by disconnecting the MAF, I was getting about 6mpg irrespective of how I drove it, so it seemed like an excuse to keep my foot down:doh:.

I think you will find LPG uses its' own ecu but I don't know much about LPG systems. You could check your MAF with a DVM, do a search i think you'll find details, if not check out RPI Engineering website, there's loads of useful information in there. If your MAF checks out, do check the wires on the plug and make sure they are all properly connected etc. ALternatives are to borrow a known good MAF and ecu and test by substitution.

Our Zavoli Multipoint Lpg system has its own ECU, with yet another big bundle of cables running over the engine!

Peter

i have a cure!!!! fit a V8 diesel

Used to work on a nice one, 14 litre Scania V8

Peter

listerdiesel said:

Used to work on a nice one, 14 litre Scania V8

Peter

Click to expand...

Ive worked on enough of them to last a life time.

123xlee said:

Ok, Ive just disconected and driven a few miles and the car is running so much better with no stalling or eratic iddling at junctions

I`m still bemused as to why it would run fine on LPG but not Petrol

Is this because the LPG system has it`s own ECU?

By disconnecting the MAF sensor will it harm the engine? The fuel economy can`t be made much worse!!

Click to expand...

My Disco has done that since the day I picked it up 6 months ago, im glad someone has finally figured out the problem, The LPG system does have its own ECU thats why it runs so much better on gas, but also the timing is setup a little bit different for the LPG system so whatever you change its going to run a bit rough on petrol.

John