P38 Missfires on Petrol but runs perfectly on LPG

Hi all

Thanks Andy. I am considering changing both LAMBDA sensors just for the hell of it. Im taking it to a renowned GURU next week. So if he doesnt come up with anything I will do the LAMBDA changes.

you can check the lambdas simply with a multimeter. Connect one probe to the black wire and the other to the chassis. Its a good idea to extend the wires up into the car so you can monitor them under real driving conditions. Expect them to go from 0v to .8v every second or so on idle. under acceleration they should go to .8(rich) and down to 0v (lean) on overrun. If they are doing the above it will be pointless changing them.

Next on the list I would suggest checking the plug for the ECU. Despite its fancy enclosure I still found some severe corrosion on some of my pins that caused an annoying miss fire under very light loads. Worth checking. Finally I think a good session with an oscilloscope may be the answer, testing with a meter is OK but a proper scope will tll you exactly what is going on.

Hi All,

Really just like to be kept informed on how Bison gets on with RR running poorly on Petrol but okay on Gas. I have the exact same fault the garage that i use is currently changing the coil packs but im sceptical as on gas its fine and coil packs should also be more demanding on gas? I had a new engine fitted 4 months ago prior to this the car ran identical on both petrol and gas its only since the engine was replaced that the fault has become progressivly more noticable. The fuel trims were reset last week and this has meant i have been off the road since as the car seems to be mising on 4 to 5 cylinders. any advise would be really appreciated. Thanks & please help

Hi lotofnames

Is the engine new, second hand/or reconditioned?

lotofnames said:

The fuel trims were reset last week and this has meant i have been off the road since as the car seems to be missing on 4 to 5 cylinders. any advise would be really appreciated. Thanks & please help

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Earlier in the thread people have mentioned that when you have your fuel adaptations reset the car will need about 100miles to get reacquainted with itself again. The reset sets the car back to its defaults like when the car was new and it will run a little rough. All the info from the sensors has to be recalculated. It takes a little time and everything should be ok, apparently!
I am seeing a RR Guru next week to see if he can tell me what the problem is. I will keep everyone posted on how I get on.
I agree with you about the coil packs! If the garage fits them and it doesn’t fix the problem make sure you get your money back! You will be entitled under trading standards law. Pay by Credit Card too, your CC company is liable if the garage doesn’t pay you back!

Hi Bison,

Coil packs were fitted yesterday has been an improvement but still not running right on petrol but as you mentioned it may need running for a few more miles yet. I purchased a re-engineered short engine from a company called turner engineering. I had the dreaded slipped liner fault this company gives life time warranty as they have changed all liners for ductile iron flanges done alot of research and they seem as good as it gets. Please let me know how you get on and I will also keep you informed on what my mechanic comes up with if anything!! Cheers

Hi Bison

have you been to see RR guru yet? Im still non the wiser with mine, still in the garage unable to diagnose at present moment

Intertesting thread... Any idea how old the LPG install is?

My 11 year old DiscoII had twin LPG tanks installed when new and then a small reserve petrol tank fitted up in the Off Side rear wing. My Disco also ran slightly rough on petrol (not too bad though), turned out the reserve petrol tank was rotten (almost rotten through) and the pump, filter and sender arm were clogged with rusty gunge and crap.

Bought a new stainless tank from Arun Autogas (£380 ouch ) and had a local Landy specialist fit it. New pump and sender unit, and she runs lovely now Plus the new one has a slightly better capacity.

Be worth giving your tank a good look over.

If you have running issues on petrol but not on gas I would check that the MAF (Mass Airflow Sensor) is clean and bright. Because it is a hot wire in the airstream over time the wire can get all sooted up, reducing its sensitivity and this drives the fuel trims awry.
I had this issue recently with my 4.0 SE; it ran like a pig on petrol when it was cold but beautifully smooth on LPG. The first thing I checked was the MAF; it is the original dated 1997 and the interior of the sensor snoot was black as coal. I cleaned it and put it back on, connected it up and started the car from cold. The idle was transformed to rock steady without a missed beat, while acceleration is now crisp and willing.
It's very easy to do; unclip the sensor from the air cleaner and removed the hoseclip on the inlet trunking, unclip the connector by pressing on the wire underneath the connector and pull it off. Carry the MAF to the bench and spray brake cleaner or electrical switch cleaner into the snoot. You will see the silver wire emerge from the soot. Do NOT USE ANY OBJECT ON THE SENSOR WIRE because it you break it the MAF is fooked. Reinstal into the air trunking, reconnect and start her up. The older and blacker the MAF the better your car will run after its been cleaned - you won't believe the difference it makes!
If the cold-running problem persists then check the ECT (engine coolant temperature sensor) and the IAT (inlet air temperature sensor) because either can also affect cold fuelling.

Hi Jon14,
Thanks for the the advise i will get the petrol tank checked out just in case, the lpg system was only fitted 8 months ago.
Grimble
Also thanks will get that cleaned immediately my mechanic has phoned me today and seems to think that its one of my Lambda sensors thats at fault. He's going to change them over to see if the problem moves over to the other bank and if all is well get a new one ordered. Im hoping that this will resolve my issue. thanks

If only 8 months old the tank should be fine... it's easy to slide underneath and check it mind.

Ha d a similar prob rough on petrol and great on gas mine was rough when restarted nt when overnight cold - was suggested that i run it on petrol for a bit as i only let it run on petrol to start when rich ran it on petrol for 100 miles ish and it sorted itself out so dont know if this will help

Hi All,

lambda sensors have not worked all faults are completly clear on ecu but car is still running rich on 4 cylinders and lean on the other 4? My mechanic is struggling to understand what to do next at the moment talking about taking the top off the engine to see what is going wrong.

Bison - have you had any joy after seeing this RR guru?

This is starting to look an expensive one.

Thanks

Hi all, lotofnames

Its been a couple of weeks now since last post. Waiting to get car in to Guru's timetable. Very frustrating! I must say there was an air of confidence when I saw him!
Well, he had the car for a couple of hours and said my right hand LAMBDA sensor was reading low which we knew from previous investigations. The Guru found that my right hand Cat was glowing red! Indicating that it had collapsed inside. May be a reason for the LAMBDA sensor reading low! Anyway I looked under the car at the cat and bugger me it was red hot! And I mean red hot! Yes RED! Had to then get the cats changed well I had them cut out. They are not needed!! It took a week to get an appointment for that! Cats being cut out didn’t seem to make any difference what so ever. So I am waiting now for another appointment with the GURU. I have also pre-empted the right hand LAMBDA sensor being knackered by replacing it.

Its getting on my nerves now. I can’t stand not knowing what the fault is. At least if you know you can get it fixed. I was a Techy in the RAF. If we had a problem we would have the kit to work the fault. Hate being a civvy and not having the test equipment to solve the problem myself. Bugger!!

SandyT - Mine misfires cold or warm/hot. I can drive it on gas, switch gas off and it runs terrible. Switch gas back on and it runs much better.

John14 – Petrol tank on my P38 is plastic. Not sure that plastic can go rotten. Filter in the tank could be an issue I suppose. If fuel was restricted why would I only get a misfire on cyl 4 and occasionally on 8, just one side of the engine?

Grimble – Out of sheer desperation I am going to clean the MAF as you have described. My P38 is on 81,000 miles so I bet it is the original and I bet it is sooty. Thanks for that info.

Tell them you want a refund on the leads, and ask them what muppet said it was them in the first place.....???? and remember the computer is only as good as the man using it ..... take out the stealers mate ....Rob

Hi all

Sorry for the rant in my last post! It was late and the problem got to me.

Removed the MAF sensor to give it a clean and it was in pristine condition, no soot at all. The car has a full service history so I guess a succession of new air filters is responsible for that. I also drove with the MAF sensor connected and then without it connected. It made no difference at all.

ROB - Had the car with Landrover for about 8 days they put new leads on but were unable to fix it. It was there that long they couldn’t find the old leads to put back on. I didn’t pay a bean. They were very good about it to be honest.

Off to see the Guru this afternoon! I will let you know how that goes.

Hi all,

The Guru has an educated guess and recons it is sucking in air from the manifold gasget. Or the injector isnt working properly.

So my next job is to change the injector and replace the manifold gasget.

Stay tuned and I will report back when I have done the job and got it all remapped.

Hi All, Bison thanks for getting back to me, Im due to get mine back today its been a frustrating 6 weeks to say the least. It seems that I had a small air leak where one of the plugs screw into the head. The head has been removed, machined and an insert fitted to replace the damage thread. Im just waiting for a call today to get the status. I think that you should definatetly check your plugs to ensure that you dont have the same fault as your sysmptoms were identical to mine. Hope this helps any more info will let you know.

Hi Lotofnames

Thats intersting about the air leak! How did your mechanic ckeck for that?

Just noticed my spelling for Gasket in my previous posts! Doh!
Cheers

Hi Bison, Just your normal run of the mill compression test and found that the thread was knackered on one of the plugs, I had the car back Friday while driving home it was running awful (major powerloss/ fluttering) My mechanic has said to me that everything from his side is 100% okay and has described it as trying to diagnose a fit patient. Basically saying that he thinks the problem lies with the lpg system? I have done about 150 miles since Friday and the car is running much better probably to do with fuel trims relearning. Anyway had a visit to LPG installers for brainstorming session yesterday and found that the lpg ECU has a fault and will need replacing? Installer has said that it has the potential to make the car run lean and rich which is what my mechanic has been trying to understand hence powerloss/flutter. I should have a new ecu by next Wednesday will have it reprogrammed and check what result this give me. Installer still thinks that my problem is petrol injector related as I still run better on gas then I do on petrol? The new ecu will eliminate LPG problem. Long winded I know but not easy to solve If you find anything that contradicts the above then please keep me informed.

Hi Lotofnames, Just picked mine up and it is running sweet as a nut. I changed the inlet manifold upper and lower Gasket and swapped one injector for another on the advice of the Guru. He said there was nothing electronically wrong therefore No 4 cylinder must have a mechanical problem. With the readings from the testbook the mixture was running very lean so he concluded that the manifold Gasket or injector was a good educated guess! What a good guess that turned out to be. I already had a problem with the left hand LAMBDA sensor so I changed it. (might have mentioned that before) But after I changed the gasket there were spurious readings from the right Hand LAMBDA Sensor! I had to pick it up from the guru, change the sensor and squeeze the car back into his busy schedule.

Finally that happened today and yipee its working great.

With regards to your problem, which was actually part of my resolution! Have you checked your catalytic converters? My guru said that a misfire can melt the cats. Superheated gasses coming from your engine at the wrong time can do this. A melted cat can give the LAMBDA sensors trouble. Obviously the superheated gasses are not the same as the normal exhaust gas or the gasses are not getting through. Its easy to check, Just warm your engine up, give it some good revs and the cat will glow red and I mean red? Quite alarming really! It will also damage the LAMBDA sensor! Have a look it might be a way forward. ECU's are expensive.

Let me know how you get on.