P38 Missfires on Petrol but runs perfectly on LPG

rewmer said:

If you've had a new o2 sensor fitted any garage worth their weight wil know that the ECU adaptions need to be reset after fitting.

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Its a main dealer! You would expect that they know what they are doing!

I will ask the questin though!

I seriously woudnt count on it. They don't get trained on LPG as no land rovers come with it fitted.

hi mate did you managed to get this working and if so what was wrong?

Hi all

Another Update

Firstly I am pulling the car out of the garage because they are stumped. I think they are quite relieved. The guys there are very good because I am not being charged. I’m picking it up on Monday afternoon. It’s in bits at the moment!

The latest is, they have been checking the wiring to the injectors.

DJ B4L - No luck fixing it yet!

Rewmer - I asked about ECU/Fuel adaptations and they said they wouldn’t do that while it was misfiring! So I don’t know what that means. May be the adaptation reset would cure the misfire! They cannot give me MAF readings because their equipment in newer and does not give print outs.

I have a print out of some real time data which I will try and transpose so it is understandable!

MAFS 28.250 Kg/h E Rough 0.000 Cyl 1 Misfire 0 Cyl 1
Eng Spd 760 RPM E Rough 2.741 Cyl 2 Misfire 1 Cyl 2
Ign ADV +12.75 DegE Rough 5.954 Cyl 3 Misfire 0 Cyl 3
Inj Time 5.328 m E Rough 0.000 Cyl 4 Missfire 153 Cyl 4
Tot MF 156 Count E Rough 1.629 Cyl 5 Missfire 0 Cyl 5
Knock +0.00 Deg E Rough 0.0000 Cyl 6 Misfire 1 Cyl 6
Fuel A 0.85 :1 E Rough 0.000 Cyl 7 Misfire 0 Cyl 7
Fuel B 0.99 :1 E Rough 3.641 Cyl 8 Misfire 0 Cyl 8

The values should be in bold. Hope that helps.

The misfire is big time on cylinder 4. I think I said 8 and 5 before, I am old and confused so that’s my excuse! Sorry! 8 apparently is misfiring too but that’s because its next to pot 4. So I am told.

Any ideas?

Is anyone in the Newport South Wales area that can help? I think I need a full on enthusiast/Guru!

Any help would be appreciated.

The bold worked really well................Not!

Newby to this forum today but owned my P38 five years is a 2000 w Thor engine on gas.
Now my car has just delevoped this very same problem. When cold it starts but is missing on 1 or 2 cylinders(numbers not specific to cylinder number). I wait about 30- 40 secs and a few revs its firing on all on petrol.
Runs fantastic on the gas.
It does stink of petrol when its missing.
I'm no v8 expert but have stripped cars and engines since I was a nipper.
I have a few oil leaks and coollant so have decided to strip the top end, put new head gaskets on maybe a new cam and big ends.
I think the trouble is a sticking injector. Mines done 120k with 70k on lpg.
As the petrol injectors are only used a short time and the tank is always low on fuel we may have a dirt problem. Have you changed your petrol fuel filter ? Mine needs doing.
Mines not a great problem as I can drive it off the drive no problems, but I am aware it needs seeing to.
I conclude its either injectors or coil as it only gives a misfire when its cold and runs great on either gas or petrol when hot.

Mines a tartarrine system and is a bit of a dogs dinner under the engine bay .

A couple of things that jump out at me are the MAF reading of 28 at idle - I would expect to see that about 20-22 kg, and the injection time of 5.3 seems to be long which is really confirming my suspicions of the LPG being out throwing the petrol trim out. I would expect to see about 3.5ms at idle on a healthy petrol idle.

GET THE ADAPTIONS RESET ! Ask them to do it before you take the car away, it takes 2 minutes on the Testbook. Ignore anything they say, they havent a clue.

Hi all

rewmer said:

A couple of things that jump out at me are the MAF reading of 28 at idle - I would expect to see that about 20-22 kg.

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rewmer - I did note the MAF reading and recalled what you had said earlier in the thread. I actually took the MAF readings with me. I explained that they were high they said it was due to the misfire. They also said that they wouldnt do the adaptations until the misfire was fixed. Like I said before I maybe if resetting the fuel adaptations is only a 2 minute job then why not try it.

I am collecting the car on Monday. It was in bits when I went to collect it yeasterday so they couldnt do a reset anyway. I will be insistant for the fuel adaptation reset when I pick it up. If nothing else it will rule it out or in!

Thanks

Hi,
Update on my issue, which sounds similar to yours......

Firstly i do not have LPG, so this maybe completely meaningless and not relevant to your issue!!!!

I took my RR to a local indy and they reported that the only thing they could find amiss was the stepper motor (unusual according to the them). It was giving a reading of 40 through the whole rev range. So i got a second hand one off fleabay for a tenner and guess what problem solved!!!!!! I had all the same symptoms as you, idled fine and reved through sweetly with no load. Apply load and i had flat spots, misfire, no power and smelling rich.

It is my understanding (though i am no expert) that the stepper works in conjunction with the MAF allowing different amounts of air in depending on other variables when the car is in use. It controls the idle settings, although if it gets stuck in the correct idle position, the car will idle with no problems, the prob' will only appear when under load as more air is required.

For the sake of a tenner and 5mins to screw it on, it could be worth a go!!

Mine was misfiring on petrol but only when restarted it would go to petrol and not gas always ran well on gas Gav suggested having it reprogrammed - my terminology not his - took it to my gas man who also has test book he fiddled about for 10 mins charged me 20 quid now it works marvelous so go with what the guys say and dont trust a stealer hope this helps

Hi all

Update!

Picked my car up. They didn’t manage to fix it! The Techy gave me a good debrief on what he had done so that was good of him. The dealer refunded all my money so I maintain a good relationship with them.

rewmer - They did an adaptions reset and it didn’t make any difference. My only gripe now is it seems to run a little rough on gas as well!

Dry Hump – Do you have any details of this stepper motor! Pt Number or anything. Where does it fit? What’s a “Local Indy” Thanks.

Here’s a recap!

Spark plugs checked and OK
HT leads replaced and OK
Coils Checked and OK
New LAMBDA sensor fitted to Bank B Checked and OK. Old one refitted.
Injectors Checked and OK
Injectors swapped with another pot and still misfires on No 4
Compression test 100%
Wiring Harness Injectors to ECU Checked for continuity and OK
Air Leaks not apparent

Food for thought!
Wiring Harness resistance check anything over 100Ohms may be an issue.

Any further ideas assistance welcome.

Any experts in the South Wales area?

Thanks to all for your help so far, please don’t stop!

Hi Bison,

Sorry i don’t know the part number, it is a round black tube about the same size as a roll of 35mm film (if anyone can remember what that is!!!!) It is on the throttle body held on with two 5mm bolts with some wires running to it. I have to admit that I am not totally sure on your engine as it is newer than mine.

Having said that…..as you have had your fuel trims re-set, it will run like a bag of nail for a 100 miles or so, as it re-learns them!!!! So I would say give it a while, try and do some different types of driving, ie motorway and round town, and give it a chance to settle down a bit.

Good Luck!!!

idle air bypass valve/idle stepper motor

Dry Hump said:

Hi Bison,

Sorry i don’t know the part number, it is a round black tube about the same size as a roll of 35mm film (if anyone can remember what that is!!!!) It is on the throttle body held on with two 5mm bolts with some wires running to it. I have to admit that I am not totally sure on your engine as it is newer than mine.

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Dry Hump - Thanks I have got the details and now know exactly what it is. I recently got an Electronic parts Catalog and I am using it more that I would Like. I have now also got an electronic and paper version of the workshop manual. Plenty of reading for me!

I still need help tho.

I will drive the car so it gets to know itself again!!

Hi all

What does everyone think about this? I found it in the V8 Forum!

quote=TFF;999654]1996 V8i. I have posted before about my misfire but I am posting the answer to my problem in the hope it may be of help. The cause of the problem was the amplifier which is situated next to the coil. Having changed it the misfire has gone and it is running very smoothly. [/quote]

Does anyone think it is relevant?

might be an idea to try changing the fuel pressure regulator,
or test system pressure
if its not holding pressure when the petrol injectors recieves a signal to fire will cause misfires/poor performance etc

maf or lambda sensors,could also be fuel pump.but id go for maf first

Man I was convinced that a reset was all that was needed. Ok, so we have to look a little deeper. I think it willbe worth while getting the fuel pressure checked. On your Thor engine this should be 3.5 bar -55psi at the fuel rail. This is a simple test that most garages should be able to do. Bad news is if it's out you have to replace the fuel pump as the regulator is integral with it on the later models.

Hi All

A couple of questions bearing in mind that the engine only misfires on pot no 4 and intermittently on No 8.

1. If a LAMBDA sensor had gone the misfires would occur on the bank of pots it relates to?
2. If the MAF sensor was duff then misfires would occur randomly at any pot?
3. If the fuel pressure was incorrect misfires would occur randomly at any pot?

Thanks for your continued interest.

Im off to get the fuel pressure checked out, I will let you know how I get on!

Hi,
I wrote a few weeks ago about my P38 mis-fire. I changed the MAF and found the car reduced to an average of 7MPG and stank of petrol! great, however, I changed both Lambda sensors which has sorted the fault. Reading the advice given earlier I was unaware of the computer needing resetting post Lambda sensor change which may explain the rough running initially on start and I mean initially, possibly around 30 seconds (must get it done).
The reason I am commenting, is my system did not mention the Lambda sensors nor MAF during the diagnostics whatsoever, so of course I assumed they were fine (assume makes an ASS out of U and ME!!). It may be worth biting the bullet and although the computer says they are fine, replace them, I got a set for £50 ea +vat.
Ref the mis fire on certain cylinders, mine came up with a fault on cylinder 3 + 4 but that meant nothing, the computer was all confused, I had the computer reset (to factory settings) and it finally started coming up with sensible readings we could work with. A chap you may like to chat to is at P38 Spares, he really helped me out, had a discussion with him on reasons (have a pen handy) and on his advice (and some from here of course) finally sorted the problem out.
mine is now returning 18 Mpg on local run about trips and I'm looking forward to higher Mpg on long trips.
Hope this is helpful, and good luck!
Andy