P38 Missfires on Petrol but runs perfectly on LPG

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Bison

Bison

Active Member
Posts
119
Location
Newport, South Wales
Hi all

I have a P38 (X) 4.6L V8.

Normally I start the engine and within 2 minutes the LPG kicks in and everything is fine.

Recently I start the engine, at idle there doesn’t appear to be a problem but if I drive the car the engine misfires. It doesn’t like any load applied to it at all. Then when the LPG kicks in, about 2 minutes, it works perfectly and as normal.

I thought maybe the fuel filter was blocked. I tried to find a fuel filter but with no success. Does anyone know where it is located? Is there a fuel filter? Some forums are contradictory some say there is on one and some say there is not!!

Does anyone have any idea as to what the problem might be?

Thanks in advance of your input.

Bison
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I am having similar problems with the petrol, i had the ECU remapped last weekend and it sorted alot of the early mis-firing out, just relocated it to 3250rpm last weekend that is, this weekend it is upto 3500rpm, the fault codes this morning were lean and rich running and misfires on all cylinders. I am led toward a MAF sensor problem. A chap from P38 Spares can provide a new sensor inner for around £80, and a full MAF from £110. It may be a similar issue with yours as the air mixture needs constant adjustment, this is computed from inputs from the MAF (for info the MAF uses a hot wire to detect the amount of air travelling passed (cooling) it and that is computed by how much power is used to heat the wire to the correct temp I am led to believe) and the Lambda sensors to adjust fuel inputs, I cannot speak for the gas running so well as I don't have it fitted, I just carry a larger wallet! However, I do believe when it runs on Gas the RR ECU is bypassed, and depending on how updated your gas system is will depend on whether it has its own computer or it will be a basic version using diaphragms etc.
If you have a friend like mine with a test book computer then reset the ECU to factory settings and see what readings you get after a hundred or so miles on petrol, if they are lean and rich faults then look at the MAF.
Hope this helps mate. let us know how you get on.
 
you need to know what the maf and lambdas are doing,

the misfires are usually the other way round as lpg is very picky about spark and mixture.

get it plugged in and read live data. you lpg kit will probably only use readings from one lambda and ignore the other on and the maf will be ignored totally.

if one lambda and/or the maf has gone then it could be that.

check also leads and plugs but again it would show up on lpg more than on petrol
 
I expect your LPG is out of tune. The engine ecu has adapted to this therfore it runs fine on LPG. When you start on petrol the ecu goes open loop, ie, not Reading the o2 sensors, and bases the fueling on the long term adaptions. As these are out due to the LPG being out of tune the mixture will be out until the ecu goes closed loop when the o2 sensors warm up and the ecu trims the fueling. The bottom line is you need your LPG tuning and your adaptions re-set.
 
Last edited:
I pesume the (X) means X reg,so its a Bosch injected motor.Very likely its a failing Maf giving a low reading,the LPG ecu may not be looking at the Maf reading so will carry on regardless.But it does depend on the type of LPG system,and how its set up.Either way a quick session on Testbook or similar should be the first port of call,live data is the way to go as DTC's may be misleading.Expect around 22-25 KG/Hr from the MAF at idle,85-100 at 3000rpm.
 
WOW! Thanks guys. I think I will get a session on the test book as it seems that live data is the key.

Many thanks for your input. I will let you know what I find out.

Bison
 
Please do. A lot of this is guess work, some guesses are more educated than others and getting feedback help everyone understand these little darlings a little better.
 
Just as a add on, here are the MAF readings from a known good one, not saying your man doesn't know what he's doing but the more info you have the better.
Idle 680rpm - 22Kg
1200rpm - 32.5Kg
2000rpm - 55Kg
3000rpm - 92Kg.

I would like to take the credit for readings but that must go to Eight NV
 
i got a similar problem mate running fine on gas cant drive it on petrol at all some times barely starts on petrol

let me know how you get on with yours iv tried chaging several things
 
Hi all

Here is my really useful (not) update so far.
I took my Range Rover to the experts! My local Land Rover dealer. Explained the problem and left my car with them so that they could put it on their test book computer. Got a phone call it’s your HT leads. OK! I had checked both the plugs and HT leads before and they appeared to be working fine. But I guess they would have checked them with something a little more respectable than my £12.50 probe from Halfords. Picked the car up and everything seemed to be fine but it usually is when the engine is warm.
Next morning decided to spend the morning with my wife, at Tesco’s! Lovely! Started the car and bugger me, guess what? It was misfiring as it had been in the first place. So it was not the HT leads and I paid £270 for the privilege!
I have taken the car back to them and have just had a call telling me it’s my Math airflow sensor! I wonder why the Math Airflow Sensor wasn’t spotted before?
I shall let you know how I get on.
Question? Is the 1 or 2 Math Air Flow Sensor/s on a P38 4.6L Range Rover? I ask the question because when I was hooked up to the LPG software it indicated that the problem was only on the one side of the engine!
 
I have a similar problem. took it to my local indy and they have reported that it is the stepper motor, it is giving a reading of 40 all the time, when it should go up as the revs increase. The symptoms are: it will idle and rev with no problems. The problems start when it goes under load, then it back fires…pops…wont rev over 2k and generally plays up.

So I am looking for a known good stepper motor (if anyone can help). I have also just ordered an Elm327 from flea bay as £50 a plug in just to tell me that seems a bit steep! When I can do it my-self, hopefully!!!!
 
You only have 1 MAF, ask to see the Reading from your MAF and compare them to the readings I posted earlier. If they are similar don't let them change it, just ask them to reset the fuel adaptions, a 2 minute job on the testbook.
 
Sorry all.

Its a MAF Sensor not a Math sensor! Doh!

Having now researched this I have found that there is only one MAF sensor!

Secondly, if the MAF sensor was duff wouldnt there be a problem with the LPG aswell?
 
Bison said:
Sorry all.

Its a MAF Sensor not a Math sensor! Doh!

Having now researched this I have found that there is only one MAF sensor!

Secondly, if the MAF sensor was duff wouldnt there be a problem with the LPG aswell?
Click to expand...

That depends on the LPG system. If it's a later system then I would say yes. My money is still on the fuel adaptions and an out of tune LPG system.
 
Thanks rewmer

I had the LPG fitted about 2 and a half years ago so im not sure if that is a new system or not. However, before I took it to Landrover I had the LPG checked out and it was perfectly LPG tuned. One thing that cropped up on the computer was the problem was happening only on one side of the engine. The LPG guy suggested it might be a LAMBDA sensor and there are 2 of those one each side! Someone mentioned that earlier in this thread.

I shal be having a chat with Landrover tomorrow and get the MAF stats before they change anything. Just like you suggested.
 
If your LPG man is monitoring the lambda sensors to set the tune then everything will appear fine as the petrol ecu is trimming the the timings based on the lambda readings.
 
Hi All

Another update.

I decided to go in and talk face to face with the really polite experts because I didn’t want to pay for HT leads that didn’t cure my problem. I am better face to face than on the phone! I'm not pretty either, that’s got to be an advantage hasn’t it? Incidentally it takes to busses to get there!

I spoke with the guys behind the desk only to find out that it wasn’t a MAF sensor that they had said it was an oxygen sensor! (LAMBDA) (Wasted loads of time researching MAF sensors)

There were none in stock so they would have to order one. It would be there next morning. I called lunchtime the next day. Sensor hadn’t arrived. I called at 4 o’clock and it had just arrived.

Today I called and the sensor had been fitted and it didn’t cure the problem! Pot 8 misfires all the time and pot 5 misfires intermittently. Apparently they are on different coils. Coils checked and are OK.

They are now looking into the wiring to the injectors and a possible air leak.
 
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. Ask the question, if they don't know how - pull the car, they are wasting your time and money.
 
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