Halfway to Caernarfon and the car has started playing up.

Feels like a loss of power, slow to accelerate and there's a vibration.

The vibration seems to lessen when the gearbox kicks down and doesn't seem noticeable when cruising. Light acceleration gives a noticeable vibration.

Revving the engine in neutral gives a slight vibration too - pointing towards engine rather than gearbox or torque converter?

MPG doesn't seem affected, hovering around 18 on the motorway on LPG. There's no weird smells or smoke etc.

Could someone give me a quick crash course on where to look in the Nanocom to help pinpoint the issue if there's no stored codes when I plug it in?

Firing on 7 cylinders? I have got a Faultmate so can't help with the Nanocom, sorry. It is able to do all models though.
I live near Chester if you get stuck. You can help me with my CV joints. Ha Ha.

I'm not too far from Chester, stopped at the Knutsford services.

If I was on my own I'd stop by and give you a hand but I've got my girlfriend and our little boy with us so I won't get away with that.

Luckily she's in her own car (she has to go home earlier than me and the little man do) so she doesn't know it's happened yet. She'd panic and flap if I told her half way there!

No Problem. The suggestion to help with my CV joints was done tongue in cheek.

Definitely sounds like you've lost a cylinder or two. What's the idle like? It won't idle any slower as the ECU will wind the IAV open to keep the revs where they should be but it's sound uneven and will be particularly noticeable when you rev it from idle.

When I left the services I ran it on petrol only and it seemed fine. Either it was so slight that it wasn't noticeable or its not there when running on petrol.

Did the rest of the journey on petrol.

I'll message Simon since I usually forget to do that with LPG questions!

That can be a sign of a coil pack starting to fail or another ignition issue rather than LPG directly. Easiest bet since its a Thor is look for misfire codes and see what cylinders are coming up with anything (might take some more running like that to get it to flag a misfire).
Single cylinder is likely a plug or lead, pairs more likely coil pack or the wiring leading to them. But at least if you know which cylinders your looking at thats a start before you spend any money on it.

Forgot to mention, there were missfires codes for cylinders 1, 3, 5 and 7. I assumed that coil was dying from that.

The cylinders aren't paired that way - thats a bank, only thing common to a bank that comes to mind is the lambda sensor. Its possible either the sensor has died, or some sort of wiring issue is present, First thing I'd suggest there is to keep it on petrol, clear the faults, see what the lamdba sensor readout is showing on petrol (should be an alternating wave your seeing) and go from there. If it looks ok on petrol at that, try the same thing on gas and see what you get then, if the lambda sensor output looks wrong under gas only its probabbly some sort of gas problem (which would possibly only effect one bank as they generally are treated independently.)

Ah, right. I couldn't remember how the coils were set up. So each coil does 2 cylinders of each bank then?

I know the lambda sensor for that bank isn't reading properly from when I did my head gaskets last October - just not got round to replacing it yet (cold, busy in general over the Christmas/New Year etc) but it's been running ok since then. Had an issue with my idle speed for a while but that seems to have cleared up.

Looks like there's a lambda sensor in my immediate future then! At least I know it'll come out easy as I had a new exhaust not so long ago.

You've got two coil packs, each consisting of a pair of double ended coils. The two coils in pack 1 deal with cylinders 2 & 3 and 5 & 8, while the two in pack 2 fire cylinders 1 & 6 and 4 & 7. So if you had a coil going down it is unlikely both on one pack so it would just affect two cylinders that are fired from the one coil. Didn't you have a problem when you first did the head gaskets with an iffy connection to one of the coils?

When I did the head gaskets I had a bad missfire that set fire to the cat. The missfire codes were for cylinders 1, 3, 5 and 7 IIRC.

I assumed it was a loose connection because I didn't change anything and the issue went away. I assumed that my checks to the wiring connectors had secured a loose one. I think that's where I got the idea that those cylinders were fed by the same coil.

I've also assumed that the heat from the cat has damaged the lambda sensor or the wiring on that side.

Another thing I've not had time to look in to properly is the stored voltage value for the TPS. It seems to fluctuate between 14.something and 15.something which I'm lead to believe is wrong.

I did take the sensor off and measure the resistance through the range but I don't have the figures to hand. I did notice that the Nanocom never sees a throttle open position above something like 70% even with the pedal pressed to the floor.

I'd check the lambda output - given the heat, its possible its melted the wiring somewhere underneath and may be that the signal wire is shorted out somewhere (seeing the output should give an idea if the lambda has gone bad, you can always swap the sensors over if you wanted to confirm before spending money on a new one, provided the old one will come out of course from the other side).

Its also possible its a blocked cat is the other thought that now comes to mind, as that would only effect one bank. But if its OK on petrol then you should have ruled that out and it seems you have.

TPS shouldn't affect only one bank either.

You can trace the ht leads and should find they go two from each coil pack to each bank, the other two going from the one further away. It might be an idea to check thats what is actually the case just incase it was put back wrong when it was apart. Though I'd expect it to run badly at best like that so wouldn't expect it to be wrong. There are some diagrams about of the connections if you can't find one then say!

I can't remember what the lambda reading was off the topmost my head but I know it was completely static. The other one was bouncing up and down.

I considered blocked cat when it didn't want to accelerate but as you said it would be the same on petrol.

Yeah, I'm confident the coils are wired properly as I've been driving it since October :P. The issue only occurred after about an hour of driving today. I think the diagrams are in RAVE, I'm sure I used them when doing the head gasket job.

First thing to do is find what the reading was, disconnect the sensor and see if it changes, inspect the wiring and if your happy it looks ok you can then either replace the sensor, or swap from the other bank.

If its showing a voltage when disconnected I'd suspect you have some sort of wiring issue which might have killed the sensor - I've had that happen on an older lpg ecu where it was putting out voltage on the lambda sensor wire (if Simon installed your system thats probably not been connected as it doesn't do anything really useful and can cause problems if it goes wrong as well).

If the lambda reading was static you were probably seeing a bias voltage from the ECU rather than an output from the sensor. That will only be there when there's no output from the sensor so the system goes into fail safe open loop. Quite why it has been running fine up until now is a different matter though. If it was dead before and running fine, why has it started to run rough?

Yeah, that's why I didn't immediately jump to the conclusion that it was the lambda sensor.

I did have quick Google last night and found a thread on Landyzone with a guy reporting similar symptoms and he said it was a worn camshaft. Fitting a new camshaft cured the issues for him.

He didn't mention a difference on petrol Vs LPG.

https://www.landyzone.co.uk/land-rover/p38-v8-vibration-when-accelerating.308979/

Doubtful that a camshaft would wear out in such a short time and it wouldn't run on all cylinders on petrol either, it would be the same on both. There would need to be something seriously wrong for the cam to wear out on 4 cylinders too, especially all 4 on one bank.

If you've been running it a lot on LPG, then the map will probably be slightly different to petrol - even if it has been tuned, so normally the petrol ECU will still make changes to the fuel trims as it's reading changes in the lambda sensors.

In my head (and from experience on my sequential system when trying to get it set up properly - and having a Motronic P38) if you have a duff/intermittent lambda sensor then it's possible adjusting the fuel trims inaccurately (if at all) and that could be causing the issue to that one bank. Also the LPG setup has a 'bank 2' offset trim, to allow for one bank to have the map adjusted slightly to the other if they aren't giving the same LPG injection timings (I'm guessing to do with if one side LPG pipes are different length for example).

One thing I noticed a LOT on mine whilst trying to get the LPG side sorted was that if the LPG map was out too far, then the petrol ECU would keep trimming the fuel trims, and then it would start to run like shite with random misfires when it hit the limit of the fuel trim, and it couldn't trim any more.

If you're getting issues on 1,3,5, 7 - a whole bank of cylinders and that Lambda isn't changing, then chances are that's your issue - it will run reasonably in open loop on Petrol as the ECU will have a backup map for that - but over time running on LPG, if it's not trimming as it should, then chances are that bank is running too rich/lean on gas and causing random misfires.

I'd sort the Lambda sensor first, reset the adaptive values, and then see how it goes.

The other thing I've found to cause the Motronic systems grief is a non genuine MAF sensor - aftermarket ones don't seem to be made/set up to the same tolerances as Bosch ones, and this again causes fuel trims to get pulled out of whack - mine could reset the adaptives, and within a few hundred miles, idle like a sack of spanners. Putting a new genuine MAF in sorted it straight out.

Genuine MAF and Genuine Bosch lambda sensors on the motronic P38s are the 2 things that seem to keep them running sweetly in my experiences!

When my lambda went pop, mine adjusted the mixture so much that from hard acceleration to nothing it would make one bank so rich it would flood them and try to stall.
When running on a level throttle it would cough and splutter badly, new lambdas sorted it out and my mpg increased..