Your LPG system will simply disconnect the petrol injectors and put a load through an emulator so the ECU doesn't know they are disconnected. If you had a faulty emulator so it wasn't switching off the petrol injector, that would dual fuel one cylinder and make it misfire but only when running on LPG, not on both fuels. It doesn't alter the ignition timing though unless someone has also fitted a timing advance processor.

If number 4 sparkplug is blackened that would suggest it is firing sometimes (maybe at higher revs), although rich, but that wouldn't make it oily or make it use oil and blow blue smoke. It actually sounds as though oil is getting into number 4 but I'm not sure how. The oil return from the heads does go down between the middle two cylinders but that would only leak into the combustion chamber if you had a leak at the head gasket. But as you have fitted new heads and, presumably new gaskets, that seems pretty unlikely. You've got new valve stem seals so it isn't getting in that way and even if it was, it wouldn't be enough to cause any appreciable oil consumption and the blue smoke would only be on the overrun. A blockage in the crankcase ventilation could push oil into the inlet manifold but that would affect all cylinders to a greater or lesser degree, not just one.

The ECM controls both the sparks and fuel injectors, so a fault with that could affect just one cylinder but as it runs a wasted spark system, a lack of trigger on one channel would affect 2 cylinders, not just one, and you have a big spark at number 4 anyway. A lack of trigger to the injector could affect one cylinder but it simply would fire not make the plug black and oily.

I've got a spare, known good, Bosch ECM (no idea why as both my cars are GEMS) and a Nanocom but I'm probably miles away from you.

 

Thanks again for your reply. It's certainly something of a brain-teaser, and I've mulled over all the possible scenarios as far as I know. The non-firing cylinder and blue smoke don't immediately seem to be compatible, but the lack of combustion pressure on the upper side of the rings may be allowing oil to be drawn up passed them (bear in mind that they aren't properly bedded-in yet. Blue smoke would indicate that oil is being burnt, though.
I may have given the impression that the No.4 plug is blacker than you think, but my wife easily saw the difference when shown the four from that bank.
Second-hand Nanocoms seem to be non-existent, so thanks for the offer - I'm on the south coast near Brighton.

 

With honed bores and new rings, even ones that haven't bedded in yet, they'd still be sealing and your compression test showed decent compression too. An inlet valve not opening would create suction to draw oil up past the rings which would explain the oily plug and it not firing but if that was the case I would expect the compression to be down as there would be nothing to compress. The only reason for a valve not opening would be a very badly worn camshaft or a broken rocker (but I would have expected you to notice that when changing the heads). Although I'm not sure if that would give blue smoke or not as you would be chucking oil out of the exhaust port into the manifold rather that burning it.

 

I specifically turned the engine over by hand while the camshaft was exposed, to reassure myself that the lobes were not worn - all OK, also rockers not showing any sign of trouble.
Having scoured eBay - are the Nanocom synchroniser tools (about £165) suitable for synching the BeCM to a replacement ECM? I'm not sure what ECD stands for. Is it another term for the Engine Management Controller?

 

Yes it does and cheaper if bought direct from the Blackbox, BeCM Sync-Mate Range Rover P38 GEMS Petrol Engine - BLACKBOX SOLUTIONS LTD

 

OK, thanks - I think that's a more realistic way to go just at the moment.

Incidentally, I've had a thought (no rude comments please). The shape of the THOR inlet manifold means that I cannot reach the No.4 petrol injector with my stethoscope probe, but it's just occurred to me that I can reach the LPG ones - watch this space!

 

Major disappointment! LPG injectors all silent when switched to petrol, and all banging away regularly while idling on LPG.

That seems to have totally exhausted my supply of good ideas - not quite sure what to do now.

 

Ignoring for the moment the fact that it is the same on LPG, do you have any stored codes, particularly a P0204 code? The output from the ECU to the petrol injector will be intercepted and fed to the LPG controller and then on to the petrol injector. If there was a break between the LPG controller and the petrol injector, that would result in the petrol injector not firing. Also, if the ECU detects a fault on one injector, it is possible that it protects itself by not trying to fire that injector. However, if that was the case, the LPG injector wouldn't fire either......

Weird....

 

I have no faults or codes showing on the dash, if that's what you mean.

Sherlock Holmes said something like "after all the other suspects are eliminated, that which is left, however unlikely, must be the culprit" - I'm not quite sure where that leaves me.

To get combustion we need three things, spark, fuel, and compression - I think we've successfully shown that we have all three. There is a fourth element, though - the three do need to occur at the right time. As the spark and the injector timing are both controlled by the ECM, is it conceivable that the timing of one, or both, could be adrift on No.4? It seems unlikely that only one cylinder would be affected to me.

 

Good theory but not possible. As the ignition is a wasted spark system it will fire the coil for number 4 and also for the matching cylinder on the same coil, number 7, at the same time. So although a spark on each cylinder is only needed on every other revolution, there is a wasted one at the same rotation but wrong stroke (at the end of the exhaust stroke but immediately prior to the intake stroke). So if number 7 is firing correctly then the timing for number 4 must be correct. If the petrol injector timing was wrong it would squirt fuel into the manifold but will still be drawn into the combustion chamber as soon as the valve opens. This is precisely how the injection on the earlier Classic with the 4CU injection system worked where it 'batch fired' the fuel injectors so all injectors on one bank fired at the same time with the other bank firing after the crank had dome a 360 degree rotation. It may not be ideal but it worked (and confused the hell out of people who tried to fit multipoint LPG systems to an early Classic and set the LPG controller for sequential injection).

You've got a spark and, when on LPG at least (as you can't easily check the petrol injector but if one is working the other must be), you've got fuel. You've also got compression and it is almost as if something is blocking the fuel from getting into the combustion chamber which brings us back to a mechanical problem which would also explain the oil consumption and blue smoke. But the blue smoke was the reason for changing the heads so it isn't as if there is a lump of gasket or similar stuck in the inlet port in the head.....

Unless you are really unlucky and have a dead petrol injector in number 4 and a blocked LPG injector that purely by coincidence is also feeding cylinder number 4.

 

Have you ever had it working properly, or was it like this when you bought the car ? Most generic OBD readers should talk to the Thor ECU, so definitely try that to see if it reports any faults.

I would also check the LPG ECU wiring especially where they cut into the petrol injector loom.

 

Richard_G - It did seem a bit far-fetched to me, but I'm now entering the twilight zone, however, another thought has occurred...
pwood999 - I've had this vehicle for over 15 years, and it has been my (fairly reliable) daily runner until this particular issue, and apart from one or two unrelated issues, the engine itself has performed faultlessly. It had the Prins LPG system when I bought it. It has covered about 60k miles since it had a replacement top-hatted block and heads from Turners about 10-12 years ago - due to liner slippage.

One thing I haven't actually mentioned previously is that I was aware of the increase in oil consumption some months before the current situation developed; i.e. heavy oil consumption but no visible smoke to start with. I did not know where it was going - parking in the garage over-night showed negligible leakage stains onto a sheet of white cardboard placed underneath, no oil dripping from the exhaust, nothing suspicious.

As we appear to have eliminated all the obvious (and not so obvious) possibilities, I'm wondering if we are looking at this from the wrong side - instead of the dead cylinder causing oil loss, is it possible that excessive oil in No.4 combustion chamber is preventing ignition (except perhaps at high revs, when the smoke also appears to diminish).
I remember commenting a while back that when I first took the Plenum assembly off, I noticed oil residue on the wall of the inlet runner for No.4 only, not any of the others.

Food for thought, perhaps?

 

You could be right, if lots of oil are going into that cylinder it could be drowning the spark. You've got the crankcase ventilation system going into the upper plenum but I'm not familiar enough with the Thor intake (and don't have one here to look at it) to know if it would dribble down into number 4 or spread around the others. You could always disconnect the ventilation pipe from the manifold and plug the hole and see what happens. When it is running, do you get a lot of pressure coming out if you take the oil filler cap off?

 

I wondered about that, too - it doesn't look like it would just go to No.4 runner from the outside, but without taking it off again I can't say for sure. Unfortunately, even if that were the case, it wouldn't explain why the oil is suddenly entering the plenum in such large quantities. The ventilation system is clear, as far as I can tell. I've had the pipes off, inspected the insides of the rocker boxes, and removed the so-called oil separator (a small plastic thingy with several shelves, looks a bit like a miniature cake stand).
It did occur to me that perhaps there was a problem with the inlet valley gasket seal and that oil was being sucked directly into No.4, but wouldn't really explain why it has suddenly started. These are not now the same heads.
If anything, when I tried before, removing the oil filler cap with the engine running shows a slight vacuum - a sheet of tissue over the tube bowed in slightly.
I can see I'm slowly heading towards some dismantling again - this has now taken over my life, as there are quite a lot of things I'd also like to get done. Hey ho! Thanks for your input BTW.

 

I know the feeling. The 4.0SE I recently bought and installed an LPG system on runs absolutely perfectly on LPG and like a dog on petrol. I can just about get it to idle and at anything below 1,500 rpm it pops and bangs until you get the revs up. Weirdest part is that the MAF should show 20 kg/h +- 3 at idle and on LPG it shows about 18, so a bit low but within limits, but on petrol at a very lumpy idle, it is showing 34 kg/h rising to 84 or thereabouts at 1,500 rpm (when it should be 60 at 2,500). Quite how it can be drawing more air when misfiring is a complete mystery to me so I keep going to it and doing a bit more whenever I've got a few minutes spare.

I can't see how a leaking valley gasket could cause your problem, particularly as it would have been replaced when you changed the heads. The valley gasket is going to be below the lower inlet manifold too.

 

As I already said - I'm in the Twilight Zone!

OK - I've come to a decision, no more messing about and getting nowhere. I think we've covered every plausible reason for this set of symptoms, so drastic situations require drastic measures.
It so happens that I've got a spare, unused Turner 4.6 top-hatted short-motor in the garage - I had intended to build up an engine for my SD1, but this seems more urgent.
So, I'm going to buy another cheap P38 as a runabout, pull out the engine from my Vogue, build the top-hat block up to P38 spec and swap them over. Then I'll dismantle the current engine at my leisure and try and figure out what the hell is going on.
I've a nasty feeling that despite the apparently satisfactory compression reading we might be looking at a cracked piston or cylinder liner.
I'll keep the forum posted as, and when there is anything to report.