P38A Misfires and Diagnostics?

Hi All,

Apologies in advance for the waffly post...

My 4.6 HSE developed a heavy misfire quite a long time ago following some work to recommission it following an extended period of non use.

Basically they put a 2nd hand pair of cats on and replaced the O2 sensors (amongst other things, but I don't believe the other things are related).

Since then it has had an intermittent heavy misfire which comes and goes, but when it's misbehaving it feels like someone putting the brakes on.

When coming to a halt it also won't idle and splutters to a stall although it will immediately restart and will continue to run if the throttle is blipped.

The rest of the time, it seems to run fine but it happens often enough to make it undriveable especially in traffic which is a fact of life where I live now.

Now I've put off getting anything done about this for various reasons (divorce, moving house, new baby, problems with the becm etc) but at the start of January I sent it to my previously trusted specialist with a remit to sort it out and service it since I couldn't work out what was causing the problem (and because I still believe they caused it).

The problem is that it is still there and they seem completely unable to resolve the misfire although they can certainly get it to happen. I suspect that they've simply lost interest in it now.

Now, I'm not very au fait with diagnostics kit, but presumably there ought to be some clue as to what is causing it other than it just reporting a misfire? So far all they've done is to replace the HT leads to no effect. I'm wondering now if they actually have any diagnostics kit at all!

Having now wasted 6 months of road tax and insurance with it just being sat in their yard, I'm beginning to get a bit irritated and really want to get something sorted but I'm pretty much stuck at their mercy now that the MOT and the tax has expired. It's quite a long way from home too. I've not pursued it too actively up until now for various reasons but I think enough is enough.

Can someone suggest an avenue for them to investigate? They seem to be utterly clueless. My original feeling was that it was related to the cats and o2 sensors change, but it's hard to see how.
My subsequent feeling was that it was fuel related since swapping the fuel filter improved things a bit and because it never did it idling on the drive.
The garage seem sure it is ignition related, although I have no clue what they are basing that on. Communication with them is like pulling teeth which is another bone of contention.

Having recently bought Georgia, I too had a bloody mystery with misfires.

I did find one of the O2 sensors had been repaired (wire broke) and the other one was a cheapie, so got a pair of OE Bosch to replace them with.

Then we found the real cause of the misfire!

MICE!

They had eaten through two of the plug leads, so I have replaced 5 and am off to do the other three today.

We did all new plugs too and the difference is night and day.

Before, on LPG, barely 30mph, now, sky's the limit, I may have inadvertently (ahem ) managed to get to 90mph yesterday to annoy a stuck up Audi driver

So, check the obvious stuff first and if it has been misfiring for a while it may have damaged the coil pack.

If there is lots of black smoke, check the MAF, but they tend to be quite robust.

Another sneaky problem can be the crank sensor (or TDC sensor) as they tend to work when cold, but when hot they either pack up altogether or send spurious signals to the ECU so it tries to run on limp... But I would start with the sparks

Definitely need LR compatible diagnostics, which most workshops don't have. Is the misfire on both Petrol & LPG ? (I assume it's got LPG based on your Sig.) Always best to get petrol mode right first. It might seem complex, but it's still basically a standard engine . . . .

• Plugs, Lead & Coils. State of the spark plugs should give some clue?
• Compression check on all 8 to make sure they're all in sensible limits.
• Crank & Cam sensors, and the wiring to them. Cam sensor wires on my 4.0 project had been bodged, and were intermittent.
• Engine temp, O2 sensors & MAF will all affect it, but ECU should switch to default fuelling if they're wrong.
• Make sure LPG install hasn't compromised the standard ECU wiring.

Pete

Thanks for the suggestions. The garage has assured me that they have checked all of the obvious, although I'm starting to wonder. It's very hard to speak to the owner as he is rarely available and has yet to return a call. Nobody else there seems to know anything about it or is unwilling to discuss it.

I did have a suspicion that it behaved on lpg, but a fault on the lpg system prevents me from trying it again now - the diaphragm has gone in the vapourizer which gives remarkably similar symptoms to a blown head gasket (the reason it was taken off the road in the first place), although its clearly progressed beyond a minor leak now. This was another reason why I was tending towards the fuel system being the cause.

I'll have to try and speak to him again and find out exactly what diagnostic system he's used on it. He agreed to try new coil packs four weeks ago but I assume from the lack of communication that he's either not got round to it or it didn't fix the problem.

Grr.

GEMS or Thor?

If GEMS I can dog-leg past yours on my way on Sunday night if you like?

kooky_guy said:

It's quite a long way from home too

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Where's the garage then ? how far from Sandhurst (you), Slough (me), Bucks (Grrrr), etc.

It's about 20 miles away which was a lot more convenient before I moved.

NSEW ?

I have a Nanocom that can read GEMS but obviously the car will have to be accessible.

On Sunday I am popping down to the Swindon area but I could conceivably dog-leg up the M4 which puts me within spitting distance of Sandhurst.

Appreciate the offer Grr, but it's not accessible.

I was beginning to think the garage had lost my number but they've just rung me to tell me it's ready.
The poor receptionist didn't know anything about what had been done though other than to tell me that after 7 months, £550, a set of unneeded HT leads and coil packs later, they've concluded that the head gasket has gone. It did apparently scrape through an MOT (the advisory was that it was very difficult to pass the emissions test although I found that out myself - she neglected to mention that little detail).
They've done none of the other work I asked for and I can only assume that it remains undriveable, given that they don't seem to have fixed anything.
If it didn't have lots of new stuff on it I would be tempted to just leave it there. I'm fuming, to say the least. I certainly wouldn't recommend this particular 'specialist' after this.

Any idea why they think the head gasket has gone?

Not the worst job in the world but definitely a weekender.

Grrrrrr said:

Any idea why they think the head gasket has gone?

Not the worst job in the world but definitely a weekender.

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No idea as I only spoke to the receptionist. I'm guessing he finally did a sniff test which I assume it would fail because the lpg vapouriser diaphragm has split so gas has leaked into the coolant. I don't believe a head gasket has gone. The misfire is completely random too.

I'm assuming that it remains pretty much undrivable as he doesn't appear to have actually fixed anything.

I'll try and speak to him tomorrow to find out more but he has proved remarkably elusive up til now.

Probably time to find a competent specialist. Any recommendations?

LPG won't show on a sniff test....it measures the presence of carbon monoxide in the coolant system....unburnt LPG, like unburnt Petrol, will not emit carbon monoxide....until it is combusted....

Saint.V8 said:

LPG won't show on a sniff test....it measures the presence of carbon monoxide in the coolant system....unburnt LPG, like unburnt Petrol, will not emit carbon monoxide....until it is combusted....

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That's interesting. Thanks Saint. I assumed it just looked for hydrocarbons. I'm still not convinced that is the problem though, after all the fault began as soon as the cats and O2 sensors were replaced a year and a half ago.
Pretty unimpressed at a bill for £550 that hasn't actually fixed anything that it went in for.

kooky_guy said:

That's interesting. Thanks Saint. I assumed it just looked for hydrocarbons. I'm still not convinced that is the problem though, after all the fault began as soon as the cats and O2 sensors were replaced a year and a half ago.
Pretty unimpressed at a bill for £550 that hasn't actually fixed anything that it went in for.

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They say it detects hydrocarbons in the cooling system....but that I believe is misleading.... it should say burnt hydrocarbons....

A quick test would be to get a dish of petrol and then use the sniff test equipment to sample to air above it and see if it changes....Petrol is a hydrocarbon.....

As far as I know, it reacts with the combustion gases which are chiefly CO and CO2 with some HC and NOx .....

Think we're back to diagnostics.

Were the O2 sensors OEM or copy? Thor engine doesn't seem to like anything other than Bosch. Not sure what the GEMS likes.

I seem to remember him saying they were oem as he struggled bit to find ones with the correct connectors.
Wondering if I should just cut my losses and bring it home, but I'm not really geared up for major work at my current house as the drive is too small.

kooky_guy said:

I seem to remember him saying they were oem as he struggled bit to find ones with the correct connectors.
Wondering if I should just cut my losses and bring it home, but I'm not really geared up for major work at my current house as the drive is too small.

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At least someone else might be able to get diagnostics on it on your drive. Useless in his garage.

Just had another 'discussion' with the receptionist who has quoted me £1500 to do the head gasket, despite not being able to give me any assurance that this is actually the cause of the intermittent misfire. She can't tell me what diagnostics have been performed or whether it is even still misfiring, yet they expect me to pay them almost £600 to get the car back in pretty much exactly the same state that it went to them in 8 months ago.
Great service from this 'specialist'.
I'm not sure what to do with it now. It's clearly not worth spending £1500 to get them to do the gaskets and I certainly don't have the time to do them myself. I'm wishing that I had just disposed of it a couple of years ago now, but I've got too much tied up in it to just bin it now!
Arggh!