Once more, I am in need of the forum's ideas.

I have posted before about a possible head gasket failure which I'm currently trying to confirm or rule out. So far I've been to a couple garages and also discussed with some other mechanics. There is still no clear answer.

To recap, symptoms are:

-A spark plug (#5) repeatedly going bad. 1st time after 4k km but with old, oily coil boot which was replaced. 2nd time after 6-7k km (same coil), 3rd time after 27k km.
Symptoms before failure, the first 2 times, were misfires during acceleration at 80-100km/h which became worse the next day.
The 3rd time was more gradual in its progression as manifestations began at 10 months, with an occasional startup misfire (seldom with a blinking cel) which promptly cleared by itself or by restarting engine. Eventual failure manifested as the other 2 times prior..
-Pulling out the spark out, the porcelain is dark brown like baked bread. The threads are dry and the ground pins and couple of treads appear somewhat rusty (?) It was also loose and this is proven by the fact that the crush washer appears intact.
-Expansion tank holds some pressure overnight (has always had) and it might or might not smell somewhat like petrol, briefly once popped open.
-Compression test came out even in all cylinders
-Leak test showed upper radiator hose slight leaking from rad coupling and thermostat / water pump coupling
-minor loss of coolant about 500-700ml per month or every 3k km
-no loss of power
-no oily sheen or particles in coolant
-no water, ever, in oil
-no white smoke from exhaust
-no water expelled from the expansion tank even after 400-500km non-stop runs in various conditions. No bubbles when observing without cap...
-Now that spark plug+its coil are replaced, I have zero startup misfires. How is that possible if I have coolant leaking in the cylinder? A new spark plug and coil has nothing to do with a leak...

Some suggest it's nothing, others it's a HG and others go as far as a cracked cylinder head.

Reality check and actual impact:

I have no significant loss of coolant and the little there is, can be attributed to the radiator hose.
There are no misfires in 10+ days
There's no overpressurization and no coolant expelled

Any thoughts appreciated because I wouldn't like to open up a perfectly good engine before it's really due. With all this crap I've also lost faith in most garages in my area...

PS: Spark plug pics in my other posts

Thank you

Have you boroscoped the cylinder to see what the piston crown etc look like?

Is the injector good on that cylinder?

If you fix the hose leak, that should confirm that you are or are not actually using coolant, rather than assuming its just the hose. 05 L322 4.2SC
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Thanks, I haven't boroscoped the cylinder but from what I could see the piston crown is just as black and sooty as the next one.
I need to find a garage that has a boroscope or buy my own.
I have purchased the hose and will be installing it after holidays.
Is there any non intrusive diy way to assess the injector? I have a GAP IID tool, can I use that to draw any kind of meaningful information on the injectors function?

Left plug is the bad one. Can anyone tell, by the looks of it, if it's water-damaged? Please note it had been loose all along (1 year or 27k km)

Click image to enlarge

is the left plug the right sort of plug, looks different to the right one to me RR 2005 4.4 petrol
Any spelling mistakes are the fault of spellchecker
Previous MG J2, CITROEN light 15 x 2 gone unfortunately
Present MERC 180e, RANGE ROVER L322, JAGUAR XJS, MERC 280SE, MG F, JAGUAR S-Type 6v 2003, jaguar s-type 8v 2004, Ford Cougar

Same type plug...The right type as per the manual.

BKR6EQUP

There's nothing wrong, just keep driving it

sounds like you had some leaked oil on the plug, from rocker gasket.. but with new plugs and coilpack, you eliminated all possible no5 misfire issue... you can have a coolant leak anywhere on this engine, they are known to have brittle gaskets, and weep a lot over time. just keep an eye on the guage, and level topped up, and you'll be fine. if its was the HG, it would be a lot more symptoms. and plug(s)
would be washed/steamed clean etc.

Thanks for this and yes, there was severe oil contamination on the old coil boots. I should have changed them all when I did the rocker cover gaskets. By the way I did a 500km trip today, in freezing conditions in the morning and rather warm at noon. It should have showed something...Did not face any issues though. Heater is also working well, almost unbearably warm at anything above the first red dot and 20C setting...
I'll have the top hose replaced after holidays and keep an eye on fluid levels and gauge.

Well, I believe I have now exhausted all possibilities. It has to be the head gasket if not the block...Beyond any reasonable doubt, I mean.
This morning with the engine cold, tried to check the coolant level but upon unscrewing the cap, water came out. So I promptly closed it back. Tried to bleed through the screw but all I got was a steady stream so off I went to work, carrying a gallon of ready made coolant with me just in case. After the 40km commute, I parked and sure enough I notice that coolant had escaped from the cap.
In lunch break I unscrewd the cap and some 2 ltrs of coolant came gushing out.
I topped it up, idled the engine and bled once more.
Before leaving for home, I will check again to be on the "Kalt" mark.
Meanwhile I've booked it with a reputable garage for a thorough investigation and a quotation on HG x 2 Vanos rebuild and chains-guides. I fully expected a hefty bill...Provided it's actually repairable and not a new engine matter, of course.

So I guess Friday will be the day it's fate will be decided, if it makes it that far...Shame, because it otherwise runs well and it has been well cared for. Oh well...

PS: Thanks everyone for all your input and help. It's been a great journey so far and a great forum to be a member of. Hope I'll still have a reason to frequent here after Friday, but even if I won't, I'll someday return...

let us know how you go RR 2005 4.4 petrol
Any spelling mistakes are the fault of spellchecker
Previous MG J2, CITROEN light 15 x 2 gone unfortunately
Present MERC 180e, RANGE ROVER L322, JAGUAR XJS, MERC 280SE, MG F, JAGUAR S-Type 6v 2003, jaguar s-type 8v 2004, Ford Cougar

Sure I just hope this is something repairable. Well basically anything is repairable but of course there's a line...

ok, so if you still have an overheating issue, then you must prove the thermostat is toast.... this could be the issue, and it might have cooked the head gasket, or, it might just still be the problem, without cooking the HG. so, search on here about upgrading the m62 thermostat, (which is cheaper than a Land Rover original,) and will regulate the temp better..(lower) and for about £50, you should be able to prove its not just a thermostat that's stuck open. you might want to watch a little overheating saga, on Youtube, with a guy that flips some half decent cars, and finds the faults, and MOSTLY fixes them etc. so here's a link to one of the episodes. you'll see his thermostat is drilled, you might want to replace it, or better still buy the modded kit, and fit etc. -unfortunatley, you can't remove it on the original, so 'Samcracc' (the YouTube guy) drills holes in his.

Thanks for this, the thermostat is less than a year old and so is the water pump therefore I doubt they could be the culprit.

Also, I do not seem to have an overheating issue, in fact never had. Temp never rises above 109C on the IID readings and the needle stays dead center on the dash.
Never in my 2+ years of ownership has the car overheated. The closest it ever got to, was when the valley pan failed (about 2 months after car was purchased) with 130km/h on the motorway and gave the bong for coolant level. It took me literally 30 seconds to stop at a safe spot and switch off the engine.

Now this is getting interesting for all the wrong reasons...Following this morning's episode, leaving work in the afternoon, I checked the level with the engine cold and topped up like 50ml to the "Kalt"
Drove 48km home through traffic and free motorway from star-stop to cruising at 130km/h and the expansion tank cap is bone dry with no sign of coolant whatsoever escaping or any hissing noises.
That's why I'm afraid this isn't a "simple" head gasket job but something worse, like a cracked cylinder head or block. Because it only "opens up" a little and at random. I could dismiss it for being an overfill issue or improper bleeding but then there's the elephant 🐘 in the room, the exhaust smell in the expansion tank every time I check...

Fit a new cap (if you haven't already) bung some steel seal in, get it bled and topped up properly and drive it just keep an eye on the header tank level. Try not to drain or top up unless absolutely necessary.

As long as its not overheating and your not dumping water into the oil or combustion gasses into the coolant, there's probably nothing wrong with it.

Cap is new(ish), a few months old. I do seem to be getting combustion gases into the coolant, I need no tester to tell as the expansion tank stays pressurized overnight and smells awful when popped open.
What concerns me the most is that the exhaust gases do not seem to be introduced into the cooling system in any consistent manner. That's why I'm beginning to think of a cracked head or a cracked block. Something that opens up at certain temperature or under certain load...And as the gases are apparently being pushed into a coolant channel by the compression stroke, I doubt any sealer liquid can help. Of course these are all plausible theories but reality could be different. It just doesn't look good though. I'll have a pretty accurate picture from the garage on Friday. Again, this is a major job and want to have solid proof before proceeding to repair or drawing a line.
I bought the car being fully aware that timing chains / guides would have to be done at some point. And head gaskets would better be replaced while at it. I just hoped the chain guides went first...And not an engine block.