Like you I bounced around for a long time with this sort of issue albeit on a TDV8.

I think the gases in the coolant is the real clue here - I didn't want to believe it with mine but unfortunately you can't ignore it. Do a compression and leak down test if you can although I think you will merely confirm your worst fears even further.

I tried Steal Seal head gasket repair without any success and then bit the bullet and lifted the heads. Bingo ! - The head gasket on the left bank was in a shocking state, Skimmed heads, new gaskets etc and rebuilt - Sorted. No gases in the coolant no over pressurisation and back to normal running.

Totally hateful job for a DIY'er, which I am, but certainly achievable - by DIY'ing it cost me about £400 all in, including the specialist tools I needed. It would have been literally thousands at a local garage.

Thanks for your input, Nick...

I've driven the car to work today, nothing happened. And I'm fairly confident nothing will happen on my way back. But as I said, the gassy smell in the expansion tank is the elephant in the room.
I only need the sniff and leak down test for myself as the requisite, definitive confirmation, of a situation warranting such major job.
I'm fully expecting a hefty bill, that's not really an issue (of course I've heard some laughable amounts from authorized garages and the stealers while I was doing some research in the past) as long as the car can be fixed properly.
I just hope it isn't a block. Because even if I decided to get such damage repaired, I just don't know who to trust. It's enough variables already with all the things to be done along with the headgaskets...

Hi, always a difficult shall I, shan't I, moment. I had a similar experience twenty years ago with a Citroen XM Diesel. It lost a little water from time to time, had a barely audible misfire , and one day, at twenty MPH, it stopped and started again reluctantly on three cylinders, very smokey. A workmate took the head off and found the exhaust valve head behind the Turbo wheel. The head gasket had a tiny weep spraying the neck of the valve with fluid, which failed, due to metal fatigue.

As long as this is something repairable within reason, I'm perfectly fine with that. If it turns out to be nothing, which I doubt, so much the better, I have plenty other good uses for my cash.

The engine block is where I draw the line...

Yes I understand your frustration. Since the pourous block issues with the old R/R v8 engines, its easy to be cautious. My guess would be a head and or Gasket issue. Still a lot of work to do, and one needs to be under cover. I had an old V8 with the Rover block with exactly the same issues as your engine, I sold it for spares or repair. The main problem with it was a bent head due to a faulty LPG system. It ran on gas as well as petrol at the same time, and overheated.

Indeed, what really bothers me is that exhaust gases are not entering the cooling system at a consistent rate or manner. I can go 300 and 400 km runs without any issues. I've even driven 800 km in a day recently. That included highway, motorway, traffic, off roading and rock crawling. I still drive 80-100 km on weekdays and 300 on weekends. Last year has been 30k km, lockdown notwithstanding.
There's pressure in the expansion tank and gassy smell every morning without fail. It seems that whatever gases are entering are at small quantities, not the usual hookah style...That makes me fear the worst.

If it wasn't for the obvious smell of petrol / gases inside the coolant tank, I could perhaps chalk it all up to a faulty cap and a leaking upper radiator hose. But, with exhaust gases present, I think the small leaks on the hose connectors are actually due to an over pressurized system There is no other logical explanation.

An often overlooked item is the water jacket leaking in an EGR valve. I had thet on a Freelander 2. They pin hole leak on spotwelds internally, so depending on what one's engine is doiung alters the possibility of a tiny leak.

This is an interesting thought. But does the M62 have an EGR valve which involves the cooling system as well? As far as I know there's a vacuum valve in front of the engine on the right hand side of the vehicle but besides the small vacuum tube, I'm not exactly sure what it is connected to. I do know that the small tube was getting brittle and leaking vacuum and that was a cause for my cel to come on despite 4 new O2 sensors!

By the way, yesterday drove back home just fine. Parked on the driveway and opened the bonnet. Nothing leaked. Within a minute of shutting down the engine, the expansion cap started hissing very very slightly. Like drawing in air. Started the engine for a few seconds and switched off again. Same story. After a minute or less, cap hissing again. I promptly wetted a scotch Brite with some dish washing liquid and it turns out this was indeed vacuum and not pressure. So I assume that air is being drawn in the process of cooling down. Probably normal, I guess?
Morning time, top radiator hose is firm but not hard, like allowing to be pinched by a third of its diameter. Got to work without issues...Again

Hi I see that that there a is an ongoing problem with the PCV valve, which fails at a certain mileage. There is a recall for EGR valves on BMW forums, but the petrol engine does not have one. I wonder if you have a collapsed water pipe sonewhere in the heater system behind the engine?

Negative on the tubing behind the engine...Had them both replaced last year.
Today I drove my employer's '08 SC to an authorized garage for a knackered transfer box. Had a chat on my issue with their master mechanic and he didn't share the common belief on M62's blowing their headgaskets easily...Anyway, Friday is round the corner...

Pulling vacuum and your cap allowing that to happen is not normal imo, your system should be pressurised after just stopping when hot, does your expansion tank have a small feed from the top of the rad (same as 4.2sc) if so check that its not blocked at that point.

I still think there's not much wrong here

Thanks for this. Yes there's the small feed line from the rad. If this is responsible for the small stream coming through the beeder screw and into the tank's neck, then yes this is free.

Ok, I refer you to my previous 2 posts on the thread

Basically

Stop worrying, stop with it.

Make sure its 100% bled properly and topped up to the required level and only top up on substantial loss of fluid or overheat.

Oh consider a new cap too

Thanks mate, I'll only open the cap again upon a coolant level bong...And certainly get another cap on Friday if this turns out not to be a hg problem.