If the reason you think you have a head gasket split is because you are losing water, have you confirmed if you have water in the oil or not?

Obviously combustion gases in the coolant system will only test for a pathway between the combustion cylinder and the coolant channels. If there is a split in the gasket between the oil galleries and the coolant channel, this would not show up in a sniff/combustion gas presence test.

If you have water in the oil, it is likely occuring from either the head gasket as you have previously suggested or the oil cooler. I appreciate you have a TDV8, but on the TDV6, the oil cooler built into the oil filter housing has been known to fail thereby opening up a pathway between the oil and coolant systems. Usually, the result is oil in the coolant due to the fact that the oil pressure is highest at this point. However, do not discount this because of this reason. Kev

MY2014 L405 RR Vogue SE 4.4 SDV8 in Corris Grey
MY2010 L320 RRS HSE 3.0 TDV6 in Stornoway Grey

Kev, no water in oil just to add to the mystery. The oil cooler idea worth a look, thanks. D

If the the oil cooler were leaking oil into the cooling system surely it would show up as a raised water level, in addition as oil floats on water would it not also be evident in the header tank as that is the highest point. If it was the reverse it would show as emulsified oil in the engine.

Cheers
Col

Yes, I agree Col, that's why I asked if Dave had checked if there was coolant in his oil.

The coolant has to be going somewhere so if the combustion gases and coolant system are fully isolated, the next best guess would be in the oil. However, he has confirmed that there is no emulsification. Kev

MY2014 L405 RR Vogue SE 4.4 SDV8 in Corris Grey
MY2010 L320 RRS HSE 3.0 TDV6 in Stornoway Grey

As far as the water finding it's own level when topped up, even my S/C does that when topped up to the level mark, no matter how many times you do it, it always drops by a small amount to find it's own level below the mark and always has done, even my old 4.6 P38 did exactly the same thing.

Cheers
Col

It could be the EGR coolers failed... water leaking into the exhaust system and being evaporated...

...sadly it's looking like the head gasket probably between the water and oil galleries. Yellow sludge on filler cap and pressure in the system pushing water out of the loosened cap. When the cap was tight it blew one of the hoses off. Performance still very good. Going to try the gunk first in the shape of SteelSeal. If unsuccessful then sell on cheaply...loads of new parts last 5 years.
Or get a price to remove heads skim and refit. The gunk in two bottles is £54, wouldn't that be a result !! Let you know.

Did you run it without coolant Dave? Kev

MY2014 L405 RR Vogue SE 4.4 SDV8 in Corris Grey
MY2010 L320 RRS HSE 3.0 TDV6 in Stornoway Grey

Probably but kept an eye on it. Got test kit coming but just thinking it may test negative if not blown on combustion chamber.

(Item ID: eBay Item No. 283852829188) on eBay. The Steel Seal worked for a short time but now looking like a heads off job. I'll get a price to do but in the meantime it's advertised at £2500.

Found my water leak was a distribution unit in the vee so all the coolant was evaporated before it hit the ground. £200 rather than the heads off 🤦🏻‍♂️👍🏻