So, following loss of coolant and pressure build up in my expansion tank, causing the fluid level of drop and trigger the coolant low sensor, I embarked on a 15 hour job to replace the thermostat housing, checking the hoses and replace the EGRs and coolers. I was previously losing around 1 litre per 100 miles.

I didn't find any hoses with signs of pink around them, corrected an EGR cooler bypass and, the thermostat housing had a little bit of pink on the infamous plastic weld but, not much of one so, that made me suspicious.

At the end of this herculean job, I filled the system back up over several hours, allowing it to bleed naturally, whilst I got the intake manifold back together and all the shieldings back on then, did the filling procedure (start, get to operating, rev 30 seconds, bleed, rev 30 seconds, bleed, heater to full, repeat).

On my test drive of about 8 miles, I already had the coolant level warning come on. When I checked the tank, sure enough, same as always, loads of gas pressure built up in the tank, forcing the liquid down below the sensor. This is exactly what it did before. Obviously, I haven't travelled far enough to know if I still have coolant loss.

What on Earth is causing pressure build up like that? If it was a head gasket problem, at a rate of 1 litre per 100 miles and 30 litres put in since the last oil change, I'd expect to see mayonnaise everywhere. 2015 Range Rover Autobiography 4.4 SDV8

Get a sniff test done at an MOT station, any signs of exhaust gas in the coolant bottle confirms a head gasket issue.

Another possibility is pressure cap itself, EGR cooling but I see you have that bypassed. BMW i3 Electric Car
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If I'm right, and that's always very debatable, if water enters the oil through head gasket failure then yes, you get mayo. But, on the other hand, if water is escaping into the combustion chamber [again through head gasket failure] then it'll simply be burnt off & out through the exhaust system, the latter however normally gives white smoke out the back.

Just my two pennyworth. 2012 Orkney Grey Westminster 4.4TDV8 with Ivory interior.........nice!
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If there is a headgasket failure to the combustion chamber, the pressures are much higher in the chamber than in the cooling system. Logic dictates that combustion gasses would enter the cooling system not water into the combustion chamber. I have seen many head gasket's failed and many people state the white smoke / steam, I'm never yet seen it only the water in oil or the cooling system being pressurised.. BMW i3 Electric Car
2012 Full Fat RR 4.4 TDV8 (now gone)
2006 VW Touareg 3.0 TDi V6

.....meaning pressurised water escaping from the nearest point........expansion bottle!
Think AJGalaxy has perhaps hit the proverbial 'nail on the head'. A cheap & cheerful 'sniff' kit are available which would probably back up AJ's post for sure. 2012 Orkney Grey Westminster 4.4TDV8 with Ivory interior.........nice!
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Exhaust gasses in the expansion bottle is a sure sign of head gasket.

Lets hope not.....

J BMW i3 Electric Car
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Expansion bottle has been changed, I just spent 15 hours replacing the thermostat housing and EGRs because that's what the garage said it was.

P***** off is an understatement. Also that I've paid garages to do things and investigate and they should have been knowledgeable enough to know that a thermostat housing is unlikely to cause pressure build up.

When bleeding, how long should it take for the bubbles to disappear? I just seem to get a constant stream.

So, I think this leaves me with only one decision - Chuck head gasket stuff in and sell it now, cutting my losses. Which is the best head gasket stuff? I read Steel Seal or K-Seal (which is much cheaper). Given my symptoms aren't major, do you think it would have a chance at working? 2015 Range Rover Autobiography 4.4 SDV8