So, despite umpteen garage visits, I'm still no closer to the cause of me losing coolant and grey smoke under heavy acceleration.

So, here's a list of the facts and observations Because I really need help on this one before wasting any more money on yet more diagnosis:
- there is little/no coolant loss or smoke if driving around town or driving the car sedately. I did a 150 mile drive earlier this week and hardly lost any coolant
- every time I drive normally and do a long trip (150 miles ish), my 'Check Coolant Level' message comes on and I have to top up around 2 litres of coolant
- if driving lightly and never going more than 50% throttle, there seems to be no issues
- the car can be Sat idling on the driveway. I rev it to the limiter a few fines, big clouds of grey smoke and the coolant level drops and does not return
- there is air pressure build up in the expansion tank - I have to release the cap (new (same issue as the old cap)) for the coolant to return
- there is black dirt accumulated on the inside rim of the expansion tank
- once the engine is off, removing the oil fill cap, there is a light vapour/steam released. Is this normal?
- the thermostat housing is leaking slightly but, the garage's view was this does not explain the coolant loss and smoke even when revving In park
- at service, the garage mentioned no issue with the engine oil
- an inspection of the EGR coolers found a broken valve in one of the cooler lines. The attached screenshot shows the little valve I mean. It sits in the coolant line. This hasn't been replaced (see point below):
- the garage have set up an EGR cooler bypass to try and rule out / confirm if either EGR cooler has failed and is mixing coolant with exhaust gases. After 2 days, the same issues seem to be the same
- there is no mayonnaise on the oil cap or dipstick
- there's no overheating problem
- there's no loss of power
- no white or blue smoke - just grey/black
- no smell of burning oil
- occasional smell of exhaust gas in the engine bay

The garage view is that the thermostat housing does need replacing but cannot be the only cause. Their recommendation is to find the source of the issue before spending any more money. If it's a head gasket, to trade the car in. If it's an EGR cooler, to replace it. Or, if it's something else...

'the thermostat housing is leaking slightly but,'

My thought is, & I'm more than happy to be shot down, is, change the thermostat & thermostat housing. If it's "slightly" leaking then when hot and/or under pressure, it could expand a knats cobbler & spew water out? I'm guessing that'll be a cheap option before you delve deeper into it. 2012 Orkney Grey Westminster 4.4TDV8 with Ivory interior.........nice!
BMW R1200GS
Alfa Romeo Giulietta
Just one Montesa now, 349 White Wonder
Austin A40 Farina MK2
1975 Morris Marina 1.8TC

A leaking thermostat doesn't explain the smoke. 2015 Range Rover Autobiography 4.4 SDV8

Correct, it doesn't. But it could be part of the problem.
I noticed you used "2 litres of water".....heck, that's 4 pints in old money!
Grey smoke & excessive water usage points to a head gasket problem.
I take it you've had a 'sniff' test to eliminate HG failure? 2012 Orkney Grey Westminster 4.4TDV8 with Ivory interior.........nice!
BMW R1200GS
Alfa Romeo Giulietta
Just one Montesa now, 349 White Wonder
Austin A40 Farina MK2
1975 Morris Marina 1.8TC

Which garage are you using? Plenty of good LR independents round Hampshire to choose from...
Sniff tests are definitely needed on this as Fox says
Remember, the 3.6 can have its EGRs blanked with a £35 ebay kit 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
New Defender D300 90 on order so "Rory" will be going to a new home....!
1974 Series 3 Lightweight = "Millie"
My preferred specialist: www.glenrands.co.uk
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If you want my (not professional, but having spent lots of hours under the bonnet of my 3.6) opinion:

Work through it methodically. Grey smoke from the tailpipes? Both tail pipes in equal measure (they're seperated in the back box)? If so, it's not the EGR coolers unless you think they've both failed at the same time. If both tailpipes match, the coolant is getting into both banks equally. I would have expected oil pressure to exceed coolant pressure and therefore end up with oil in the coolant, but it doesn't sound like that is the case for your motor.

There's a lot of places coolant could potentially mix with other fluids.

If this was my car i'd be:

- Finding a garage who really knows these motors. Trust me i've been to a lot who didn't and it just cost lots of money to get nowhere. There are very knowledgable independents out there who will get to the bottom of it.
- If DIYing, taking the methodical approach and investigating places where coolant and oil could mix. Oil cooler in the V springs to mind as one of the many heat exchangers.

Filler cap mayo normally takes a while to develop. Would be interested for you to drain off a pint of oil from the sump and see if it seperates. Sniff test sensible to do.

I could well be talking **** but hopefully of some help. 2008/58 FFRR 3.6 TDV8 Vogue

The garage have done a block test today and have a negative result so, this means a problem with the head gasket. The lack of any other symptoms such as steam or mayonnaise probably means it's early days.

To verify, which is the best head gasket sealer product? I know they only work short term but, this is the point for me. I want to see if the sealer makes a difference and if so, I need to consider my options.

I have warranty on the car which says it includes head gasket failure and block skimming however, there will, I'm sure, be some kind of catch. I may have to consider selling and accepting the loss on this car. 2015 Range Rover Autobiography 4.4 SDV8

Well, if you’ve got a warranty, with all due respect......use it! I’m guessing that any excess would outweigh any faffing about with sealers etc etc.
If you go down the warranty road, I personally wouldn’t put any sealer in her, if the garage report to the warranty company that “sealer has been used” they could well cancel the policy......that’s just a guess.
I would also try & use a garage that fully understands the RR’s & any work attributed to the head gasket & also .......how can I put this......”sympathetic to the owner”.....in other words convinces the warranty company that said work is necessary & complies with the policy wording.
First thing to do is to study the warranty terms & conditions then make a plan. 2012 Orkney Grey Westminster 4.4TDV8 with Ivory interior.........nice!
BMW R1200GS
Alfa Romeo Giulietta
Just one Montesa now, 349 White Wonder
Austin A40 Farina MK2
1975 Morris Marina 1.8TC

I'm way worse than that...

Just finished having the head gasket repaired, at a major cost, and...first drive.... still losing water!!!
Every time I park the car it smells like ''coolant''

I'm convinced it's not.

The reason I don't like using garages is because they don't talk you through their work in detail. It's always a rush and just get handed a bill. I mean, what do they mean by "block test"?

If they're testing for CO2 in the coolant in the standard "smell test" then, this can be caused by a faulty EGR/EGR cooler or, anywhere exhaust gases and coolant mix! If you do this test on your driveway, you know exactly what test you did and under what conditions.

I'm still not convinced it's the head gasket, as per your experience.

I've decided I'm going to replace the EGR coolers and thermostat housing then see how things go.

After that, if I'm still getting issues, I'm going to do a smoke test. 2015 Range Rover Autobiography 4.4 SDV8

Hi Gary, I am getting grey smoke at high motorway speeds ( but I have a petrol V8 ) , but just curious if you found what was causing your smoke issue ?

Blown head gasket(s) 2015 Range Rover Autobiography 4.4 SDV8