Hi, thanks for the add to the forum, I have owned my L322 4.4TDV8 for 6-7yrs now and up until now I have pretty much done all the jobs needed to keep it going of which there have been plenty. Recently it has developed and issue or issues which I'm struggling to resolve however.
This may turn into a bit of a saga so apologies up front.
About a year ago the car developed a slight coolant loss which gradually got worse, it ended up being a thermostat housing that had cracked so I replaced it. At this point the heater had developed a problem where it would only work as the car warmed up, then it would only blow cold. Having tried many ways to bleed the system I put it down to the heater valve(s) sticking which is a known issue I believe.
The coolant loss issue seemed to be resolved for a while although after a month or so it needed a top up and this seemed to go on, needing a top up every month or 2. During one of these top ups I noticed that the header tank had become very contaminated with some kind of red/brown substance resembling brick dust, definitely not oil. Whilst I have only used OAT coolant during top ups (halfords brand) I came to the conclusion that maybe some antifreeze had had a bad reaction but I am unsure what would have caused this. I took the header tank off and flushed the system as best as I could.
Fast forward a few more months and another leak from a hose prompted some further investigation work. I was concerned that the cooling hoses were getting rather pressurised and this could be causing some of the failures I was experiencing. A lot of the plastic joiners seemed to be crumbling so I swapped out all of the hoses with plastic connections on top of the engine. I also replaced both heater valves (LHS & RHS of engine bay) and the header tank which was still somewhat contaminated despite having tried to wash it out. I flushed out as much of the system, heater circuit etc with a hose before building back up.
Having built it back up the heater still does not put any kind of warmth out and the hoses seem to pressurise up as before, there are a lot of bubbles present in the header tank. I have run it with cap off, bleed port open, as well as filling the 2x heater lines that come off the top of the engine (RHS when facing engine).
I was able to borrow a 'sniff' tester, well the kind that the fluid changes colour if there is CO2 present in the header tank, this seemed to show that there is exhaust gas present in the header tank.
So having got this far, assuming the test is correct I am assuming its either headgasket/head which seems pretty much unheard of or could it be EGR cooler related - could that be pressurising the cooling system or would it just let coolant into the EGR? I cant explain the heater not working, it may or may not be related. I was kind of hoping that was still a bleeding issue.
The car has done around 135k miles, I have never seen the temp gauge go past the middle point, it certainly hasn't been cooked up in my ownership (had it at around 80k miles).
Can anyone add any useful suggestions? I have an EGR cooler on order as it looks easier than pulling the heads off and I'd rather not be tackling that job.
Thanks!

Well my first forray into the world of online forums didn't throw up any responses, I've decided to get a second opinion so for the first time in many years Ive sent a vehicle to a garage for them to take a look. Will update with any findings. Searching online and having rung a few independants I can't find anyone who has experienced a headgasket failure on a 4.4tdv8 but there's always a first for everything.

As you say, virtually unheard of with these TDV8’s, most oil coolers seem to fail between 80-100k so if yours is the original, it’s done well. 2012 Orkney Grey Westminster 4.4TDV8 with Ivory interior.........nice!
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I am a newbie to this too and I am in the middle of replacing my oil cooler and thermostat for oil and coolant loss. My heater didn't work and that was fixed by replacing the valve on the RH inner wing.

As above, in my research a head gasket failure is extremely rare so a second opinion on your sniffer test is a good idea. 2012 Vogue TDV8 in Aintree green over sand

If a sniff test has indicated combustion gasses then it's either head gaskets or EGR cooler. I'd start by either clamping off the supply and return lines to the cooler or ideally bypassing the circuit. This will have no short term detrimental affect, but will allow you to run the engine to temp to see if it pressurises the system.

If it still pressurises then it can only be a head gasket. I posted somewhere about when I had my heads skimmed, think 4 thou was the max we could remove as thicker gaskets are not available.

With regards to no cabin heat, definitely this will be the coolant control valve stuck, very common on both 322 & 405, albeit they are different parts on each model!

Hope this helps in the diagnosis path! L405 SV Autobiography LWB SDV8, finished in Aston Martin China Grey by ETO division.