Hi Discomania.
you have my attention. would you expect same on light throttle then just after overrun? and subsequently no smoke when back on medium/high throttle. the smoke at junctions is significant (when ticking over, and will plume with short bursts of throttle at junction).
I have re-read your original description of the problem just to help with this remote diagnoses and you made the following points.
No issue starting and no smoke at start.
Smoke only appears when the engine has warmed up.
Smoke appears when you come off the throttle - puffing smoke at this point and then when you apply throttle you have a lot of smoke.
It clears soon after this episode until you repeat.
Looking at that data I can surmise that the issue is not fuel related - it starts fine, it runs fine and with no loss of power and nothing happens until it is warm - therefore I will assume the fuel and the fuel system are OK - if you had a fuel issue then even at cold you would see added smoke or wisps of smoke from cold. Starting would possibly be affected.
There are two stories here, the intake and the exhaust. Both sides behave differently during different engine situations. So to paint a rough picture lets take the inlet side, you are going to have a bit of vacuum, usually boost and about atmospheric pressure at some stages. On the outlet you have pressure from exhaust gasses exiting, however immediately after the valve closes and when you let off the throttle this changes for a split second where the momentum of the exhaust gases will actually pull a slight vacuum. Imagine, going along with high revs, lots of air going in, air and gasses and heat and sparks and flames shooting out the exhaust valve, you then take your foot off the throttle the engine starts to slow, but the outgoing gasses don't get this message, they want to batter on and create a vacuum behind a now slowing engine and of course, a closed exhaust valve.
The fact it only happens when hot suggests things to us here too, the expansion of the valve guides is greater than the moly-chrome alloy of the valves, so the gap opens up a bit with a wonky seal you end up with more oil bypass. It gets sucked in under the circumstances described above and you end up with a squirt of oil above your exhaust valve, nice hot oil, it smokes, give it some throttle, burning fire and gas picks up the oil, ignites it all, blasts it through a turbo and before you know it, you have a nice stream of smoke, once you get the engine rolling again it burns if all out fairly quickly, on light throttle with the turbo sitting on the edge it is possible more oil is being fed in or it just takes longer to burn it up, if the leak is not too severe then it may have no smoke at light throttle. A tiny amount of oil can make a LOT of smoke. As someone who has spilt chainsaw bar oil onto a red hot chainsaw exhaust I can attest to this.
So how to proceed... you would need to get it hot(the reason for this will become apparent), then without removing your fingerprints loosen off your exhaust manifold and pull it back (I would bet on this one first) and have a look, if the valve stems look oily and wet then that is a good indicator, start her up (why you want her hot) (be careful!) and have a look, at idle with the vortex the exhaust gas will create it will pull on the valve stem you may actually be able to hear a sort of wet oily noise and see oil splashing about.
You can change the seals insitu - the problem is the guides are probably shot if the seals are leaking, but for £5 for some cheap ones (I would not recommend long term) you could change them all out, and see if that fixes it for a day, week month etc. If it does then I would suggest head off, order up new guides and good seals, take the lot to an engine shop (I always use a marine diesel place) and have them fit the replacements. The procedure for this head is to heat it up to I think 120°C if I recall correctly.
That is from my thinking and personal experience with Land Rover engines - sitting behind a laptop and not getting hands on with your engine always puts us at a disadvantage and it could easily still be injectors or turbo (but your sort of ruled that out) but there is some food for thought.
