The turbo charger has been removed, rebuilt and refitted. The rebuild cost £280, and required a new set of seals and bearings, replacement of the scored and corroded shaft and polishing out of light internal scoring of the core casing that the shaft runs in. The soot in the turbo suggested the suspected oil loss from the turbine – the soot in the hot side of the turbo was very thin, fine and dry, but in the cold side (the elbow to the down-pipe) was thick, slightly moister and pasty. The good news is that the engineer from The Turbo Centre called me at the end of the strip down with the findings, prior to rebuilding it, with his diagnosis – it was all just age and wear and tear. It seems that the turbo had done quite some time in service on another engine as the amount of wear was not consistent with the 15,000 miles it has done in my 109, but was also consistent with a healthy engine, not one that had any oil contamination, heat or pressure problems. The fact that it looked new was probably due to a shoddy “reconditioning” that consisted of little more than media blasting of the casings and a good clean.
The unit has been refitted (no photos – the camera kept fogging up in the cold, and the turbo looks the same as when it was fitted first time around anyway) and run. Time will tell if it sorts out the oil consumption, but two 50-odd mile round trips to work show no oil loss yet.  It may be an illusion, but the engine also seems to have a bit more urge to it and the turbo whistle is certainly a little more noticeable. Unfortunately, it has made no difference at all to the oil pressure once warmed up.
So, while the turbo is now as-new, and seems to have marginally improved performance and hopefully stopped the oil loss, and while the low oil pressure issue has been shown not to be affecting the most susceptible and sensitive part of the engine, I still haven’t found the cause. The fact that fitting a new oilstat (thermostat in the oil filter housing that regulates flow through the oil cooler) slightly reduced the stabilised oil temperature but also dropped the warm pressure by about 5psi now points me in the direction of the oil cooler system.
I now suspect that flow through the oil cooler is somehow being restricted by the oilstat system, so full pressure is given with the oil cooler system shut off, but the pressure is lost when the oilstat opens. To my knowledge, the oil cooler is a full-flow type that takes oil from the pump and cools it before distributing it to the engine oil galleries along with any oil that goes via the standard cold route, not a bypass system that takes a proportion of oil flow and dumps it directly back into the sump like on oil cooler equipped Series Land Rovers. I need to investigate this further, but no-one I have asked yet seems to know the exact sequence of events within the oil filter housing. There are only two oil ways between the housing and the engine block – one in and one out. Obviously, pressurised oil comes directly from the pump through one of the oil ways into the housing, and eventually exits through the other oil way to the oil galleries, but the sequence of pressure sensing, temperature sensing, filtration and cooling are important – if the pressure is being sensed after cooling, then it would seem that when the oilstat expands and closes off the cold bypass route and opens the oil cooler route, flow is being impeded and the pressure is dropped. If, on the other hand, pressure is sensed before the oil is cooled, then the warm and thinned oil flow seems to have too little resistance to hold its pressure (which would not be affected by routing through the oil cooler) , which could be a sign of excess bearing clearance elsewhere in the engine or a regulator not performing correctly. This could be an incorrect size head oil feed hole in the head gasket (I have one of the new composite gaskets rather than the original single piece type with black coating) allowing too much oil flow into the head, dropping the pressure, a broken oil jet at the bottom of one of the bores or perhaps a leaking skew gear assembly (the one that drive the oil pump and brake servo vacuum pump) allowing oil to leak directly back into the block rather than forcing it through the filter housing assembly. The change in temperature and pressure readings after replacing the oilstat seem to suggest the former, though.
I hate to be the bringer of bad news but going on the basis that it is a 200tdi with reasonble milage, low oil pressure when warm can be caused by camshaft bearing moving as i have found out and seems to be common tends to be the rear one which you should be able to see though plate in side of engine if you can see it then it rebuild time. .
Hi Tom, thanks for the reply.
The bad news was that while the new turbo has increased engine response and torque quite noticeably, it didn’t make any difference to the oil pressure. The good news is that the cam bearing drifting issue that Tdis are known for is not a problem here; that fault will bring the oil pressure light on at idle and show almost no oil pressure at all – it’s an all or nothing failure.
the oil pressure issue is to do with the oil cooler circuit. All the readings that I have managed to get from other Tdi users on various forums and the advice I have been given from the local LR specialist garage is that these readings are pretty normal. As an experiment, I have removed the large washer from the oil thermostat, and while the oil temperature is now fractionally higher (2/3 of the way up the black “normal” segment of the gauge as opposed to just over half way), the oil pressure is back to 50psi at medium or high rpm and about 20 psi at idle, despite the slightly higher temperature. The only slight snag is that after a long run at high rpm, slowing down to low and mid rpm causes the oil temperature to rise to 3/4 of the normal arc as the oil flow through the cooler reduces (slower pump rotation) but the engine is still warm. It’s acceptable, but this was only an experiment – the next stage is to bevel the outboard side of the washer and refit it so that it creates less of an obstruction to the oil entering the filter from the cooler side while still blocking the direct bypass when the oil is hot.