Turbo Diesel Failure
Clean combustion resulting in clean oil (assuming it has the proper grade, not some cheap concoction) is essential not only for the engine but for the turbo as well. Most people are not aware that a turbo can be--and actually should be--cleaned every 150,000 km or so. This procedure will make it last forever. I can tell you that I looked at mine when I changed the head gasket, and even though the car has been tortured before I got to own it (I am not to certain it had its oil changed more than twice in 120,000 km, injectors were still the original ones, never touched), it was in excellent condition and could probably have gone at least that much longer without any problems even if I hadn't touched it.
To clean it, the turbo should be removed from the manifolds, dissasembled to pieces which are then cleaned thoroughly of carbon and crud deposits. This involves mostly the bearing case, the bearings themselves, the shaft, and the heat sheild on the hot side. The shaft should also be re-balanced when the whole unit is put together again, although with some attention to detail this can be avoided. It is essentially the same process a turbo rebuilder service would do except you need no spare parts.
If you have a turbo seizure, don't think of driving with it even temporarily. Also, for obvious reasons the turbo should not be removed (open manifolds), nor should the intake be bypassed and the turbo left to blow into the atmosphere (over-revving will occur because it will not reach waste gate pressure at any rpm). If a working turbo needs to be disabled it's actually safer to put a piece of pipe at the pressure output and plug it at the end, leaving a very small hole (say 0.5 cm, unfortunately this will result in lots of noise) thus creating a small reservoir and something for the turbo to do without revving itself apart. Merely pulling out the manifold pressure tube that goes to the compensator on the pump will not disable the turbo--it will still work, albeit at a much lower efficiency.
If exhaust gases end up hitting a seized turbo, they eventually burn off the blades of the turbine and the debris ends up in the converter, not to mention the overheating of the turbo itself. There are people who overhaul turbos: this will mean a new set of hydrodynamic bearings, oil seal rings, a new shaft and a new set of turbine (and possibly compressor) blades. In any event it will be cheaper than a new one.
Alternatively, you can look at scrapyards for the same basic model, and have the refurbishing people do a 'make one out of two' job, which actually only involves exchanging the parts that attach to the manifolds, plus cleaning the salvaged turbine.
The main reasons a turbo can fail are the following: it is not allowed to spin down before oil pressure drops. This results in the spinning shaft crashing into the hydrodynamic bearings. This in itself may not damage the bearing or the shaft, unless it happens repetitively. The oil that remains there offers some lubrication for a while until its temperature reaches a very high level because of drag to the shaft; this will produce a local carbonisation. This builds up until it either damages the soft bronze bearing the next time the shaft drops or until enough carbon builds up to seize the turbine. The situation is much worse if the turbo was under a great deal of strain just before this, the temperature of the shaft being quite high. Always allow then engine to run on idle for a minute or two to let the turbo cool down before you switch the egine off, especially after highway runs.
If the oil seal wears out on the hot side, the oil will pour out between the seal and the heat shield, getting burned. The resultant crud produces drag and more heat on the shaft, and as it builds up, it may actually end up pushing out the heat shield as it expands under heat. The resulting impact on the hot side rotor will brake it down. The added (signifficant) drag may then literally burn off the rotor blades. This is usually caused by contaminants in the oil carbonising on the oil seal and wearing it out, This is responsible for most turbo failures on a diesel. The occasional cleaning of the turbo will prevent this failure completely (unless the engine and the turbo are already on their last legs).