Originally Posted by jonnys_walkers
If you shut a diesel down too soon the turbo is still spinning at a high rpm without any oil flow. This will cause the oil to coke and minimize the oil flow.
It's not the turbo spinning that cokes the oil, it's the heat. The turbine usually stops spinning with 10 seconds or so, and there's enough oil in the bearings to keep it lubed as it stops. The problem starts after it is stopped and the oil stops moving too.
Heat soak from the hot manifold and downpipe (a little worse on V8 diesels due to the more complex piping) will cause whatever oil is left in the turbo to cook and leave deposits (coke). This is bad beacaue the turbo normally gets fed clean filtered oil. The deposits will, over time, eat into the seals on both ends of the turbine shaft and eventually start leaking. A leak on the exhaust side can cause issues with the catalytic converter, and DPF (if equipped). Worst case is an exhaust pipe fire. On the intake side, oil can coat the inside of the aftercooler, reducing its efficiency. If there is a substantial leak, the engine can experience diesel runaway. That's where the engine will run on a different combustible source of fuel, other than diesel. The engine can easily exceed its redline RPM, and usually self destructs.
Diesel engines in high risk environments (oil fields, chemical plants, ships) are fitted with intake air cutoffs, that chokes off an engines air supply and brings it to a stop if it does runaway.
Idling the engine before shutdown will prevent this problem. If unloaded, a minute or two is sufficient for turbo cooldown. Towing or heavy loads on the engine can take 5 to 10 minutes of idling to cool the turbo and manifold sufficiently. If you have a EGT gauge installed, you have it easier. If the pyro probe is installed in the manifold between the engine and turbo (pre-turbo), you can safely shutdown when the gauge hits 350º or lower. This seems to be the happy temperature for turbos, large and small. It's the same temperature we shut down our engines at work (USCG buoy tender). The turbos on our main engines are 2 feet wide and weigh about 800 lbs.
Sorry for being a little long winded but I figure this is good information for anyone that runs a diesel in either a pickup truck or a Class A.