It's to do with the possibility of the turbo being red or white hot, and if you switch off the engine straight away, the oil in the turbo can carbonise,

A problem that might theoretically occur if you were to switch off immediately after completing the Pike's Peak hill climb.

In reality, the only opportunity to do sustained high speed driving is motorways/dual carriageways. Under what circumstances would you imediately switch the engine off after cruising at 90+mph? Even 90mph cruising is a far cry from the full power dyno runs you see on youtube that cause orange red manifolds.

You see your exit sign, ease off the throttle, roll up the slip road, find a parking space and switch off. All the time your diesel engine is pumping thousands of litres of cold air straight through the exhaust system. You've already done your cool down. By all means sit there for an additional 30 seconds if it provides reassurance

No sorry Unthrottled, my previous post was ambiguous, as usual.

I was also referring to the huge mileages old school lorry drivers have clocked up with never a whisper of trouble from their lorry turbos, modern fleet lorries driven by all sorts who don't look after their vehicles in quite the same way do suffer from more regular turbo failure, unheard of even 15 years ago...though i wonder if the move to starship mileage oil change intervals has some bearing (ho ho:-) on the problem, as i'm convinced it has with cars.

I'm aware that it's beneficial to let a turbocharged engine idle for a while (I leave it about half a minute) before turning off), but isn't good practice to do this with all engines?

Edited by Sofa Spud on 22/06/2013 at 18:59

Going back to the 70's I can remember de-coking an even older 6 cyl petrol engined Bedford coach. Job finished, decent test run, back to the workshop and lifted the engine cowling (inside the bus) and in the semi darkness the exhaust manifold glowing red hot. It was getting late, so checked a few things, ign timing etc and called the main dealer next day.

Oh yeah - normal, they all do that. And that was after a leisurely unloaded test run.

The company I worked for made turbo's for automotive diesels with watercooled bearing housings and for marine engines, watercooled turbine housings.

For some high duty marine applications, instead of watercooling the turbine housing an insulating jacket was fitted for better thermal efficiency.