Originally Posted By: Thermo1223
The temp will never be at 250F in the exhaust stream. Turbo's spin down instantly (matter of a few secs). The exhaust is going to be well over 1000F when under load. You should let it idle and cool a bit but it will never reach 250F.
While pyrometer is a good idea it isn't needed, you just have to use common sense. Never shut down immediately after a high load like climbing a hill or freeway speeds. Let it idle for a minute and then shut down.
I beg to differ. They absolutely will cool to 250... or less. I've owned three diesel trucks and all of them were nearly the same in that regard, as were some I didn't own. If I start my truck and let it idle from cold, seldom will the exhaust temp even reach 150F, let alone 250F. This is the case for a truck I've owned for a quarter century, almost. The hotter they are, the longer it takes to cool down. I found that, while towing, a good coast-down run down an off ramp was enough to bring it down to 350 or so (from 6-700F) and a short idle brought it down to around 250F and I would then shut down. Even my gas truck will cool to that temp (yes, I have a pyro on it for tuning purposes).
As to spinning down, I don't really know how long it takes the average turbo to spin down, but I checked mine and it took between 30 seconds and a minute to stop spinning when shut down. Before writing this, I did a quick web search and note many people report largely the same thing, with 20 seconds being the lowest one I found (on a Bimmer gas turbo car). I know that in diesels, they can spin from 120,000 to 140,000 rpm at max boost and imagine it takes a while to spin down from that! I have several Garrett and RotoMaster turbo manuals that lead one to believe they spin at a fairly high rate of speed, even at idle and they caution that if you shut the engine off turbo off when, a) the turbo is spinning at high speed, it loses all its cooling and lubrication and at a high speed, with no lube, that's a lot of wear happening, and b) when it's too hot, it loses it's cooling and lubrication but that when the constant flow of oil stops, the heat carried in the turbo housing will instantly reach a very high temp. According tot eh Garret book, the oil temp in the center (bearing) section of the turbo runs at 180-250F... pretty close to oil temp. If you shut down when the turbine housing it at 350, for example, you can get an almost instant 100 plus degree rise and that's one way the coke is formed... massively, instantly oxidized oil.