They say after driving a diesel engine, your supposed to let it idle for about a minute or so before shutting it off.
2 questions:

-Should you do this in a gasoline engine?

-What happens if you DON'T let it "cool down", (shut the engine off as soon as you are parked)? I know about letting it warm up before pulling off right after starting, but what can happen if you never let it cool down?

It can be hard on the turbo, not letting the oil cool down inside it. But an intercooled turbo cools down quicker. The turbo gets much hotter than the motor, yet shares the same oil. If you dont let it cool and recirculate out, it can bake, and leave hard crust inside and cause wear on the bearing surfaces.

If you don't let the truck "cool down" properly, say goodbye to your turbo. Only thing worse would running down the road with the hammer down and shutting the truck off, nearly guaranteed to lose the turbo that way. I see a lot of construction equipment that lose turbo's due to inept operator's not letting the thing cool down. They just shut it off with the throttle still wide open.

As long as your pyrometer is reading 300 or below it is safe to shut the engine down. The bearing technology has changed, and the newer engine oil doesn't bake on the bearings like in the old days. Most time by the time you exit the highway and drive at low speed into wherever you are going to park the turbo will have cooled sufficiently.

this is true.

In turbo cars they sell whats called a turbo timer. After the key is turned off and you walk away the engine will remain running for 2 minutes or so then turn off once its cooled down to prevent oil coking inside the turbo. Also another trick would be not running the car hard for 2-3 miles before you turn it off.

I had a car (can't remember what it was) with a turbo. It had an electric oil pump that kicked on after shutting off the engine that circulated the oil solely through the turbo for about 10 minutes.

Worked real good - I had to replace the turbo about 6 months after buying the car (it was used when I got it). It's a great idea though.

gasoline engines with a turbocharger usually have the turbo water cooled, so it's a moot point on those engines.

Besides, EGT's on gasoline engines don't vary as much as diesel engines, even at idle they're way over 1000 F due to more constant A/F ratio.

my MACK will not shut off. When you take the key out of it it will run for 4 mins and shut off on its own. Mack did it with the comptuer i hate it. It shuts off after 5 mins of idleing also. If it ever goes back to the dealer am gettin them to take that crap off. But you can get past the shut off thing. If you turn the key off and turn it on and off real quick again it will over ride it and shut the truck down. Back to the turbo timer thing they sell it for my diesel pickup but i think if you really needed one they would put them on right at the factory and it wouldnt be after market.

I usually let my engine idle with the fan on until it reaches the 150* range (ambient) when i shut it down. Shutting off a hot engine is never a bad thing.

I consider shutting an engine off hot is like not letting your engine warm up before a run.

The drive off of the highway and into the parking area or fuel pumps is plenty of time to get the things cool enough to shut the truck off. it is actually much longer to get off the highway and into the truck stop than the idle cool down that some say is necessary. Cooling down a turbo after the time it takes to get to the parking is just a waste of fuel for the most part.