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Discussion Starter #1 (Edited)
Page 179 of the 2018 F-150 Owners Manual....talking about a diesel 3.0L engine says this:
SWITCHING OFF THE ENGINE
Allow the engine to idle for three to five
minutes before shutting it down. The larger
the engine, the greater the need is for this
idling period.
Note: Try to limit engine idle to 10 minutes.
Excessive idling reduces fuel economy.​

I've gotten feed back on other forums and have done some internet reading and here is my distillation of what I've read:

The Turbo spins down in a matter of seconds....so idling the engine after stopping is not necessary to keep the turbo lubricated...BUT... the turbo does get really hot, especially when the engine has been run hard. Idling the engine will allow the turbo to cool down...and doesn't "Cook" the oil in the turbo. E.G. Your driving at highway speeds and are quickly stopped due to an accident, etc. Don't stop the engine immediately (or let Auto Start-Stop do it) until you've run the engine for a few minutes to cool the turbo down. In city start-stop driving, if you've slowed down the last 1/2 mile or so to your garage, idling for a few seconds might be all that is needed.

Also, the Auto-Start-Stop feature seems to run counter to the page 179 recommendation. In fact I might buy a 4Dtech.com device version 2 to allow me to toggle this feature off/on and have that "stick" between startups.

Any experts out there that can dovetail what the owners manual says to reality for a good recommendation on when to let the engine idle before stopping the engine?

As a side note, I have a Sprinter 3500 3.0L Diesel (a class B RV). It's basically always towing because the RV is 11,000lbs. So I'm going to start leaving the engine idle for 3 or 4 minutes every time I stop the RV.

On my Sprinter, I'm posting this label on the dash: Let Engine idle 2 to 5 minutes before shutting off. Longer time for higher speeds/loads/heat. This gives the turbo time to cool down.

On my F-150, I'm posting this label on the dash: Let Engine idle 2 to 5 minutes before shutting off. Longer time for higher speeds/loads/heat. This gives the turbo time to cool down. Auto Start-Stop: Turn-Off
 

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I have no experience with the 3.0L (mine is a 7.3L), but I think this is a thing with turbo-charged diesels. You're right, the turbo spins down quickly @ idle. I believe the issue is the heat on the exhaust side. As you said, hard driving, hot exhaust. This causes the turbo to heat soak. It's about preserving the turbo. I have A-pillar gauges, one of which is the EGTs. After driving I usually (not always) idle until the EGTs show 300°. Sometimes, if I driving slowly before parking the EGTs drop down to near 300° so I end not having idle as long.
 

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Correct, it is a cool down time and is to protect the turbo. Also correct is the driving just before shutdown will dictate how long it should idle. Coming in after a run on the interstate or with a heavy load, the engine should idle longer. In my case heading home, I drive down small side roads for about 1 mile at about 20 mph. I don't let my truck idle when I get home because there is no need. The turbo isn't going to get cooler

My experience is also with the 7.3, but also a 5.9 and a dt466 that do a bunch of heavy towing.
 

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Idling before shutting down a turbo diesel has been around a long time. I had a 1983 Audi 5000 turbo diesel, and the owners manual stated to idle before shutdown. I've been pretty careful to do that with my 6.0, but sometimes it doesn't happen. Must be Old timers disease because I forget.
 
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