I can only tell you what i know from what diesel mechanics have told me.
Idling an diesel engine causes Carbon Build Up
Carbon build up is not at all good for the engine
Detroit Diesels are built differently than other diesels, because after you idle it for periods of time, then start down the highway,it blows the carbon right out the stacks.
No other diesel had that ability to blow out the carbon.
Any diesel that has been worked hard & has not had time to cool down is also bad, but not only for the diesel engine
It is even harder on the TURBO(s)
As the engine is being worked at any RPM, the turbo is spinning at a high rate of speed.
The inside of the turbo is made of thin blades. These blades are constantly being kept cool by a liquid being squirted on them.
Big trucks have a Pyrometer that indicates the heat level of the Turbo
When truckers pull into the place that they will be spending a couple hours or more, they don't shut off the engine for a while before shutting the engine off, they are waiting for the turbo temp to drop from something like 5 or 6 down to 3 or a time period of 3 minutes or a bit more
The reason for that rule is because the liquid only sprays onto the blades of the turbo while the engine is running.
Shutting off the engine causes that liquid pump to shut off & the hot turbo blades begin to deteriorate quickly without the liquid cooling them down.
What i can't say is, if the Ram Cummins diesels have that cooling
necessity
The next time that you happen to stop at a Truck Stop for what ever reason.
Look over at the trucks diesel isle, it is likely that you will see a sign over there that mentions the 3 Minute rule
Sorry, but not one thing in this post is true.
Old Detroit's were dirty in general, and the black smoke coming out of those stacks was unburnt fuel. All old diesel engines did this, not just Detroit.
There is no type of coolant cooling down the turbo blades in the diesel engine in question or any diesel for that matter. The Cummins does have coolant going to the turbo, but that is to keep the VG actuator cool. Some turbos use thermal syphoning which pulls in coolant (even when the engine is off) to cool down the turbo shaft to keep oil from coking on the shaft.
Oil coking on the turbo shaft is the reason why turbos driven engines(not just diesels) may need to idle for a period after hard use and the turbo is hot. The shaft that holds the blades is fed with engine oil to keep it lubricated. If you turn it off after hard use, the heat will heat soak into the center of the turbo causing the oil to basically cook on the shaft.
OP, diesels generally don't use as much fuel when idling as gas engines. On older pre-emissions diesels, idling was not that much of an issue if the outside temp was not too cold to keep from wet stacking, however, with today's emissions equipped diesels idling has negative effects on emissions systems. I generally idle if it is really hot or really cold outside to keep the cab warm.
I also will idle if I need to make a stop(like a convenience store) and the engine oil has not got up to operating temp yet. I would rather idle than put my engine through another cold oil start which creates a lot of wear. In fact, most engine wear is due to the first 10-15 minutes after a cold start because your engine oil is not hot and thin enough to properly coat the entire engine especially in a diesel running 15w40.