Since I am new to driving a diesel I have a question. Is is better to leave diesel engines running or turn them off for short periods?
For example, if I pull up to the house and am going to run inside to grab something (let's say five minutes or less), is it better for the engine (300SD) if I leave it running, or should I just turn it off to save gas?

 

doesn't matter.
you can turn it on and off just like a gas engine.
i have owned 2 mercedes with diesels., and then the large Detroi 671 on my 50' fishing boat..., doesn't matter.

kevin

 

Basic diesel (even Detroit with a blower) can be turned off without restrictions, however turbo on some might require some cooling before shutting down.
Mercedes have an excellent turbos, but when you run them hot and turn off right after, the hot temperature of turbo can cook the oil in it into charcoal, what is not good for future lubrication.
For short stops like mentioned I would leave it running. Diesel idle almost on vapor and it is better to be on safe side.

 

When I went to school on the 9.0 L international (many years ago) they told us to leave them running. They said that the diesel engine is at its most efficient at idle and only uses enough fuel to overcome the internal friction of the engine.

We were told on our turbo engines to allow a 2 to 5 minute cool down period after extended highway use.

Picture this: your turbo is at 1200 to 1300 degrees (I know thats a hair high but extended WOT could easy get you there) and you turn the engine off. The turbo has to wind down from whatever speed it was at when you pulled up. With the engine off the turbo is spinning and the bearings are being oiled by the left over oil sitting in them since there is no oil pressure with the engine off. This oil is getting hotter and hotter and as kajtek1 said "the hot temperature of turbo can cook the oil in it into charcoal" ------------ Its easy to see how.

Another thing to consider is hot spots in the engine. Letting the car idle a few minutes allows the coolant temp to even out the overall engine temprature reducing the hot spots. I've had a few cars that you could hear the coolant boiling a few minutes after they were turned off. I live in the south where it gets HOT. We like to call it hell on earth.

With all that being said, I like a little cool down period on my engines but I'm not sure you have to. I consider it cheap insurance.

 

I've been told if you have been driving hard, or have climbed a hill, definatly leave it run to cool the turbo. Likewise, if you park at the bottom of a hill, let the engine warm up before driving so the oil lube the turbo easier.

My 300SD is very sluggish at first in extreme cold weather until it warms up. The oil is too thick to allow the turbo to spin correctly (I'm probably using the wrong viscosity oil).

In any case, make sure the auxilary electric fan is working properly if you are idling for an extended period.

 

One exception.......SYNTHETIC oil will not turn to burned-on thick chunky tar that clogs oil passage to turbo bearings..........only standard dino type oils will do that when not let run a few minutes before shut off after highway stops!