what is the long term harm that may occur if a large portion of your driving is around the 10 mile distance mark. most of the time
my truck gets to normal operating temp[the 90 degree c mark]
but with cooler temps coming sometimes it only gets to between the 60 and 90 mark...... i have heard in the past that this may be harmful long term with a diesel. Is this true and should i worry
about it? what exactly hurts the motor?

 

That is true with any type of engine. The whole system needs to reach normal operationg temp in order to properly displace the moisture in the system. What yo may have to do is what I do, I usually leave a little early and go a bit out of my way to help the system warm up porperly. Those few extra miles I put on my truck IMO outway the effects of improper engine temp

 

I have a 15 mile drive...In the winter(starting about now)I plug in my heater into a timer..it comes on around 2:00am and has a bit of warm air when I leave at 5:30am. I have the cord anchord to the wall and the plug fed out under the grill so when I back out it unplugs itself. It sure makes it a warmer drive to work...and I agree short hops are hard on all rigs so I might as well be comfortable!