I have a 2004 Duromax with 30000 miles on it. The last time I towed with it I noticed the temp. rising when I was climbing a hill. I slowed down and turned off the A/C and it went down. Anyone having this problem, and what is causeing it?

Oh boy, how much time do you have?

First you need to know which motor you have. It's either a LB7, or LLY. If it is the latter, go to the 'Second Generation' threads under the Diesel Engines header, and do a search for "overheating". Be forewarned, there are probably over a 1000 posts, on multiple threads.

Have fun!!!