My 99 TD overheated this weekend sitting in traffic for 20 minutes and then a few hour later going up a steep grade into the mountains while pulling about 5-6,000#. While leaving the mountains in 75 degree heat going up about a 1/4 mile moderate rise I saw the temp guage rise like a speedometer until I crested the hill, then it went back down. I never heard a fan turn on. On the flat with 106 degrees there was no overheating. Any suggestions are welcome.

 

The subject is covered very well in the Towing and Hauling / RV fourum. Do check it out.....

 

When was the last time you flushed your coolant? I did mine this weekend and the truck runs a helk of alot cooler now. It will still get warm, but as soon at it hits ~195*, the new t-stat opens and it drops back down to 180 or so. Haven't driven w. a load yet to see. Also might want to check the radiator for any trash/debris clogging airflow. It wouldn't be a bad idea to wash and rinse the thing too. You'd be suprised what kinda junk comes outta there. Good luck.