In my 2002 Dodge 2500 Cummins Diesel my water temperature gauge fluctuates to both ends of the gauge (from 140-240). When it first happened I stopped and popped the hood and could tell the truck wasn't at 240 like the gauge said. I limped home and let it sit all night. Went outside in the morning and turned the key on and the gauge immediately rose to read the temperature at 240. This showed me it was truly not overheating. In order to try to fix it, I have replaced the water temp. sending unit and the thermostat. When neither of these helped I installed a mechanical temp. gauge which reinforced that the truck was in fact not overheating (or dropping to 140) as the stock gauge would occasionally say.

Any ideas on what is wrong and how to fix it?

 

Welcome to the site. I will move this thread to the Cummins section,
FF