Going down the interstate (not towing, running ~70) and looked down at the the temp was rapidly dropping off to about 150 then started climbing back up to almost 190 and it started dropping off again. It took about 25 seconds to go from 150 to 190 and about ten seconds to drop back down. Called my dad up and he said his 94 does the same thing. Well I got back out on the interstate a few days later and the gauge was pretty much steady with the needle covering the "1" on 190. My dad said his is sometimes steady as well. What's goin' on here?

 

Quirk of the system. The thermostat opens, the hot water gets out and the engine is flooded with cool water from the radiator. The water isn't heated sufficiently to keep the thermostat open and it slams shut. By the time it heats up enough to open, the water in the radiator has cooled enough that it will close the thermostat again when it gets into the engine. It's unnerving, but normal with these huge radiators.
You can let it idle and the water will not cool as fast in the radiator and it should not close the thermostat after it opens.

Chris

 

The more fuel, the higher the load on the engine, the more heat they develop. It is common for the Cummins to cycle when the engine is being run with a light load or at slow speeds.

There is a new (about a year now) revised thermostat from Cummins that has dual valves that will eliminate most of the fluctuation. However, the fluctuation does not hurt a thing, and actually serves to tell you your thermostat is working.

 

Thanks guys. I figured it was just a quirk but wanted to make sure.