Well I noticed this when I bought the truck, the temp gauge bounces from about 1/5 to about 4/7ths (sorry if those fractions are way off), pretty much it likes to go up and down almost in a pattern from about 1/5th or so (normal idle temp), whatever the average idle temp is to a bit less than half. Is this normal? It doesn't just change when giong uphill, etc, its almost like the thermostat slows the flow, lets it warm up a bit, then decides to open all the way up and let it cool back down. Its not getting hot or anything, but it definately is a noticeable change. The guy I bought it from said he had other Dodges that did this, but my 2500 doesn't do that at all. Is it anything to worry about?

 

Totally normal.

What's happening is that the cooling system is designed for heavy loads, it's very large for a pickup. T-stat is closed, air is still flowing though the radiator, coolant in the radiator getting colder, engine coolant getting warmer till the stat opens and gives a big shot of very cool coolant to the engine, gauge drops, the stat closes, temps start rising again. The cycle keeps going on and on...
If it didn't do this then when you got on a hill with a heavy load the cooling system wouldn't be adequate.
I ran an oil temp gauge for awhile, it stayed rock steady even when the coolant temp was going up and down. This indicates to me that metal of the engine is staying a pretty constant temp even though the coolant gauge is going up and down.

 

Thanks thats exactly what I suspected. It gets up to about 170*, I watched it on the way home today. Thats ok correct? So if my 2500 is staying constantly low, that could actually be a problem, such as the thermostat being partially stuck correct?

 

There is an internal rubber seal in the stock stat that has a habit of coming off and holding it open. Very common. I just remove the stat and trash the seal, seems to work just fine without it.

 

If you go to a 5.9 cummins deal (like Frieghtliner trucks) they can give you a thermostate that fixes this problem.... Its about a 34 dollar piece though. I used the old rubber gaskets....seem not to leak at all.