While driving my temp gauge goes up and down.and heat never gets hot just warm.new thermostat

 

Mine died the same thing, only difference is my heat blows hot as hell

 

You've got a bad thermostat or a whole lot of air in your cooling system.

 

Betcha money a cummins thermostat will fix it. We see a lot of folks installing stants and come to us with this problem.

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what about having this issue before the tstat was replaced like on my truck?

 

Could be a number of things. Anytime you have wide fluctuations that are persistent we would look for as mentioned above air in the cooling system, air entering the system via a leak (pretty rare with these trucks) or possibly a faulty gauge. In theory a water pump can cause it but unless the fins are missing a faulty pump is pretty hard not to notice. In very cold climates a big cooling system will experience variations.
I would start with a coolant funnel and verify there's no air in it and move from there. Obviously a faulty thermostat could cause similar issues as well.

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So I've replaced my thermo,added an aftermarket gauge, and water pump seems fine. I will give the funnel trick a shot.

 

Did you use a cummins thermostat?

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no, i used a failsafe from autozone, that way if it does fail it will fail open

 

We sure do replace a bunch of the fail safe thermostats in otherwise normally functioning cooling systems. Might be wise to do some research on those thermostats and you might find that they cause more trouble than their worth!
Did the thermostat you installed have the "jiggle" valves? They say they have a "bleed notch" but they just don't work well. Often cause drastic temperature swings. If I were in your shoes I'd order a cummins thermostat and see if that fixed my problem.

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