I notice my temp gauge moves quite a bit, almost like the thermostat is opening and closing a lot. Is That normal on a diesel or should i be concerned? Its never gone to H but bounces back and forth just over and under the middle marks. Original radiator, 337K.

 

Also check for an air pocket in the cooling system. Sometimes the coolent overflow tank is full but the system is not pulling coolent over when it gets hot and needs it. This will allow hot 'air' pockets. The gauge reads normal until an air pocket hits the sender and the gauge jumps.

 

That is normal operation. Some have reported a steadier reading after changing the thermostat to the most updated Cummins unit. Mine cycles the same as before.

 

After I replace my t-stat to the cummins one mine stays vary steady.........untill i start to pull a steap hill! got to get a bigger radiator!