Not always...there was a issue at one time called "tick-tocking" where the temperature would bounce up and down, and it was caused by a faulty/failing thermostat.

I know what they are talking about as I had a truck do this. Normally, I see a swing of three degrees and that doesn't even register on the in-dash guage. On one that is tick-tocking, you will see a 10 to 15 degree swing, and it does register on the in-dash guage. A new thermostat (from Cummins) fixed my problem.

 

Well my 12v did this from day one and still did it when i sold it @475k with stock thermostat, my 05 does it when it gets hot out and since the temp probe in right next to the thermostat it just makes sence for the temps to flucituate while it's opening and closing. JMO

 

Just because it did it, doesn't mean it was right...

 

Well when the therm. is shut the temp is gonna go up and when it opens its gonna go down, right? w/ the temp sender right there its gonna have a change in temp when it opens and shuts,pretty simple. If you dont belive me put a new thermostat in and put a mechanical guage in and tell me the outcome.

 

I have a mechanical guage, and a thermostat should open/close smoothly, not "jerky"...already been down this road. Have seen several trucks this way, and its always been the thermostat.

A fluctuation of a couple degrees is normal, a fluctuation of ten plus degrees is NOT normal. A fluctuation of a couple degrees WILL NOT REGISTER on the factory guage, however; ten plus degrees will.

 

so my thermostat may need to be change, i currently have a fluctuation of about 10 degrees