HELP! Rapid and large changes in water temps!

[AdidasCU]

Drove the truck for the first time in a month, and the temp is jumping from 1/4 mark to over half within seconds.  It used to stay steady at 1/3rd..

 

What's the deal!?!?!? please help...  97 tahoe BTW

[gave20]

is the coolant low? If not the thermostat could be stuck closed.

[BlueRado]

It could also be that your gauge and/or sending unit are bad.  I'd check my coolant and thermostat as posted earlier, and add when the engine is cool, if needed.

 

I've seen pretty explosive movement of the temp gauge, but only upward, and you'll see steam, or other noises, etc that signal overheating, if that's what it is.

 

Good luck!