2002 Monte Carlo temp gauge bouncing from high in the red to low

I have a 2002 Monte Carlo what recenty had a new radiator, thermostat, intake manifold gaskets replaced. I was spewing coolant everywhere. Since the repairs with all new OEM parts, I still get the 'Hot Coolant Temp" warning on my dash. The temp guage is pinned at 260 but minutes later drastically drop to under half way after which it seems to balance out at slightly under half way. I also see the 'Low Coolant" as well even though I am full. All of this ocurring while driving. I checked the oil and underneath the oilcap. No frothy or milkiness. Exhaust seems ok, no white smoke. The heat inside my car when i run it seems to go from warm to cold and back again. I’m praying it’s not a cracked heard or blown head gaskets. Help!!!

Was the system properly ‘burped’? If so it’s a pretty certain indication of a failed head gasket or cracked head. If the thermostat had a bleed hole in the base the wild swings would likely be lessened and possibly forestall catastrophic failure for a while. But more than likely the head will be coming off soon.

That’s what I was afraid of hearing. Thanks for your input!

Good luck.

It was sounding like a head to me too, though you would certainly want to verify this before doing anything at all. There are gadgets that can check for combustion gases in the coolant. And this is a way that a head can fail without any mixing of coolant/oil or coolant into cylinders - just a kind of breach where the biggest symptom is exhaust blowing out into the cooling jackets.

You could also hope that the system wasn’t fully bled or perhaps that your water pump is going squirrely.