'02 Chevy Venture / on & off overheating

'02 Chevy Venture minivan; 110K miles; suddenly overheats (pegs @ red, back down while driving, up again); husband (mechanic) replaces thermostat, figures it’s stuck. However, the temp gage continues to fluctuate while driving; has pegged @ red twice, but always returns to “normal” (half-way). Ideas? Suggestions?

Engine temperature changes aren’t sudden! The problem is electric/electronic. My crystal ball sees an automotive electrician in your life.

Ideas? Suggestions?

Faulty temp sending unit. (he can easily replace it)

Air pocket in the cooling system. (he can easily bleed the air out of the system)

Bad head gasket. (not so easy)

Pray that it’s the temp sending unit or an air pocket.

Perhaps the start of the infamous GM leaking intake manifold gasket is happening to your engine. (Coolant leaking into the engine oil)

Do not keep driving it if it keeps overheating as you won’t be driving it very long.

Unless there is a plugged rad and you’ve replaced the t-stat with no change, that is likely the problem.

Do you see the coolant slowly but steadily disappearing from the coolant reservoir with NO visible leak?

Also, check the bottom of the oil filler cap for light brown sludge.

If you see these things, take the vehicle to your tech/shop for a coolant pressure and leakdown test.

Could be a bad gauge. Same thing happened to a car of mine with your symptoms.