Boiling Coolant

My daughter has a 2002 Chevy Malibu. Not all the time, but at random times, the coolant in the overflow tank boils. I have replaced the thermostat. It does not seem to be overheating when this happens. Any ideas?

There might be some debris (rust scale, etc) floating around in the coolant. Every so often it gets lodged in a radiator tube and impedes the flow of coolant. [Plausible theory, huh?]. Drain the coolant. Use a garden hose to flush the cooling system. Refill with your favorite brand.
I hope this work because the theory of quantum cooling states isn’t well understood.

These are notorious for intake gasket leaks. I am not sure if that is your problem, but I will check that before any engine damage happens.

There may be nothing wrong. When the engine is running and is not fully warmed up, some expansion takes place and could make it seem like the coolant is boiling. The coolant is constantly expanding and contracting if the fan and thermostat are working right.

I tend to think that Galant may be correct about the headgasket theory. I would have a block check done to see if there are exhaust gases in the coolant.

Agree, with a GM car of that age, always check for intake manifold gasket problems first. I had an oitherwise great Caprice, but when it developed problems the shop immediately identified the intake manifold gasket.

If headgaskets were shot then the engine would probably pass combustion to coolant if blown on a coolant passage. But this usually causes overheating. But cyl combustion in coolant may apear to be coolant boiling. This is easily verified with compressed air in each cyl at TDC or as cougar said, verify exhuast gas in coolant at your local reputibal shop, if your blue test liqiud turns green you have a head problem.

this model came from the factory with dex coolant.have you changed your antifreeze on this car.