The most likely cause is a leaking cylinder head gasket. That would allow combustion gases under pressure to sneak into the cooling system. The radiator cap will limit pressure to 15 or 16 psi, then you'll see excessive bubbling in the reservoir.
Usually the additional symptom is you have to add coolant periodically. The best test to verify a leaking head gasket involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.
Here's a link to an article that describes this test better:
https://www.2carpros.com/articles/head-gasket-blown-test
Mar 21, 2019 at 6:42 PM
