What likely causes bubbles to form in the coolant when the radiator cap is removed and the engine is running?

When bubbles form in the coolant with the radiator cap removed while the engine is running, it is often indicative of a leak in the head gasket. The head gasket seals the engine block and cylinder head, and when it fails, combustion gases can escape into the cooling system. This intrusion of gas into the coolant creates bubbles, which can be observed when the radiator cap is removed.

This situation usually suggests that the combustion process is allowing high-pressure gases to enter the cooling system, leading to visible signs of bubbling as the gases rise along with the circulating coolant. Addressing a head gasket leak is crucial, as it can lead to overheating, coolant contamination, and potentially severe engine damage if not resolved promptly.

Other conditions, such as a faulty thermostat, low coolant level, or a blocked radiator, may cause overheating or coolant circulation issues but do not directly lead to bubble formation in the coolant due to gas leaks.