With the radiator cap removed and the engine warmed up, seeing bubbles in the coolant likely indicates what issue?

Seeing bubbles in the coolant with the radiator cap removed and the engine warmed up is a strong indication that there is an issue with the head gasket. When the head gasket leaks, it can allow combustion gases to enter the cooling system. This intrusion of gases results in bubbles forming in the coolant, as the gas displaces the liquid.

This situation typically arises when the engine is under load or has reached operating temperature, as the heat can exacerbate leaks. Such leaks might not be immediately visible outside the engine, but they create visible signs like bubbling in the coolant when conditions are right. Detecting this symptom is critical because it can lead to overheating and significant engine damage if not addressed promptly.

The correct identification of this issue as a head gasket leak is vital for effective diagnostics and repairs, ensuring that appropriate measures are taken to resolve the problem before it escalates.