Question

Why is my coolant bubbling in the reservoir even though the engine isn’t overheating?

If your coolant is boiling, then the issue is probably with the air in the cooling system. But shouldn’t it cause the overheating of the engine?  

Firstly, there is no need to worry much if you are planning to address the issue immediately. The main reason for the coolant to bubble in the reservoir without engine overheating is air bubbles in the cooling system rather than actual boiling.

This is mainly due to the following reasons.

Trapped air in the cooling system, after a coolant change, can block the proper flow of coolant. However, if unresolved, it could eventually lead to overheating. 

Blown Head Gasket or Faulty Radiator Cap can allow combustion gases to enter the cooling system, causing bubbling. Similarly, a bad radiator cap can fail to maintain the proper pressure, leading to air intrusion. 

A malfunctioning thermostat can disrupt the coolant flow, causing bubbling even at a steady temperature.  

It’s essential to address the issue promptly to prevent potential overheating or engine damage.