The expansion tank performs the task of absorbing the excessive coolant. It also reduces the pressure in the engine cooling system. An expansion tank reduces coolant hammer and protects the engine cooling system by maintaining a minimum pressure rise during heated water expansion. 

If the expansion tank is missing or malfunctioning, the boiler's pressure relief valve may vent water due to the pressure. The signs of a coolant expansion tank gone bad are coolant leak, coolant odor, overheated engine, low level of coolant.

You may temporarily repair the leak of an expansion tank, but, we advise replacing it eventually.