This is a typical automotive coolant system with the heater core in the upper right. There are generally 2 typical issues with our wrangler heater cores, the most common is a plugged core that either restricts or stops proper flow thru the coil and a failing coil that commonly presents itself with a sweet coolant smell in the cab along with a coolant drip from the AC drain on the firewall near the battery. If it's performing well then just salute and keep going. System performance is greatly enhanced by a well maintained cooling system and most issues/failures can be attributed to a lack of maintenance other than wear items such as the water pump and T stat.
One sign your heater core is bad is if your getting water stains or wet spots in the cab Or No heat when the heater is turned on. For example my TJ heater core was plugged (when the heat was on it was blowing cold air). So, I disconnected the two heater core hoses in the engine compartment. Took a garden hose and flushed one hose at a time and this blows/circulates the fresh water through the heater core (that mini radiator in the TJ cab) with fresh water. Some soot and crud came out of the opposite hose I was not flushing and eventually clear water came out (the car heater is a closed loop system). I Re-connected the hoses and the heater works fine now.
