The combined heater and A/C unit in my 1974 145 doesn't put out the heat that it used to. The air temperature is warm, not hot. I replaced the heater control valve with a piece of tubing because I wasn't sure that the valve was fully opening. That didn't change anything. I replaced the thermostat on the engine just because. I disconnected the hoses at the firewall and reverse flushed with a garden hose. No scale came out. As far as I can tell, no outside air is entering the heater core in Recirc mode. The Recirc control does open and close the door to the passenger compartment. The core isn't leaking. My only theory is that the core has failed in some non-visible way such that the coolant is bypassing part of the core and flowing from the inlet to the outlet without going past as many of the fins as it used to. Do cores fail like this ?
Thank you for your suggestions.