I would suggest it is more likely that the heating element of an older thermostat is deteriorating. The amount of current being passed through the circuit changes in time to achieve the same effect on system temp (either by pulse width modulation or voltage).
Similar to a slowly clogging injector, the electrical effort to maintain the status quo is slowly increased over time. Meanwhile, no other difference is engine data would be observable. Eventually the electrical effort able to be delivered maxes out. In the injector analogy, you are pulsing the injector for the whole duty cycle and it can deliver no more - if the system deteriorates any further NOW you will lean out. Back to the thermostat case, NOW you can no longer control overheating and you get the A/C shutoff business.
When you install a new thermostat unit, there are conceivable programming scenarios where the old learned duty/voltage to the heater element will be maintained until the ECU learns that it can get better fuel economy by incrementally increasing the "standard" operating temp to the hotter end of some of the figures discussed here.
*EDIT - except that less electrical effort to the heater element equals higher temp the way it has been described above...? That makes no sense from an electrical engineering perspective. I'm going to try to look into how its controlled.
Similar to a slowly clogging injector, the electrical effort to maintain the status quo is slowly increased over time. Meanwhile, no other difference is engine data would be observable. Eventually the electrical effort able to be delivered maxes out. In the injector analogy, you are pulsing the injector for the whole duty cycle and it can deliver no more - if the system deteriorates any further NOW you will lean out. Back to the thermostat case, NOW you can no longer control overheating and you get the A/C shutoff business.
When you install a new thermostat unit, there are conceivable programming scenarios where the old learned duty/voltage to the heater element will be maintained until the ECU learns that it can get better fuel economy by incrementally increasing the "standard" operating temp to the hotter end of some of the figures discussed here.
*EDIT - except that less electrical effort to the heater element equals higher temp the way it has been described above...? That makes no sense from an electrical engineering perspective. I'm going to try to look into how its controlled.
