Heater starts out find ,blowing a lot of heat,drive about a hour have to put the fan button on high just to feel a little heat,I put a thermostat in it ,that did not help
The system either has air in it or you've got the beginnings of a clogged heater core. Try doing an aggressive flush on the system, and when you refill make sure there is absolutely NO air trapped in the upper hose.
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