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When I drove school bus many of them had electric recirculating pumps. The best method was to shut off the engine and turn on the recirculating pump, as it only pumped from the engine to the heater cores. The radiator cooled off very quickly and the thermostat closed, but the hot engine would heat the cab for an hour or more. The running engine kept coolant going to the radiator and would actually cool it off faster, circa 15 minutes for an idling bus engine.
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