I have a '95 windstar, heater blows cold, rev the engine you get heat, heat circulates entire system even rear heater, changed thermostat - no help, changed heat sensor - no help, changed water pump - no help, elevate front end to refill radiator and aid circulation - no help, engine runs great - no warning lights, at wits end - please help.
The flapper door that allows the flow of either hot or cold is not opening fully , caused by a vacuum leak. Once you rev the engine you are creating more vacuum and this fully opens the flapper door until you release the accelerator. Locate and repair the vacuum leak and this will cure your problem. *note*This is a common occurrence.