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Sounds like the thermostat may be stuck open. Buy the original temperature thermostat that the parts book calls for, to help eliminate condensation build-up inside the engine(brown foam stuff under the oil cap).
Go to your local auto parts store or Ford parts counter. Tell them the year and engine size of your truck. Let them supply you with the correct thermostat. It will probably be 195*. I've found that the Motorcraft thermostat will let my truck warm up a little faster than aftermarket brands. The Motorcraft thermostat has fewer and smaller relief holes drilled around the outside flange.
Went to the nearest Ford store... Surprise! They were out of stock. Drove to the next Formoco. They had one; but sold me the wrong gasket. No wonder the after market people do so well. Real weak supply system. Put the new one in after modification of the gasket. Now I have a heater! Truck seems much better. Thanks. Webster