How do you fix 1991 Ford Ranger 4-cylinder heater?

Answer 1

Replace the heater core under the dash board. Fairly easy job if no A/C. But first before you start this, see if is there a coolant leak inside the cab? Usually onto the floor mat and/or what appears to be vapor exiting defrost windshield vent (leaves greasy like film on glass. If above is not happening then check thermostat for proper operation and heater hose connections for leaks. Heater cores are usually no more than $40-50. You should be able to buy at after market auto supply store.