Nissan Frontier heater
4 Answers
Most common causes are air in the lines and heater core getting ready to go kaput
Could also be that your blend door is still allowing cool air to come through
Thermostat may not be keeping the operating temperature up to the correct temp.
Can almost guarantee that it's air in the radiator/heater hoses. A good sign is whether the heat increases under load/acceleration. Park facing up an incline and let the truck get to operating temp at idle with the radiator cap off. Squeeze the upper radiator hoses multiple times to get bubbles moving. Top off with coolant and replace cap.
