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I posted a thread a while back about this, and yesterday finally tried to fix the problem.
Problem: Even after driving 10-20 miles heater is only mildly warm.
Attempt to fix was, replace thermostat, flush out heater core and radiator. I drained the radiator. took 1 of the heater core hoses loose, and attached a garden hose to it. Started the truck and ran water thru the system with the engine running and the thermostat removed. Then swapped the garden hose to the expansion tank for a while. Did this back and forth until clean water ran out of ever possible opening I could open and still run the engine.
Then I put in some prestone coolant flush. Let it idle a while, drove it a few miles. Came back drained radiator again and flushed with clean water some more.
Refilled system with new coolant/antifreeze and water mixture, after reinstalling a new thermostat. I then let it idle a while to warm and mix the new coolant. Turn on the heater and its blowing good hot air. Park the truck.
Got in the truck a little later last night, drove it for a little bit and the heater never got hot. But the heater hoses are hot to the touch, but not sure if hot enough to heat the cabin. The system is full of coolant.
Anyone have any clues?
Am thinking air is still in system. Keep running it with the rad cap off, and heater on. When I replace hoses and belts on my 02, was very difficult to purge out all the air. In a normal functioning system both heater hoses will feel just as hot as the top hose. (Return from heater a little cooler). Can you see if the blend door actuator move full travel when actuated?
If the engine is at operating temperature, and both heater hoses are hot - as in 'difficult to hold onto for very long' - would suspect the temperature-blend door is not swapping over or sealing off completely.
One check may be to move the temperature selector from hot-cold-hot several times to see if there are any changes or any unusual noises like thumping or popping. Wondering if it may be on the edge of sticking or binding - as you said it did get hot once.
May have to bite-the-bullet and lay some eyes on the blend door actuator. Not for sure on your model year if there's an access cover to remove to be able to stick your hand up in there to feel around.
If you see your temperature gauge making some unusually wild swings - a typical sign that there may be air still in the system.