As stated above:
1. Check all hoses for proper fitting, holes, etc...
2. Check the coolant level, do not open when the engine is hot
(top off if low, CHANGE if nasty)
Down the road you may want to invest in a coolant filtration kit
3. Ensure that the hoses on the heater leading all the way from the outside air
vent to the under-dash unit are connected properly, tight, and hole-free
This is also a spot where critters like mice enjoy building nests......
4. Check the heater set up, ensure that the hoses are properly connected to
the engine in the correct locations in the cooling system.
(If incorrect, disconnect and re-connect in proper locations (refer to pic below
Otherwise, you will most likely be robbing the engine of needed heat when
you operate the heater.
5. Make sure the valves in the sytem are all in the OPEN position
6. Turn truck on, open air damper, turn on heater. Truck will need to be near
operating temp. for you to get decent heat. You can place a cardboard cover
over the radiator to allow the engine to warm up more quickly in cold temp.
7. While running, crack open the small valve on top of the heater to ensure you
have a steady flow of coolant to the heater.
This is exactly, step by step, how I diagnosed my heater after I recovered my truck from Mechanicsburg PA. The hoses on my truck were INCORRECTLY connected, I found this out while barreling down the Turnpike in January with no heat, during a blizzard. LOL
I hope this helps, if you have any other questions feel free to post or pm, good luck!!
PS!: I installed the 1/4 Turn valves recently, when I recovered my truck, they were the old Petcock style twist valves, identical to what you will find if you look at the bottom front of your radiator, just behind the front bumper.
