Farmboy, depending on the year of your truck, there is flap that either closes off the water to the heater core or redirects the fan to blow through the heater core. this flap, if in the heater core hoses, can get stuck due to age and even if you are putting the controler to heat, the cable wont move it enough. If it is a door to redirect the fan to blow through the heater core then it could be stuck due to the fact that most people throw stuff on thier dash and it falls through the defrost vents and into the duct work. The most common problem is the thermostat, even if you have changed it. The only way to know is to actually test it in a pan of water with a thermometer. if it opens before 192 degrees it is bad. You need at least a 192 thermostat to get good heat in our area. It also improves fuel milage. I have changed many more than once to find out that the new one was bad. In our economy the cheap part isn't always the best part. The water pump is not the problem. The water pump does exactly what it says, pumps water. The reason we have a thermostat is to restrict the water from pumping untill the motor reaches a certain temperature. So the theory that the water oump could be bad is wrong. Did you have this problem before you replaced the heater core? What is the rading on the temp gauge? If it isn't around 200 degrees then you have a thermostat problem, the correct running temp should be around 200 to 220 degrees. The clutch fan only cools the water in the radiator, not in the heater core, and the thermostat has to open to let the cooler water in. the clutch fan is not the problem either. First thing I would do is check the thermostat and the duct work. Cheap and easy fix. If you want info on how to do this, hit me back.