Have installed the thermostat in my 02 Ram 1500 4.7l making sure it's correctly installed and bled the system of air as described by Remerson, but once the rad cap is replaced the engine is almost overheating after 10 mins. of driving. Is a faulty thermostat the reason or should I try something else first? I ran the truck for 3 days without the thermostat spring assembly with no problems, other than no heat.
Could be a faulty thermostat out of the box. Have you checked for exhaust gas in the coolant? Any bubbling from the overflow bottle? Try starting it cold and revving it with the radiator cap off. If any bubbling comes up, you're getting exhaust gas where it shouldn't be.
It's entirely possible you still have air in the system. the 3.7/4.7 engines are notoriously hard to get the air out of. Is the heater going cold at any time while you're driving it? If so, you still have air in the system. Another possibility is the cap could also be bad. You installed the stat with the spring in the block and the valve up, right?
It's a special tool sold on tool trucks and they run around $100-150, so they really aren't for the do-it-yourself guy. There are a couple of models available but they all work by attaching to the filler neck and pulling a vacuum on the cooling system via shop air. When the gauge reaches about 25 in. hg you close the air supply and open the valve to allow the coolant to get pulled into the radiator.