Originally Posted by Remerson228
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?
Second thread I've heard Remerson228 mention using vacuum to bleed air out. Any more detail on procedure? What type of vacuum pump do you use?