I have a 2002 4x2 Sport Trac with 153K miles. Recently, I decided to do some preventive maintenance and install a new fan clutch, water pump, radiator hoses, bypass hose, and heater hose. Obviously, this required draining the coolant from the radiator. When it was time to refill with coolant, I filled the radiator up with 50/50 mix, turned on the heater to full heat with the fan on low and then cranked the engine to let it warm up. The coolant level in the radiator dropped a little as air was pushed through the system, but then the coolant level began to rise until it was overflowing for a while. (This was unexpected and I don't know what would cause that to happen.) Once it stopped, I put the cap on. I also added a small amount to the reservoir, but it was near the fill line. The next day, I checked the coolant level in the radiator after driving it and letting it cool down. It was not full, so I added a little more coolant to the radiator. Same thing happened today. The level in the reservoir appears unchanged. The truck is not overheating and I don't have any obvious leaks. The heater works well, so there is no air lock. The thermostat housing was replaced about a year ago, so I don't think that is an issue either. Do I just have air in the system? If so, how do I get it out? Should I just keep adding coolant until it stays full? Any ideas why the coolant level rose and overflowed with the cap off? Why hasn't the coolant in the reservoir been pulled into the radiator? Thanks for any help you guys can offer!