I just bought a 2005 with 4.2L 167,000 miles. It was running great for a week and then it overheated. I had to put about 2 gal in the system. It is using coolant and I do not see an external leak. The passenger side floorboard is damp but not 2 gal damp. It doesn't appear to be coming out through the exhaust (no smell or white smoke) or dumping into the oil pan (oil level has not changed). Tonight I noticed that the upper radiator hose gets hot and the bottom one stays cold even at operating temp. The heater core hose from the intake manifold gets hot but the return hose that goes to the water pump stays cold. There is no signs of hydrolock and the engine starts and runs smooth. At first, I was leaning toward intake gaskets leaking into the head, now I wonder if it is a water pump problem. Maybe the impeller has come apart or come loose from the shaft and is not spinning fast enough. May the radiator plugged? Has anyone run into this problem? Thanks.