I'm working on a high mileage Chevy 2500hd with the 6.0 gas engine. It seems to overheat when it's worked more than just cruising around empty. I was plowing snow a couple weeks ago and it was getting hot in a short time and I was not plowing any different than I ever have. Today I was pulling a trailer with a tractor on it down the highway and it was getting hot and it's pulled this load many times without trouble. This truck has had a new radiator and thermostat in the past couple of years. The fins don't seem blocked. I have also changed the fan clutch before. My question is can water pumps wear out without having a leaking? Can the impeller wear out? This truck has 460,000+ miles in it and it's still the original water pump.