Due to cavitation, over time they will sometimes lose the vanes. They are prone to turn loose and spin on the shaft. Cummins uses a lot of phenolic impellers in replacement pumps. If the bottom is a lot hotter, it is not circulating. A quick test for a suspect Cummins water pump is to attempt to lay your hand on both top and bottom radiator tanks, when the engine is showing hot.
If no manual switch, jump the solonoid with a test lead to make the fan operate, and test drive it. If it is air operated look for a manual switch. Whether viscous or air operated the fan clutch would be the first suspect. Joe, on models with Horton, or similar fan clutches, the same Kysor shut down also controls the fan clutch. He replaced the water pump with no luck and is wondering what to look for next. Bobtailing it does fine, but even pullin his savannah log trailer around EMPTY it'll try to heat up. Just the least little incline it'll heat up then shut it's self down. The truck is a low mileage truck around 400,000, and the fuel is set on the factory settin. He was haulin full time with the truck but bought a Western Star and only uses the cabover to pull a reefer trailer around a 100 mile round trip on saturday. A buddy of mine has a '86 International cabover that has all of the sudden started overheating on the least little hill.
