The waterpump usually either works or doesn't. Most of these problems are related to poor circulation in the radiator and partially opening thermostats. If the pump is quiet and not leaking, I would look elsewhere. BTW, flushing with a garden hose will not remove scale or sludge in the core unless you have a lot of pressure and flow. It may seem the flow is adequate, but the real problem is heat transfer not necessarily flow. Actually more heat is transferred at lower flow rates all other things being equal.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.