My 94 gas 6X4 has started overheating. I replaced the muffler and decided to check the water pump while the exhaust was off. I removed the cover and all inside looks new and very clean. Impeller looked new, vanes on impeller perfect And the pin is in place on the impeller. You spin the engine over and impeller rotates. Cleaned cover and pump surfaces put on a new gasket on a closed it up. Topped off radiator and all seemed fine. Ever since the temp light comes on after about 15 mins. After cooling off I removed the radiator cap and it’s full. Ran with radiator cap off and it’s moving coolant. Put cap back on and it did it again. Changed thermostat, no help. Found bleeder plug, bled it and fluid moving with cap off. Put cap on, ran 15 mins, hot again ! Took thermostat out, same thing. Removed new exhaust and checked pump again. All seems well. Bled again, same deal..15 mins and hot again.Elec fan comes on but engine doesn’t cool off. Frustrated !! Any ideas??