Check the thermostate. Is the Thermostat staying closed or is it opening at running temperature. Is the cooling fan operating on low speed and high speed. To check the high speed operation, switch the ignition on and pull the Engine Coolant Temp Plug off the sensor on the termostat housing, the Cooling fan will run at high speed. Run the engine to running temperature and then operate the interior heater at maximum temperature with the interior fan on Maxinum speed and see if the Engine Temp cools down.