I have a 2003 ford F-250 powerstroke 6.0 diesel and the heater will not work and over heats under a load. I have tested for a blown head gasket and replaced the raidator heater core thermastat and water pump and did and egr cooler by pass and no hoses are sucking shut and the heater doors and valves are working properly any ideas?

Reverse flush the heater core, engine, and radiator. If the overheating problem persists, replace the radiator. These links may give you some ideas:

https://www.askmehelpdesk.com/cars-t...tml#post235973

https://www.askmehelpdesk.com/cars-t...ml#post1923468

If you have an Electronic Air Temperature Control (EATC) system, find out how to perform the self-test diagnostics. On my Ford, you turn the ignition to RUN, simultaneously push OFF and Floor buttons; then, within two seconds, press the AUTO button. The test may run as long as 30 seconds. Record all DTCs displayed. The self-test is completed when 888 appears in the center of the climate control display window of the ICP. To exit the self-test and retain all intermittent DTCs, press the "-" (cooler) side of the TEMP button. Always exit the self-test, before powering the system down. Many control problems can be solved just by running the self-test.