Overheating 7.3
 

I have a 2000 7.3, I have washed out the radiator and changed the thermostat, fan clutch seems to be good and the fluids are good,
My truck over heats when only in the afternoon after about 15miles of highway driving. I can turn on the heater and it will cool down, I don't have this problem on the morning when it's cool outside. I figure it's not the water pump based on when it overheats. Please help

You could have a bad thermostat. You can test it by putting a thermostat in a pan of water bring it up to a boil and see how far it opens up

I tested the old thermostat and it was still good and haveing this problem still.

Do you know how to check the fan clutch and it could also be due to have to much dirt in between the fins on the radiator and condesor easy way to tell is hold a pease of paper infront of the grill and it should pull it towards the motor and it being a 7.3 they have few over heating issues and the other thing is have you put the additive in from ford to keep from building calcium and cavating the heads if not you could have ports in the head clouged up

i ran into an interesting problem a while back on a 7.3 it slowly got worse over a couple years but wouldn't go far in the end with out getting hot. i replaced all including thermostat and still did it so i finally pulled the radiator to have the tanks pulled off and roded. the center of the radiator from top to bottom was plugged so not sending heat to the fan clutch to activate it. i went ahead and bought a new one and problem solved. the guy that tested it is a radiator shop. been doing this for 30 plus years and said plugging down the middle like that is a common problem on diesels. never knew that or would have guessed that but took him at his word.

that and he showed me the plugged section lol