I have a 1993 f350 7.3 when I put a heavy load behind it, the coolant system will have so much pressure it blows the resovor cap of and will cause the truck to over heat. What can be causing the back pressure?? When I do not have a load I don't have any problems can drive it all day and will never over heat only with a load behind it??

 

sounds like a headgasket combustion leak. I would pull the heads and get them checked for cracks by a machine shop. Good luck.

 

Um, you'd probably want to TEST for the combustion leak before assuming it's there and pulling the heads.

What anti-freeze concentration are you running? Did you try replacing the radiator cap?

 

I'd look at the radiator cap, too. And probably replace the thermostat, mine was stuck part way open so it ran too cold with no load, and way too hot with a load. But the cap may not be holding the correct pressure, it's common after a certain age.

 

welcome to FTE.

first thing i would do is drain the coolant and flush out the radiator.
preferably removing the rad and flushing it in the bottom and out the top.
my bet is a partially clogged radiator.

this can easily be tested with an infrared thermostat. shoot various places across the radiator surface. it should be hot on top, and progressively cooler as you go down. if you find areas where it is hot all the way down, or almost all the way down and cool almost from the top down, those cool areas of the core are plugged and not working to dissipate heat.
this will cause the areas that are not clogged to not let the coolant cool properly, giving you an overheat condition under load.