I am having trouble finding out what is the problem with my 1991 ford 300 I6...it will overheat on hot days (90+). It seems to do it when it wants to kind of cause i let it set for 5 mins came back to it and it was boiling over, but it wasn't boiling when i parked it.

I replaced the thermostat because i though it was sticking and not opening but now i don't know what could be causing it.

 

Fan clutch.

I had the same problem that got even worse when I turned on the a/c. I replaced the fan clutch and now it runs cool.

 

Also check your radiator cap.

 

^x2

Have you flushed the system?

 

ya i flushed the system when i replaced the thermostat. I have another fan clutch sitting somewhere so Ill try that.

 

As Warlockk mentioned - don't overlook the importance of the radiator cap.

A 50/50 mix of coolant has a boiling point of 227degF. Each PSI pressure adds about 3degF to the boiling point, so our 13psi caps will raise the boiling point up to around 260degF, give or take.

That being said - the fan clutch, coolant flow restrictions inside the radiator, air flow restrictions outside the radiator, or a tired water pump are also suspect.

A collapsing lower radiator hose (suction side of the water pump) is a bit more unlikely, but possible.

 

i replaced the radiatior cap...its a little less hot out now and i dont drive the truck that far very much so ill have to see later on