I bought a 1978 f150 with a 6.9 idi swapped in it. But im having a problem with overheating at idle. When i drive the trucl stays no higher than 198 or so but when im sitting at a light pr just idling itllget very hot over 215 anx i have to shut her off or start drivimg again for the air to cool the radiator. I replaced the thermostat already with a motorcraft one and didnt really change much. It is the original radiator from the 78 so im wondering if that could be the proble, not big enough or clogged up. The fan seems to be in working order. Please help thanks

 

There's a good chance that radiator isn't up to the task. The radiators in my 93 and 86 were both really clogged and still wouldn't let the trucks overheat. Do you even know what size motor that radiator was cooling? The IDI cooling system should hold 8 gallons of fluid as well.

 

I dont know what motor but i had to make a fan shroud because the fan from the idi is alittle bigger than the radiator. Im thinking mabe an all aluminum radiator from a 460 or 7.3 should work. Is there any way to tell if its clogged?

 

I could tell mine were clogged by looking in the cap. Most of the tubes I could see had crud in them. If your fan is bigger than your radiator, it sounds like you have a small block gasser unit. A big block radiator might work as I am sure you would have to do some modifications to fit the diesel unit in that core support.