I just finished up an engine and tranny swap on my 1991 f150 4x4 XLT. It originally had a 302 with a mazda 5 speed. I swapped in a built 302 I had and swapped to a C6 auto. I can get it to run and idle but when I drive it the temp just goes through the roof and blows out the over flow when I shut her down. I replaced the thermostat with a 185. Flushed the radiator before I installed it and checked all the hoses. I pulled the water pump off last night and I took it apart and cleaned it. There was some gunk in there but nothing major. I am going to replace it tonight when I get home with the new gasket.

I can't seem to find anything else that might be causing this. Any ideas?

Thnaks in advance!

 

Maybe you didn't completely fill it with coolant. It can be tough to fill all the way because it won't seep into the block. Keep me up to speed on how the conversion goes. I'm swapping a 351W for a 94 302 and have some of the same questions you have posted (Knock sensor and TAD solenoid).

 

Like frenchie said, it might not be completely full of coolant.

The closed thermostat keeps the coolant from filling the block when you fill the radiator. It holds an air bubble in there so the new coolant can't equalize with the top of the radiator.

To prevent that, leave the radiator cap off and idle it until the thermostat opens and the gauge reads 'normal', then top off with coolant.

Or... leave the thermostat off until you fill the block.

If the new engine sat around a long time, the coolant passages in the heads could be restricted too. Power flushing the block might help. But you said it was a 'built' engine, so maybe it's fresh.

Also: Have you confirmed that the fan clutch is working?