I just bought a 91 f-150 ext cab short bed 351W w/e40d transmission brought it home on a trailer due to a rusted coolant line (on intake) I fixed the line went for a ride (45m-1hr) and it over heated I limped it home got a thermostat went to change it and the housing had bits and pieses of a thermostat nothing big enough to create that much blockage what could be causing it to over heat (the heater core is bypassed and I had straight water bc I was checking for leaks. The temp gauge hasn't gone over the "L" on normal but the tank over flowed ... I need Help !!!

 

Sounds like it is time for a compression test. And/or a hydrocarbon test.

Are you getting any steam out of the exhaust after it is warmed up?

I did an Explorer for a friend not too long ago, had a cooling problem at highway speeds. Pulled the radiator and cleaned the outside from all the feathers, sticks, dirt, and mud. then I straightened all the fins so air would flow through. Put in a new thermostat and all was good.

Another thing it could be is the water pump, even if it isn't leaking. The impeller could be rusted apart and just not moving the coolant like it should.

Good luck.

 

Radiator could be clogged up and needs the passages cleaned out. Basically a radiator shop removes the top of the radiator and shoves a cleaning rod through the passages.

If water is not flowing well through the radiator due to corrosion, you will overheat.

 

Bad rad or water pump or blown head gasket or cracked head or ??

One test you can do for the gasket/cracked head is when the engine is cold, remove the rad cap, start engine and see if you get bubbles coming into the rad.. If you do, it's most likely a head gasket or cracked head.

If no bubbles, start thinking new rad.

You can pull the pump and see if the impeller fins are still okay.

Good luck!

Mitch