need a new thermostat or water pump
 

i have an 89 gmc r3500 with a 454 and it overheated on my drive and could only drive a few miles before you had to let it cool off some.so i ended up removing the thermostat and it made it the final 5 to 7 miles back home without overheating.i looked into the radiator and you can see water coming from the driver side of the rad flowing into the passenger side with the thermostat out does that mean the pump is ok?

If your truck did not over heat with the thermo out then it's the thermo. If it did over heat with it out then its the pump.

ok so since it didnt overheat any more in that 5 to 7 mile drive with it out it more then likely the thermostat.

Yup. 99.999% chance it is the thermostat.

Water pump pumps coolant to take excess heat from the engine and deliver to the radiator. The radiator transfers a bunch of that heat to the air. The thermostat restricts flow of coolant to keep the engine at an efficient operating temperature for power, emissions, efficiency, metallurgy-concerns, HVAC heat stability, etc. When it works correctly it opens at the specified temperature to allow the coolant that is warm in the engine to move to the radiator. Sometimes, however, a thermostat will get stuck closed. That appears to be the problem you had before removing it.

What if an air pocket was created by a coolant leak?

Hmmmmmmm . . .

Good point.

For it to start that quickly it would probably have needed to be at least a small puddle on the ground where it was parked before he drove it this time. If he missed the puddle and/or it was parked on dirt/gravel/etc. then removing the thermostat allowed the air pocket to move to the radiator and hopefully to the overflow tank. If that happened then putting in the new thermostat won't fix the leak (unless it was leaking at the thermostat housing).

wes104 , after you install the new thermostat, keep an eye on coolant level and look for leaks.

ok will do.

see it might help some if i explain some.when i took the trip i drove to the town thats about 20 miles away and it didnt overheat on the drive there but it did on the way back.i was about 5 miles away from home so i just took the thermostat out and it made it home without a problem.also i dont know if it matters but the top radiator hose had alot of pressure on it and when it overheated the radiator cap let out some water.thanks for all the help so far.

if it was the water pump or a air pocket it would be all the time wouldnt it not just sometimes

Yeah the whole system should be at the pressure rating of the cap. After warmed up and on a warm day the hose should feel like my kid's bike tire or thereabouts.

A couple of questions I've been waiting to ask: How old are the thermostat, hoses, and cap?