you may have the wrong water pump. most serp belt systems have a reverse rotation on the water pump. if it has a smooth pulley for the water pump than the water pump will spin back wards from the crankshaft. if the water pump pulley has grooves than the water pump should spin the same direction as the crankshaft. check what you have and the rotation of the water pump compared to the crankshaft. if you do need a reverse flow water pump try like a 1990 Chevy c-1500 pickup with a 5.7l. hope this helps

 

to get the correct water pump you ll need to get the casting numbers off the intake, if thats the original intake. BUT, since this motor isnt the original do you know if its any good? HEad gasket? Other internal cooling issues?

16psi radiator cap on my 350s.

 

^^^^^^ very posible, all it takes is 1 time to overheat and u could have warped heads

 

One thing to look for on the Vortec V8s is the waterpump gaskets. Apparently they benefit with replacement by a good name like Fel-Pro, and a fair amount of Hi-Temp Premium RTV sealant.

 

I had the same thought Rusty had - it sounds like the coolant isn't moving through the system properly. Rust or built up deposits, a gasket blockage or maybe even a gasket from another motor with different porting. You can test the coolant circulation, but that only gives data at the hose location, not around the cylinder walls. I've had this overheating problem, and it went away when I tore the motor apart, boiled the block and re-manned. Also, low compression will sometimes cause engines to work too hard and overheat. Just cruising along on the highway is about the easiest work a motor does. For what it's worth.

 

As to the vigilant guy, I am also thinking restricted radiator.

 

Rusty-454 has a good point. Could it be a warped head or possible blown head gasket? the easiest way to check for symptoms of this is check your oil on your dipstick. Is it milky, or very thin? also, you can start up your car and get it at running temperature, and then look at your exhaust pipes for coolant, or water coming out. if you put your hand where the exhause is flowing out and it feels damp, it is most likely a blown head gasket. another sign is white smoke puffing out the back, but it must be damp!


if this is the case you might wanna rent a compression checker at auto zone or any local auto parts store, and run a test on your cylinders. this will definitely let you know if you have a head gasket problem.

 

here is one...i blew my head gasket this summer pulling a trailer with an engine i built...i didnt enlarget the water passage holes in the fel-pro head gasgets (they come with 1/8" holes and the block has provisions for 1/2" holes) it always had heat problems and i spent big $$ trying to mask it for years with aluminum rad, big fans diff water pumps diferent timeing...etc..etc...i blew the head gasket, realized i didnt enlarge them, so i did this time and i cant get it hot if i tried, if cooling is dissapearing u may have blown head gaskets...sometimes it doesnt mix in the oil sometimes it can go directly to the cyl (like mine did) take a compression test on it (when i did mine the first plug i pulled let out a bunch of antifreeze)
good luck