I probably don't know something but here's the info:

Sometimes, for hours or days the engine runs fine, but sometimes (not very often) the engine overheats like crazy.

I use the heater core as a test element and when it blows hot air all is fine but sometimes it starts blowing cold air. 2002 Express 5.7

Heater blows cold air and engine temp rises close to the red line so I crossed out the fan clutch as that would make the coolant even hotter. I also crossed out the thermostat because a thermostat blocks the main coolant flow and not the small diameter heater core bypass. The two remaining things are the serpentine belt and the water pump. The serpentine belt seems tensioned because when I turnb on the AC for a while I hear it functioning. So looks like no belt slipping.

Is there something I don't know about the water pump. To me it seems like a pulley on a shaft and on the other end of the shaft there are paddles to pump water. This is a solid connection so once the pulley moves, the paddles move as well, right?

So what might be causing such a scenario? Once the van overheats I need to get the RPMs really hig (like really high) and I can gain a quarter mile of peace. I currently have no garage so I am trying to pinpoint the problem before tearing down the van. I am really puzzled

 

starting with the simplest things first : Do you have enough coolant?

 

What make you think it is not the thermostat ?
The symptoms you describe seems to be a bad thermostat to me ?
A thermostat can be stuck open, closed or at any point between closed and open...
It can randomly operates fine and end up being stuck once in a while.

See here : https://www.yourmechanic.com/article...ing-thermostat

 

Could be a thermostat, could be low coolant. First reply posted is the basic thing in troubleshooting anything.

 

Get a new thermostat and carefully drill a 1/8 in hole in the wafer. Mine drove me nuts .a small air bubble

 

I’ve got a ‘76 g20 van, and I had to redo the cooling system almost as soon as I got it. I replaced the radiator, water pump, thermostat (180 degree) and also did all of the belts and hoses.

I was running into a similar issue with the van overheating, and it seemed that it wasn’t sucking up the coolant in the reservoir. I couldn’t figure out what was wrong for the life of me, until I started squeezing the radiator hoses and saw the coolant level drop in the main radiator. The culprit was a locking radiator cap. It was preventing the overflow from letting coolant in, and as such no coolant was making it from the top radiator hose to the thermostat. As soon as I ditched the locking cap, it ran cold and smooth and hasn’t given me any issues yet.

 

reverse flow started on my year.92. I'm fixing that,tbi,sensors and all with a simple little 327 from 1965

 

this isnt how the system works. starting at the water pump, the pump sucks fluid from the lower rad hose pumps it through the engine block and to the thermostat, once the thermostat opens its lets it flow back though upper hose back to the rad where it cools. untill the thermostat opens the pump will only cycle water through the block only.

if water pressure rises above the rad cap pressure rating it will flow up through the overflow tank. once the pressure in the cap starts to go down the rad cap will allow the reverse flow of fluid from the overflow back to the rad till the pressure drops.
sounds like you had air trapped at the thermostat preventing coolant from reaching the bottom of the thermostat. normally when i have coolant problems and go to refill the system i will pump the upper hose to ensure coolant is above the thermostat

 

Unless someone at the parts store gives you the wrong pump.
https://www.jalopyjournal.com/forum/...ly-sbc.945259/

 

I wanna thank yall. You reminded me I have been running the old van without a fan for over a year now without a cooling fan. May be ok for old tbi but about to put in fresh motor with more than 150 hp more. Tried a few fans from a few cars. Settled on dual electrics from a park Avenue. Almost perfect size for the van! Now to find a big block radiator to slip in......