Your radiator cap controls that. It may be bad.

 

the hose from the radiator needs to be submerged almost at the bottom of the tank. the other hose is tank overflow. any small air leaks in the hose from the radiator will kill any vacuum that pulls the coolant back in. as mentioned above the cap needs to function properly.

 

The hose from the radiator attaches to a right angle nipple near the bottom of the overflow tank. The hose needs to be free of any defects and the fluid level of the tank should be at the level indicated at the embossed mark of the tank. If the fluid level is below the nipple (and the radiator cap is functioning correctly) a vacuum cannot be created when needed.

 

I refer to that tank as an expansion tank and it should have coolant in it to at least the min level. I don't think having it a little over will hurt. The radiator also has to be full, full, full. As the engine heats up, the coolant in the radiator expands and gets pushed to the expansion tank, which IS NOT under pressure. Then, as the radiator cools and coolant contracts, it siphons coolant out of the expansion tank. The radiator should always be full. Before there were expansion tanks, radiators were not full to the top when cool but were when hot.

 

An expansion tank is under pressure. An overflow is not. All S10s have overflow tanks, no expansion tanks. Well, from the factory anyway. Mine among others have the cooling system upgraded.

 

@BRANDONCUOZZO

Pull your radiator cap. Flip it over and inspect the rubber seal on it. If the rubber is hard and cracked, or if it's missing, your cap won't draw coolant back into the radiator from the overflow. It'll suck air through the broken/cracked seal.

Also check the mating surface on the cap opening of the radiator itself. Make sure there's no corrosion on the surface the seal engages. If it's damaged there, it won't draw coolant back. The fix for that is a replacement radiator.

If the cap is over 3 years old, I'd just replace it. The caps are supposed to be replaced every 5 years when you change the coolant. Nobody ever does that. They can fail to draw coolant back from the reservoir, or they can let too much coolant boil off through the overflow reservoir. Either way, the results are improper cooling, often manifested as high operating temperatures and overheating.

 

So if radiator is supposed to pull water out when needed then how dose that work

When the system heats up and is flowing it creates pressure. The radiator cap controls the pressure with a spring. When the pressure gets too high, it pushes the spring open and lets coolant flow into the overflow tank. Thats why the level in the overflow tank is higher when your truck is at operating temperature. Then when it cools down, the system loses pressure creating a vacuum that sucks the coolant back into the radiator.

New radiator cap is cheap and easy enough, just make sure to get one with the same pressure rating as the old one.

 

Should my radiator still be under pressure after sitting all night? In the morning when I go to check water in radiator it is still under pressure.

 

No, it should not have any pressure left in it and the only explanation I can think of would be that it cooled off at night and warmed up a little before you opened the cap, causing it to pressurize a little. What happens is, the coolant expands, pushes on the spring on the pressure cap and gets released to the OVERFLOW, tank (gotta keep that nomenclature 100%). Then, as the coolant cools, it sucks a vacuum and draws the coolant back out of the OVERFLOW tank. As the last thing to occur is a vacuum, there should be no pressure in the radiator. You won't get burned by any escaping coolant but I'd have to wonder why it would be pressurized at all. It should be full to the top and maybe spill a little though.

 

If it's sat all night (multiple hours) not running, then you're fine. It's cool enough and not under pressure. In that case, you can remove the rad cap safely to check rad coolant level and or to check the rad cap condition.....

 

After it sits all night the radiator is still under pressure. Wondering why that is?

 

@BRANDONCUOZZO
For the stock 4.3L engine/ 4x4, the original cooling system in the truck, in an intact and properly functioning state, is fine for typical daily driver use - a system upgrade really isn't needed.