Stupid Question About the Radiator Overflow Tank

[MylesNT]

I ordered several small parts from LMC the other night, one being the overflow tank. It took a little modification to get it mounted up, the hole on the inner fender was pretty far off. I guess the 8 year old in the factory in Taiwan that made it was new to the job. Anyway, I want to make sure I understand how this recovery system works.

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I did some searching and Googling, but I still don't really get it. The only hose off of mine, as you can see, is on the neck above where the cap should seal. I always thought this was just for burping off pressure if it got too high. I understand how other auxillery resivoirs work on all my other vehicles, but not here.

Is this set up right? The hose off of the neck runs down into the recovery tank and it expands and contracts there? How? It seems to me that the cap would work as a relief valve, and only pop open momentarily over 16#s. Though I suppose the system could operate >16psi and as it cooled down suck coolant back in, but I don't know.

It also seems that rubber nipple could cover another hose barb that is before the cap, and as coolant expands and contracts, it could ebb and flow into the tank without involving the cap, but then the cap wouldn't need to be pressure rated, as excess pressure would vent through the recovery tank.

If that's the case, what is the barb under the nipple for?

I'm probably over thinking this, so any help would be appreciated.

 

[75gmck25]

The upper nipple is for the overflow, and it allows water to flow in and out of the overflow tank as the radiator heats up.

The lower hose nipple is for the heater return hose. On my '75 GMC that is the stock location for the heater return, but on some GM 350's the return runs to a fitting on the water pump. If I start up my truck with the radiator cap off you can see the water gushing out of that return hose and into the radiator. The heater supply hose comes off the intake manifold.

Bruce

 

[4WDKC]

The radiator cap is a valve used to release excess pressure into the overflow tank, then when the radiator is low or cooling off it creates a vacuum in the system that allows coolant from the overflow back into the radiator. Thats why the old cars that didnt have an overflow were always not full, they would puke out the coolant to maintain pressure then when it cooled off it would be 20% ish low.

 

[77 K20]

To add a little bit to what 75gmck25 said about the heater hose return... if you hook up the return to the water pump then you are bypassing the radiator and not cooling off the coolant. If you live in a hot climate and are worried about maximum cooling ability then it is recommended to hook the return up to the radiator instead.

For that matter I live in a cold climate and really should do the opposite- and put the return to the water pump to let it warm up a bit quicker.

 

[donnieray]

To add a little bit to what 75gmck25 said about the heater hose return... if you hook up the return to the water pump then you are bypassing the radiator and not cooling off the coolant. If you live in a hot climate and are worried about maximum cooling ability then it is recommended to hook the return up to the radiator instead.

For that matter I live in a cold climate and really should do the opposite- and put the return to the water pump to let it warm up a bit quicker.

Good stuff. I've wondered why my 86 C10 heat only cab has the return to the radiator and my 84 K10 ac truck has return going to the intake.

 

[75gmck25]

My K25 has the heavy duty heater and A/C option, so maybe that was set up differently than other trucks. It has the heater return hose to the radiator, which I have seen in many other trucks. I also have a large 7-blade metal fan and an HD thermal fan clutch.

The pulley on my water pump is dual groove so that the alternator belt goes around the crankshaft and the water pump (as most do), and also the power steering belt goes around both the crankshaft and the water pump. If I lose/break either one of those belts, it will continue to turn the water pump and keep the engine cooling. When I look up parts online it shows an option to use shorter PS pump belt that goes only from the PS pump to the crankshaft, but that would not engage the water pump and help with cooling.

Bruce

 

[MylesNT]

Thanks for the replies and info everybody.

I always thought the cap just snapped open and closed. I understand now.

Thanks again.