Yes, That is how the bypass works. The heater hose from the pump back to the intake is constantly passing water from the rad back to the rad without going through the engine. I don't know the flow data for a 5/8 hose but it is better off going through the block.

 

If the rad is totally full, no air in it, the coolant will expand, have no where to go, and easily overcome the pressure cap. Unless, of course, your rad is made of latex, or your hoses balloon up at low pressure

If there's an air pocket at the top of the rad, the water expansion has somewhere to go, compressing the air. It expands a certain amount, stops and the air at the top of the rad will be pressurized to below the cap rating (usually). Water is an amazingly strong fluid, just look what it does to engine blocks when it freezes, or how it can push a steam-engine locomotive.

As to the original post, after it initially pushed water out, and you let it cool, what was the level in the radiator? Is it possible it still had more to push out?

I was moving my friend's 40ft trailer one day with my '74, in a very tight space. Had to keep moving back and forth, riding the clutch,etc. Suddenly, it was pushing coolant out (I had overfilled it even though I know better because I had a leak) and he started laughing because he thought it was overheating. Well, I just kept going because the temp guage was right in the middle, not overheating. Made a mess on his driveway, but he stopped laughing after he realized I just peed on his driveway and it had nothing to do with loss of control !

 

when i got the rad i was told that it had been flushed but that could be it i will get some flush and see if that helps it
thanks again.

 

Humm...that is interesting. The cap goes on only halfway with an airless system? Can someone please explain why?

Also, is an airless the same as adding an over flow tank? What are the differences?

 

I too would like to know what "cap goes on only halfway" means?

That's usually a hack to overcome a leaky coolant system...

art k.

 

bump...to the last questions...since spring is here and summer is on the way...

 

Interesting Replies . Wow Im Impressed . Now I Know Why My Truck Likes To Let Some Water Out After I Fill The Radiator Up. Been Overfilling I See. You All Are Great. :-x13

 

Airless system caps are plainly marked "only twist halfway." Or some such.

Airless systems require that coolant passes both directions. Tighten the cap down, and it only passes one way - spew.

An airless tank is more than an overflow. Coolant can be sucked back into the system as needed.

 

Loosen the heater hose at the intake, run the engine, when coolant leaks out tighten the clamp. This will let out any trapped air. You'd be surprised how much air comes out. All the previous posters are right! If it still overheats after you bleed the air out, consider that you may have the thermostat in backwards. These are usually marked (radiator side) - (engine side) but if not the spring goes inside the intake. I like a 13-14 pound cap, 50/50 mix, 1 container of water wetter, tight belt, new hoses, brass spring in the bottom hose. Now lets talk about headgaskets!!!!

 

Ok...from what I am reading...if I install an overflow tank and/or airless tank system on my 390FE, then I should only tighten the radiator cap half way. Correct???

-- OR --

Should I purchase a new radiator cap that is designed for an airless tank system?

I'm looking for a best method solution here...

Thanks!

 

Get a overflow tank with a sifon tube and a new radiator cap that allows water to be sucked back in when it cools down. This is how all modern cars handle the expansion of the water.

 

i had the exact same thing you described going on with my 360 a few months ago. i tried three different caps and installed a reservoir and vented cap. it would get hot then go back down to normal. it would fill the reservoir then cool down and suck the coolant back in. i thought i had it beat. no such luck. about a month later i was driving up the pass near my house and started blowing coolant out my right side exhaust. turns out the intake gasket went south and let water into the cylinder. i think the cylinder was pressurizing the coolant system. no serious damage thank goodness. after i replaced the gasket no more problem with overflow or temp changes. i hope your problem isn't that serious but it could be something to watch out for.

 

"Get a overflow tank with a sifon tube and a new radiator cap that allows water to be sucked back in when it cools down. This is how all modern cars handle the expansion of the water."

Exactly what I've been saying. An airless system. You fill the radiator to the top, and then fill the expansion tank about half full.

 

Good advise...now that I know to get a vented cap for the pressured cooling system. I'll be installing the complete system sometime soon...

 

universal coolant tank with mounting bracket and hose at local kragen's 14.95. vented cap 6.95. Don't spend alot of money on some "kit".