Radiator overflow tank - Do you use it? Why? Mine is overflooding

[Frank818]

While filling up my coolant from the engine head and engine expansion tank, I realized the radiator expansion tank fills up and overflooded on the ground.

I scratched my head and it made sense, cuz the top of my VR6 engine is about 6" above the radiator.

I have an expansion tank close to the engine, it uses the highest head hose and tees it from the top into the top nipple of the expansion tank. No fluid will go there by gravity.

Do I really need the radiator expansion tank? If not, what do I do with the rad fill cap, I simply block the nipple and if there's too much pressure it will push all the way to the top hose of the head and into the engine expansion tank? Or I need to route a hose from the fill cap nipple and tee it to my engine expansion tank?

If I do need the tank, uh, how on earth can I design that setup...

Did you use the rad expansion tank and if yes for what purpose? How is your coolant system setup compares to mine I just described?


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[Hindsight]

Myself and many others delete the rad expansion tank. Just cap off the barb at the radiator (use a clamp too) and ensure the pressure rating on your REAR cap (at the engine) is lower than the one on the radiator. Ideally, you run a small 1/4 or 5/16 hose from top of rad to rear reservoir, and rear reservoir is highest point in cooling system.

Also, something is wrong if your front reservoir is filling while you fill coolant from rear. I bet you have the bleeder hose going into it instead of the hose that comes off the radiator fill neck under the radiator cap.....

[Frank818]

I figure out the unwanted filling up. The cap wasn't tight enough.

But my questions are still valid, or a little different. Since I am using the VR6 tank at the head's level and that the VR6 tank doesn't work the same as the Subaru, I'm questioning what would happen if I block off the rad cap. If there's too much pressure it'll do like on the VR6 (which has no cap on the rad), the head small hose at the highest level will overflow to the rear tank. When it cools off, the bottom of the tank will flow through the 5/8 hose at the bottom and back into the system. But is that theory going to work, not sure. I have to think...

If I still use the front tank connected to the rad cap, I fear that when the coolant will cool off and the vacuum valve in the cap will open it won't suck fluid back in due to the higher pressure from gravity, since my engine is higher than the rad cap. Hum, experiments...

[wallace18]

Coolant resv. tanks work on pressure differential not gravity. They are located high on Volkswagens because they are part of the pressurized system on them. This way air gets out of the system easier. The plastic tanks on the Subaru's are just for expansion and contraction. they are not pressurized. JM2CW.

[Frank818]

Yeah I just understood that, they are 2 different things.

I called my mech and he says the way I did it is fine (assuming I tighten the rad cap enough) and that if the cap opens, whether under vacuum or not, it will not interfere with the rest of the VW system.

I also learned that the black plastic cap (sometimes it's blue) I have on my pressurized VW tank is actually a relief valve! It will open up to atmosphere if there's too much pressure. Then I control coolant level in the system by adding coolant to the tank if my pump sucks up some from time to time, from the bottom of the tank.

Learning new things, I guess cooling systems I wasn't good at yet.

[wallace18]

Sounds like you have a good handle on them now. Best of luck.