Coolant system issue on VF500

Hoping you guys can help me out with a coolant circulation issue!
1984 VF500

-Coolant is in the radiator.
-The reserve coolant tank is empty.
-Once the engine heats up, coolant comes OUT of the reserve tank hose and fills up the reserve tank.
-Once the engine cools down, the coolant in the reserve tank is sucked back into the system.
-If I fill the reserve tank with coolant, what I added overflows once the engine heats up, due to the issue mentioned above.

I have observed air bubbles coming out of the coolant reserve tank hose when the engine is hot.

Any ideas what the problem could be? Thanks for reading

 

The overflow tank level should be inspected at normal operating temps and be between the "low" and "high" marks per the owner's manual. If you fill it when cold, you could be losing some to the ground out the overflow tank, overflow tube.

This is assuming everything else in your cooling system is working normally. Have you replaced your 35 year old radiator cap? Easily had at Napa for like 8 bucks. I think the 500 uses a 0.9 bar cap, which is about a 14 psi cap. Just because it's not "leaking", doesn't mean it's doing it's job correctly.

 

I will look into replacing the radiator cap. Hopefully it's as simple as that, but I have my doubts. I don't see why there would be pressure coming OUT of the coolant reserve tank hose. That's the mystery

 

"-Once the engine heats up, coolant comes OUT of the reserve tank hose and fills up the reserve tank."

Please clarify this statement. Doesn't make sense.

 

I did some research and I realize I was over simplifying the cooling system in my mind. Sorry if this is confusing!

So one of the hoses in the reserve tank starts to move the coolant that is already in the system back out, back into the reserve tank. It's basically moving the coolant that is in the system into the reserve tank. That's why when I fill up the reserve tank to the full line, it overflows when the coolant that is already in the coolant system gets moved back into the reserve tank.

To put it a different way, I have seen the coolant reserve tank absolutely full to the brim as air bubbles come out of the reserve tank hose. A constant stream of air bubbles coming out of the hose and bubbling to the top of the reserve tank. Once the engine cools, the coolant returns to the system, emptying the reserve tank.

So this may just be an issue with the radiator cap? Do you think the lack of a proper seal is causing this?

 

Or it's just old and weak and no longer is rated at the pressure it was when new. Get a cap, you need one anyway. Fill the overflow tank to the lower mark when still cold and give it a whirl. Eliminate the cheap easy stuff first which need to be done anyway. Be sure to check the 2 sealing surfaces on the radiator (one is the main seal on top and one is lower seal surface for the relief function) for any deposits or corrosion that may prevent the new cap from sealing properly.

 

Thanks for the info. I flushed, filled, and bled the system with new coolant, which seems to have fixed the problem. I had no issues on a 1.5 hour ride today, and the engine cooled down much quicker than before. I'll replace the radiator cap as well, just to be safe. Thanks for your help!