Coolant spillage

Rubio

Rubio

Joined
May 26, 2025
Messages
45
Age
65
Location
Ontario, Canada
Bike
ST1100A
Why does coolant spill from the expansion tank vent/overflow hose when I remove the rad cap when cold? I’ve been running the bike up to temp then letting it cool and topping up the coolant if air has surfaced. When I remove the rad cap a bit of coolant drains from the expansion tank hose from the cap. It’s always concerning when you find a puddle under the bike and I want to know why this happens. Ideas?
 
These bikes are old, so is the hose coming from the thermostat neck to the expansion tank brittle? Most are, crack, and develop leaks. This is an easy to change item and well worth it to stop a leak like you describe. I guess the bigger question is do you see more substantial coolant loss in the expansion tank after a few rides? This could also be from the brittle line and I would change that first. Just a guess on my part. How many miles on your ST?
 
Erdoc48 said:
These bikes are old, so is the hose coming from the thermostat neck to the expansion tank brittle? Most are, crack, and develop leaks. This is an easy to change item and well worth it to stop a leak like you describe. I guess the bigger question is do you see more substantial coolant loss in the expansion tank after a few rides? This could also be from the brittle line and I would change that first. Just a guess on my part. How many miles on your ST?
Click to expand...
The expansion hose has been replaced. This coolant comes from the hose out of the expansion tank cap as soon as I open the rad cap.
Last night I ran the bike to synchronize the carburetors and I laid a paper towel under the bike to show if it drips. Nothing this morning until I removed the rad cap. The coolant was down a bit in the thermostat housing, not out of view.
Is it just atmospheric pressure pressurizing the expansion tank that is under a slight vacuum from the overnight cooling and siphon cycle?
 
How is the coolant level in the expansion tank, is it increasing with each radiator top up? Isn't there a hole at the top of the expansion tank cap? I am not near my bike to confirm. If that is plugged that could cause this burping.

Edit: if there is a hole or break in the new overflow hose, it will allow the fluid to flow from the radiator down to it, but the lack of vacuum will not allow it to go back to the radiator, just another thought.
 
Last edited:
Rubio said:
The expansion hose has been replaced. This coolant comes from the hose out of the expansion tank cap as soon as I open the rad cap.
Last night I ran the bike to synchronize the carburetors and I laid a paper towel under the bike to show if it drips. Nothing this morning until I removed the rad cap. The coolant was down a bit in the thermostat housing, not out of view.
Is it just atmospheric pressure pressurizing the expansion tank that is under a slight vacuum from the overnight cooling and siphon cycle?
Click to expand...
Respectfully, is the level in coolant bottle correct, as fluid needs to be able to get to the overflow tank outlet to drip from it. If it was too full, when you release the vacuum by opening the rad cap, you may allow enough coolant to flow from the tube at the height of the thermo housing down into the resevoir, increasing the level to enable some fluid come out the tank outlet.
 
New expansion hose installed tightly and nipples on tank and thermostat housing are clear. Coolant level just below the high line when hot. Down to the lower line when cooled. The expansion tank isn’t filling. Coolant is moving back and forth as intended. No coolant loss while running or cooling, just when I open the rad cap.
 
When the engine heats up and the coolant expands and is pushed into the expansion tank and after shutdown and cool down the coolant is drawn back into the radiator. I am sure some of the coolant can get trapped towards the top where the hose attaches to the radiator top. It would be easy for some of this coolant to spill out when removing the cap. It's probably nothing to worry about.
 
When the engine rises to full temperature some of the coolant is expelled to the expansion tank due to pressure build up in cooling system. When cooling system returns to ambient temp coolant is drawn from the expansion tank because the cooling system is now under a vacuum. yes there will be a small amount of coolant trapped by the radiator cap that will drain out when you open the radiator cap breaking the vacuum seal.
To test your system, with engine cold check the level of your expansion tank. Make sure it is not above the max level. Place some paper towels under the radiator area. Bring the engine to full operating temperature and let it cool to ambient temperature. DO NOT open radiator cap! Is there a leak on the paper? If yes replace the radiator cap with new. If not.... go riding.
Dan
 
Rubio said:
Coolant level just below the high line when hot. Down to the lower line when cooled.
Click to expand...
Is that normal for an 1100 (or any bike)?
Leaky head gasket overheating the coolant and blowing bubbles into the overflow line (with a dip in it)?
Overactive imagination (mine)?
 
I think @diferg nailed it. You are seeing a bit of residual pressure in the expansion tank pushing the coolant in the hose out. It is my understanding that you fill the radiator after flushing the system, burp it and check the fluid level and then do not remove the radiator cap to check the level. You check the expansion tank's level. If the hose is intact - no cracks or splits, the radiator will suck more fluid back into the radiator as needed, and push some fluid into the tank when the engine is hot. The expansion tank has a high and low mark, as long as the fluid level is between these lines when cold, you are good to go.
 
You must log in or register to reply here.
Back
Top Bottom