Coolant Leak from Reservoir Overflow/Breather Pipe

tankerman10

Joined
Jun 11, 2011
Messages
5
Location
UK
Bike
1998 ST1100AW
1998 ST1100AW CBS/ABS/TCS

I parked my bike on the sidestand while I was out yesterday and came back to find a pool of coolant underneath. There wasn't too much, so I rode home keeping an eye on the temperature, which remained normal. Within about 5 minutes of arriving home, coolant began to pool under the right side of the bike again.

Today, I took the lower fairing off and the leak seemed to be coming from the coolant reservoir overflow/breather pipe. I topped up the coolant, then started the bike and ran it for a while; as soon as it warmed up, coolant began to drip from the pipe.

The level hasn't risen in the reservoir and the temperature remained normal, so I don't think it's the problem of coolant not siphoning back into the rad when the bike cools; also, it began to overflow as soon as the engine began to warm.

Any ideas, anyone?
 
Well first I'd check the overflow tube from the rad to overflow tank... they tend to crack right by the connection tube to rad neck... this usu stops the return suckion to rad though.
As the engine heats, the fluid expands and go to the overflow bottle... if the bottle is filled, it will piss excess out.

Was it possiblely overfilled ( the rad/overflow bottle ) to start with recently... extra will leak out till stable level is reach?
 
Haven't had chance to check the overflow pipe yet but:
  • The bottle hasn't been topped up for ages and definitely wasn't overfilled.
  • The level in the bottle hasn't increased, as it would have done if coolant had entered from the rad and not siphoned back.
  • The leak began today as soon as the bike began to warm up up and way before I would have expected coolant to expand from the rad to the bottle.
 
The overflow bottle is *not* full?
Then check the tubing; if it is leaking (vs. overflowing) then there might be a hole in the over flow line.
 
The coolant is coming from the open (bottom) end of the overflow pipe, the other end of which is attached to the cap of the coolant expansion bottle. It isn't from a split in the pipe. The coolant in the bottle is up to the top mark - ie, the level to which I filled it - so there is still plenty of space for expansion and it shouldn't be overflowing. The flow from the pipe begins as soon as the engine begins to warm up; the temp guage is still at the bottom end of the scale, and it never goes much above that anyway.
 
Guess I'm not seeing the picture, but does the bottle overflow tube start lower than the top on the 1100...?

All I can think of at this time is if if isn't overheating BUT your getting the antifreeze pissed out to the overflow bottle , is it a chance yr rad Cap is bad, maybe rubber gasket cracked.
 
The bottle cap on my 01 had a tube down into the bottle. Digiamo told me this waswrong, there is NOT supposed to be a tube on the bottom side of the cap.

If there is no tube inside yur cap, there's another problem. Only two ways fluid can come out the overflow:
Bottle is over full (radiator and bottle are overfilled);
Bottle is pressurized and blowing fluid up to the cap. (It would take a bunch of big bubbles to do that.)
 
George said:
The bottle cap on my 01 had a tube down into the bottle. Digiamo told me this waswrong, there is NOT supposed to be a tube on the bottom side of the cap.

If there is no tube inside yur cap, there's another problem. Only two ways fluid can come out the overflow:
Bottle is over full (radiator and bottle are overfilled);
Bottle is pressurized and blowing fluid up to the cap. (It would take a bunch of big bubbles to do that.)
Click to expand...

That's interesting, George. There IS a tube on the underside of the cap, going down to the bottom of the bottle. As far as I know, it's always been there and it definitely hasn't been put on since I've owned the bike. It's never overflowed before, though. Perhaps I'll just take it off and see what happens.
 
The standard model and ABS model coolant reserve tanks are different. The standard's cap doesn't have a port in it:
1998 standard model Reserve Tank ST1100

- the ABS's does, but the fiche doesn't show a tube from the port in the cap to the inside of the tank:
1998 ABS model Reserve Tank ST1100A

Considering the design/operation/function of the standard model reserve tank, I concur that there is no need for a tube inside the tank connected to the nipple on the ABS cap's port. As long as the coolant enters the tank from the bottom, the tank needs a vent-to-atmosphere in the air space above the normal coolant level, and of course to expell any coolant should the system fail and overfill the tank. [edit: IMO. I'm trying to understand how tankerman's worked with the tube, prior to the problem...(?)]

John
 
Could be it's been pissing green since the tube was put on John.

Gotta give much credit to Danny for catching that problem on my bike at a gas stop in remote south Mexico last spring.

Anyhoo, Tankerman, remove that tube and see how she reacts. (Notice on the "fitting" on the underside of the cap, there is no shoulder to hold a clamp as there is on virtually every other hose fitting on the bike. Admit I double-checked Danny's advice after I got home and he [and John] are correct, no hose on that nipple.)
 
Agreed the fiche doesn't show a tube on the underside of the cap. However, I couldn't really see how that could be the problem as the tube's always been there and the coolant only started to overflow a couple of days ago. I spoke to a tech at a Honda main dealer; he's worked for Honda for over 20 years and said that although the fiche doesn't show the tube, some bikes have it and some don't!! He also thought it was unlikely to be causing the problem.

Anyway, it's all been overtaken by events now; the overflow has stopped just as suddenly and inexplicably as it started. I'll let you know if it returns; meanwhile, thanks for your help.

Karl
 
Before you sigh in relief I'd check the coolant level in the radiator...

Mark
 
I agree with Mark. That coolant had to come from somewhere. The only way I can see that this could suddenly happen is if the over flow tube, either at the radiator or coming out of the bottle had been clogged and suddenly opened.
 
I had my overflow tube vomit one time last summer. The PPO had bought a spare radiator cap for it, then the PO thought it didn't need it, so I had a brand new radiator cap to play with. I trimmed the hose to the overflow back at the radiator cap to make sure it wasn't cracked, then installed the new rad cap. No more problems - make sure you don't have any nicks or cracks in the seal on your radiator cap, as this is what my old cap had.
 
I was wondering if You ever figured this out as I am having a similar problem. I have replaced the rad cap, thermostat, head gasket, hose running to reserve tank and I still have the reserve tank overflowing onto the ground. Not sure what to do next.
 
Last edited:
Big E said:
I was wondering if You ever figured this out as I am having a similar problem. I have replaced the rad cap, thermostat, head gasket, hose running to reserve tank and I still have the reserve tank overflowing onto the ground. Not sure what to do next.
Click to expand...

Did any of you guys Burp the system of air when you either flushed the system or replaced the thermostat?
 
Big E said:
I was wondering if You ever figured this out as I am having a similar problem. I have replaced the rad cap, thermostat, head gasket, hose running to reserve tank and I still have the reserve tank overflowing onto the ground. Not sure what to do next.
Click to expand...

Are you sure it is coming out of the overflow on the tank? Or could there be a crack in the tank (could have happened if it ever froze).
 
Yep burp it many times as I thought there may be a air lock but it has not helped. Thermostat, hose and cap all replaced.
 
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