Loss of coolant - why?

Last night while boarding a ferry, I was queuing on the car deck with engine running to waiting get parked and strapped down. I didn't notice or feel that the engine was running particularly hot. About 30 seconds after switching off the ignition, the bike took a pi$$ all over the floor. Taking a pi$$ is a good analogy because that's exactly what it looked like, the same rate of flow, the same length of time, and the same quantity of fluid, as a small dog taking a pee.

My coolant level in the expansion tank has always been a little high, it's been like that since I bought the bike 18 months ago and I've never had to add any coolant. But if this were simply overflow, why would it not have happened before now? It's certainly not the first time the bike has been that hot.

The coolant level in the tank has dropped slightly, but is still acceptable between min and max levels. The bike is getting up to 104 degrees at which point the fan is kicking in OK, but it's now only taking about 3 minutes to get there from stone cold, whereas I would be expecting 5-10 minutes at tickover normally.

The coolant hoses to the radiator are getting hot.

It doesn't seem like just a hose popped off or split as it is not leaking any more, unless there just isn't any coolant getting to that part of the system anymore.

Theories from the recovery agent are a blown head gasket causing combustion gases to pressurise the cooling system and trigger the expansion bottle overflow, or a stuck thermostat, resulting in the fluid not getting to where it needs to go, so back pressure causing the same overflow.

The water did come from underneath the rear of the engine, where there are some rubber overflow/breather pipes, and the expansion tank is under the seat.

Any ideas what the problem might be? Thanks


(Mods - Put this is BTI because TTT is SV related tech stuff only, though a cooling system is a cooling system whatever the bike, so the answers in the thread could well be relevant to the SV also. Happy for it to be moved - we are nearly a Triumph forum nowadays anyway.)

[dizzyblonde]

Kitty pi$$ed on the floor like a tomcat from the breather hoses, when she was filled a little too well. She did it after a damn good run.....obviously couldn't cross her legs anymore and widdled on the patio.

She did it once or twice after that, then after regaining a natural happy level, never did it again. Pressure has to go somewhere, preferably out via a hose breather.
I think iIRC if an air bubble gets trapped on the thermostat it can make it over heat, had that once on Dave the SV, took the cap off the rad, and it was fine after that. The fan kept kicking in which was exceptionally unusual for that bike, and I was worried about it blowing up, took the cap off the rad to see if there was coolant in it, and after that, the problem went away!

Originally Posted by dizzyblonde

Kitty pi$$ed on the floor like a tomcat from the breather hoses, when she was filled a little too well. She did it after a damn good run.....obviously couldn't cross her legs anymore and widdled on the patio.

She did it once or twice after that, then after regaining a natural happy level, never did it again. Pressure has to go somewhere, preferably out via a hose breather.

Maybe just normal overflow, but the bike's had plenty damn good runs, like a 1000 mile weekend to Ireland last year, and the coolant level in the expansion bottle has never moved in 18 months I've had it.

It's also been up to 105 degrees plenty of times before, by which point the fan kicks in and keeps things in check (normally you can let it tick over as long as you like without it overheating, I've never seen it above 105 degrees). Normally when switching off the bike when hot the fan would continue and I'd notice it running (it's like an aircraft taking off), but there was a lot of ambient noise on that car deck, so it could have been running and I didn't hear it or feel the hot air blast.

So why now?

[dizzyblonde]

Not sure tbh, I couldn't understand why Dave overheated, when it had never happened before. I was sure its fan was broke, as it had never kicked in, then one day it just did it, and not even on a blazing hot day...a little bubble could have been the cause apparently.


VRaps fan kicks in at 106, and its exceptionally easy to get it up to that temp too. I get worried as I know the damn radiator is so full of glued up holes its likely to explode. The fan does sound like an aircraft taking off too, and the hoses get hot, piping hot, surely this is normal as they are full of very hot liquid?
Perhaps you didn't hear the fan on the deck because you are used to hearing it, so dismissed it amongst the noise. Last time I was on a car deck with the bike I couldn't wait to get upstairs, they give the heebies!!

[Dave20046]

Parking it next to an intimidatingly beautiful silver sv?
Hope you get it sorted, subscribed.

[Sid Squid]

Check the radiator cap for sealing.

As you undoubtedly know the system contents expands and contracts as it heats and cools, and the system is pressurised as this raises it's boiling point. The cap isn't just a lid, it's two spring loaded one way valves, one in each direction in and out of the radiator, and each with a different load spring. One valve is relatively heavily sprung and typically holds about 1 bar of pressure, when the engine heats up and the pressure rises, that sprung valve holds the cooling system's pressure at said 1 bar-ish stopping the coolant boiling, (that's the rubber bit you'll see around the edge of the cap and the spring you work against when fitting the cap), should the pressure rise above the set value the rubber is pushed off it's seat and vents excess pressure, (and coolant), to the expansion tank. When the system cools down it's pressure falls, it contracts and now atmospheric pressure is great enough to overcome the very lightly sprung return valve - the small round button in the middle of the cap's rubber - and return the excess coolant back to the cooling system.

When the engine stops the system contents effectively stalls as the pump stops turning and no cooling takes place, the coolant in the block and head now gets very hot as it stays in there longer and absorbs more heat, if the cap does not seal fully the pressure rise can be enough to push the pressure cap off it's seat and force coolant out of the overflow. Some bikes, (Triumphs for instance), start the fan when the ignition is switched off, this cools the water in the radiator which causes it to become denser and sink, pushing hot coolant up to the top of the system and into the radiator, this is called a thermosyphon, and is deliberately used to help even cooling when the engine is off and not circulating coolant by means of the pump.

Thanks for that Sid. A couple of people on the Triumph forum suggested the same, but it was along the lines of "Check your radiator cap", the time taken to explain why is appreciated, always easier to diagnose if you have some understanding of what is going on.

[NTECUK]

When was the last time the coolant was flushed and re filled.
When the pump stops the heat of the engine can cause localised boil that then overwhelms The cap if the thermo siphon action is impeded by partial blockage.

[Mr Speirs]

What Sid said.

A thorough flush wouldn't go amiss either, especially as it's been sitting a lot of it's life before you got it.

Hope you get it sorted easy mate.