Coolant from overflow tube

[JackDG]

2002 CBR 600 f sport, 41k miles

Stopped by shop on way home, when I came out there was a puddle of coolant from under the bike. Reservoir level looked fine, and i let the bike idle for a bit. It got up to temp for the fan to kick on, temp dropped and fan stopped. No coolant leaked so rode it 2 min home.

Left it to idle again at home and noticed no coolant leak. No exhaust smoke, sight glass didn't have any milk.

I've already ordered a new radiator cap as it's cheap enough to try out. Anything else I should try? Reluctant to ride it until I figure out what's wrong but I can't recreate the problem!

Thanks,

Jack
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1992 GPZ 500

[jeffyjeff]

99% probability your issue is with the radiator cap. If the cap in 100% holding pressure and the surge tank is still overflowing, you could have a blown head gasket. But if your cooling system is well maintained (antifreeze changed occasionally), that probability is low. There is probably an automotive equivalent available from the auto parts store that you could use, instead of getting raped by Honda. Stant 11233 is one that fit my Kawi EX500, Honda PC800, and VFR800Fi, available for a fraction of the OE price.
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History shows again and again how nature points out the folly of men - BOC

[jeffyjeff]

The symptom that points to the cap is that there was no leak before you parked the bike. When the cap begins to fail, the cooling system hemhorrages when the engine is stopped after running at temperature. Heat soak from the engine into the water jacket causes coolant to expand. The cap cannot withstand the pressure created by coolant expansion, and the coolant fills the the surge tank to capacity and out the overflow tube. As the engine cools, coolant is drawn back from the surge tank into the engine. Usually, enough coolant returns to the engine that the level appears low in the tank.

The pressure cap does more than keep the coolant level proper in the surge tank. Pressurized coolant has a higher boiling point than coolant at atmospheric pressure. The boiling point of coolant rises 3 degrees F for each psi increase in pressure. A 15 psi pressure cap raises the boiling point of the coolant by 45 degrees, from 212 F to 257 F.
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History shows again and again how nature points out the folly of men - BOC

[bikenut]

Thermostat, bike been idle for long ? Thermostats can and do stick, they need excersice too.

How high did the temp go, enough to activate the fan, but that was after a run, check from cold, does the rad get warm/ warmer from the bottom, suggesting a sticky thermostat maybe ?

Feel around the rad until the thermostat opens.

When up to temp, any cold patches in/ on the rad matrix ?
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nuts about bikes

[JackDG]

Jeff many thanks for the detailed answer. Yes I got one off eBay for a fraction of an OEM one! Just need to wait for it to arrive.

Bikenut, fan kicks on about 109c and was fairly consistent, and cooled it down pretty quickly. This was only the 2nd or 3rd time this year the bike has been run, I will have a look at the rad in detail as you suggest this bank hol. Thanks for the suggestion very helpful
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1992 GPZ 500

[Robby]

The bike has hardly been ridden. Things get sticky, rubber seals dry out.

Luckily, coolant is quite good at lubricating and restoring the cooling system.

New rad cap is a good idea, but the problem may well have fixed itself anyway. I wouldn't go poking at it any further unless the problem happens again. You're more likely to break something that doesn't need fixing.

[steve the grease]

Liquids are incompressible , so whilst they transfer pressure ( Hydraulic brakes) you can't pressurise them . The pressure is in the ( compressible) air space at the top of the chamber. If the space is too small - maybe it's been over filled a little - or if the cap isn't sealing, or holding pressure, it will vent a small volume of coolant until the pressure drops a little. Cooling systems are normally quite reliable, unless you have head gasket problems , pretty unlikely in this day and age.
So it's probably nothing to worry about.
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All the above is my personal opinion, you can see my lips move, but I'm talking out of my arse.
I've been riding, and fixing , bikes for 50 years, in that time the more I learn, the less I am absolutely sure of.....

[martin734]

[Fizzer Thou]

I had a similar problem with my Exup-R,even after a short journey.Luckily the radiator cap is the same as on my R1 so I borrowed it as a test to see if that was the faulty component.When the same thing happened I looked more carefully at the expansion tank and found that the overflow pipe had worked its way further into the tank so that when coolant did make its way there it spilt out onto the floor easier.Once I had adjusted the position of the overflow pipe I had no further problems.
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Just talk bikes.What else is there?

Always have a 'Plan B'

[bikenut]

2 or 3 times for an outing this year, idle/resting for most of last year ?

The bottom tank on the rad will stay cold/cool until the thermostat opens, then the top of the rad gets hot quite quickly(5 seconds ish) when the thermostat opens, then that hot water is cooled by the rad and the bottom warms up.

Cold patches in a rad matrix is not good.

Thermostats don't usually fail safe...

If the top of the rad stays cold/cool but the bottom heats up, then this hot water can reverse flow, arrive at the thermostat and help it to open.
If this is happening, renew the thermostat.

Like most things they need excersice...
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nuts about bikes

[stinkwheel]

Worth feeling the coolant hose while reving the engine, if it goes hard as revs increase, that might indicate a blowing head gasket pressurising the water jacket. Symptoms don't exactly match this but its a free and easy test.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Robby]

[bikenut]

The expansion tank that collects/holds to expanded coolant from the radiator gas to be vented to atmosphere, for the coolant to be allowed back into the radiator as it's coolant cools down and contracts...thus keeping the radiator as full of coolant...

Any leak in this system and it won't work.

The rad cap has 3 seals, the pressure seal and valve, the contraction seal and valve and the seal between the Cap and radiator.

Anyways, any indicationn that the thermostat is not working as intended/ being lazy ?
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nuts about bikes

[JackDG]

Took the fairings off to check the fluid level and give the rad and pipes are proper looking at.

Radiator level was full, and still healthy amount in the reservoir. Cap seemed fine? I gave the spring a few pushes and it felt fine. The lower left side outlet (inlet?) of the rad was looking a bit crusty but this is a 40,000 mile bike and it wasn't leaking.

Let it run with cap off, no bubbles to be seen. Revved it up while feeling the hoses and it didn't go hard (could feel the coolant being pumped faster with higher revs)

Today wasn't hot enough to let the bike get to fan-on temp in any reasonable time so it seems to be cooling the bike just fine.

Honestly cant find anything wrong at all. Gonna go with the theory of thermo/cap hadn't been exercised in a while and to not poke further while all seems ok....


Many thanks for everyones input
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1992 GPZ 500

[MCN]

Make sure the radiator is clean. Flies and tar and shite can plug the core. That restricts air flow through the rad and allows over heating.

And. Water pump not circulating water around the engine will allow overheating.

https://www.freeasestudyguides.com/engine-radiator-cap.html

A rad cap that is only working one way will allow coolant to be 'pumped' out of the system. But you will notice that the expansion tank is overfilled if that is the case.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.