Radiator caps

Just a random question about radiator caps. My WR boiled over a few times when I was riding the clutch a lot in first (duh) and second. My fluid levels seemed alright and I could see it being sucked back into the radiators once things cooled down (under 5 mins). Either way, it had me thinking about getting a higher pressure cap (I think stock is 1.1 and Tusk sells a 1.6 one). 


So my question is mostly technical: wouldn't a high pressure cap be counter-productive? I mean, if the coolant is already boiling hot inside the radiators, then how is it effectively cooling the engine? You're basically keeping hot liquid inside/postponing a boil over, aren't you? Or is the idea behind a high-pressure cap: better than having no liquid at all? Meaning, keeping the liquid (even if it's hot) longer in the radiators better than no liquid at all? 


Secondly, is there a way to know if you're boiling over without actually stopping to check overflow bottle level? Once when it was boiling over I heard it once I stopped the engine, but it kind of scares me to know that it might be happening while I'm riding without me being even aware of it. Then again, a few days ago I went riding and it never boiled, so I know it's mostly due to me riding the clutch at very low speed. More of a theoretical question again... 


Sorry if these are stupid questions... Just wanted to check before pulling the trigger on a different rad cap.  

The higher pressure radiator cap is what most do. The only way you will have problems is if you boil out too much fluid. Check to see if your antifreeze mix is at least 50/50 or more.

We ride very technical trails and serious hill climbs to the tune of running straight anti freeze with bikes that do not have fans.

Most likely the cap and 50/50 will work for you.

Testing the cap is also a good start.

Thanks guys.

I had the same problem with my wr450f '09 and solved it with the help of richer mix (needle,  mix screw) and 1.8 radiator cap. I bought moose racing cap from ebay, it's installing was terrible because of bad fit, but it works nice.

Most of my riding is slow and technical, 1-2 gear, log hopps, obstacles and technical uphills with lots of clutch. Don't know how my bike will be behaving in higher temps in summer, but now everything is ok. Before cap and mix modes it was boiling 5-6 times a training.

As for me, main idea of high pressure cap is to avoid coolant system from air bubbles appear and coolant loss. Air bubbles and low liquid level could lead to local overheating.

Edited by Markin

I guess you forgot your chemistry and science lessons from junior high and high school.  Boiling point is subject to pressure.  Example, water boils at 212 degrees F at sea level.  At 10K feet (with lower atmospheric pressure), it will boil at about 200 degrees fahrenheit.    So the higher the pressure cap the higher the boiling point becomes. The limitiation to all this is that at some point the cooling system becomes so pressurized that  it causes leaks elsewhere in the system such as at the head gasket, waterpump, coolant hoses, etc.  This is why it is commonly understood not to open a hot radiator cap because when you release the pressure, the coolant can boil instantly(turn from liquid to gas) and create a very hot steam geyser from the radiator.  Its not that it was boiling in the radiator before you released the cap, but as soon as you release the pressure it instantly boils (and sometimes explosively)

Edited by SilvFx

Thanks guys. Great info. New to liquid-cooled bikes (I know), so getting the hang of it.


This brings another question out of me: 

How often do you guys find yourselves topping off your coolant? Before every ride? Every few rides? Hardly ever?

Thanks guys. Great info. New to liquid-cooled bikes (I know), so getting the hang of it.


This brings another question out of me: 

How often do you guys find yourselves topping off your coolant? Before every ride? Every few rides? Hardly ever?


If you stop boiling never.

Interesting. After boiling over a few times I was able to go out on several rides and it was always full. Reservoir level looked a bit high, but it was within the lines. My last ride was roughly 6 hours and the next day I checked and the coolant level was low. Topped up this morning. I can'y see any visible leaks by the waterpump (thought I have a seal/rebuild kit coming just in case). I also drained the oil before the ride and there were no obvious signs of coolant in it. If the system is self-contained and "sealed", then were is it escaping from? Evaporating slightly at every boilover through the cap? 

If you have an overflow catch tank (which also has and overflow hose) and you never overflow the coolant catch tank, then you should never have to refill.  My guess is that you boiled over so much coolant into the catch tank that it overflowed a bit.  When it overflows, it just dumps the overflow on the ground.

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