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Radiator Cap Annoyance

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When riding around slow (with minimal air flow), I noticed that when my 2011 KX450f gets hot enough to start overflowing a little coolant, that a tiny amount of coolant comes out of the actual radiator cap, and ends up getting drops of coolant on my plastics near the radiator.

Does this mean I have a bad radiator cap?

My overflow hose works just fine; definitely not clogged at all. Coolant runs freely and easily through the overflow, just by purposely over-filling the radiator (with no cap on it.)

So yea, definitely no problem with the overflow hose being clogged etc.

So why does some coolant come out of the radiator cap when it gets hot enough to overflow a little?

I just purchased a new CV4 (20 PSI) radiator cap.

Should that fix the problem? Has anyone else ever had this issue?

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Simply a bad cap or, the surface of the filler neck where the cap sits isn't true.

 

Stock cap is likely a 1.0 (14psi) or 1.1 bar (16psi)

increasing the pressure rating to a 20psi will increase the resistance to boiling over but,

also put a bit more stress on your cooling system's components (hoses, seals, gaskets etc.)

 

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3 minutes ago, mlatour said:

Simply a bad cap or, the surface of the filler neck where the cap sits isn't true.

 

Stock cap is likely a 1.0 (14psi) or 1.1 bar (16psi)

increasing the pressure rating to a 20psi will increase the resistance to boiling over but,

also put a bit more stress on your cooling system's components (hoses, seals, gaskets etc.)

 

Thanks for the reply mlatour.

According to the service manual for my KX450f, the stock radiator cap relief pressure is 16.2 - 20.6 PSI

So I think the 20 PSI cap (1.4 Bar) should be fine.

I saw CV4 caps "For KX450f" that were 2.0 Bar / 30 PSI.

Sounded tempting, but the HECK with that. Just seems way too hard on the cooling system, to be worth it.

By the way, if the surface of the filler neck on the radiator isn't true, then I'm guessing there is no easy way to fix that.

I hope that isn't the case, because my bike has really nice Mishimoto braced radiators on it. It was $200 for those I believe.

I guess I will find out soon enough. My new cap is arriving soon.

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Try your new cap before attempting to 'straighten' the top of the filler neck.

I've never had to mess with that but imagine careful use of a sanding block could fix surface imperfections.

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The Kx250f of same year came with a 1.6 or 1.8 so I wouldn’t sweat using a higher psi cap.
Try fresh coolant and Water Wetter as a first step.

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