Hi all, had my NC21 on the road for a little over a week now, done 700 miles so far and everything seems solid.

My only concern is after a decent ride the majority of my coolant ends up in the expansion tank and not in my rad...

It doesn't make it's way back to the radiator but I dont appear to have actually lost any fluid from the system (level hasn't changed when I syringe the coolant back into the radiator).

I'm starting to suspect the radiator cap is knackered and letting pressure out prematurely...

Does anyone know what lb pressure the 21 caps are?
Can anyone shed some light?

Cheers
Jesse

1.1 bar

and yes it sounds like rad cap is knackered or water pump is busted.

i had one once where rad cap was knackered, water pump was busted too

Having had a look at a few parts fiches, it seems a lot of the rad caps have been superceded by 19045-MY3-621, including the NC21, NC35 and MC22 to name but a few. The cap has 1.1 written on it, which equates to 16 PSI. The specs for the NC35 that I have dug up, plus the NC30 service manual, state that the pressure relief on the cap starts to open at 16PSI, so I reckon that's your answer.

As for what it going on, I am slightly surprised that the coolant doesn't make its way back into the system as the bike cools. I thought the whole idea was that the expansion tank simply catered for exactly that, and wasn't a one way journey. Does the tank get filled above the max line? If not, I would be tempted to top up the rad from a fresh supply rather than the tank, and see if it blows anything out the tank overflow.

Thanks for the replies!
I saw the 1.1 on the cap but wasn't sure if that was the pressure rating.

Speedy I thought the exact same thing, slightly alarmed that the coolant isn't getting back into the rad.
Took it on a 120 mile round trip on Sunday and the expansion tank was filled to the top, way above the MAX mark.

I'm positive the ammount of fluid in the system is correct, just seems like something is stopping it from getting back where it needs to!

Fingers crossed it's just the rad cap, nice easy fix

JesseDT wrote:Thanks for the replies!
I saw the 1.1 on the cap but wasn't sure if that was the pressure rating.

Speedy I thought the exact same thing, slightly alarmed that the coolant isn't getting back into the rad.
Took it on a 120 mile round trip on Sunday and the expansion tank was filled to the top, way above the MAX mark.

I'm positive the ammount of fluid in the system is correct, just seems like something is stopping it from getting back where it needs to!

Fingers crossed it's just the rad cap, nice easy fix

there all 1.1 bar...

fit a new rad-cap and see what the crack it... you could have knackered head-gasket pressuring the system forcing coolant out, the excess pressure is stopping it being drawn back into the bike.

Get rad cap and go from there.

Thanks for the replies guys

Problem appears to have been rectified!
Swapped rad cap for my old man's ZX9R one but to no avail.
Drained the system, checked thermostat was functioning and hoses/radiator where not bkocked then put it back together.

Took my time refilling the coolant to make sure no air was trapped (what I believe was the problem) and after 30 odd miles everything seems how it should!

Expansion tank is no longer filling up and level in radiator hasnt budged, chuffed!

ah could have been airlock in system.

proper bleeindg proecdure for this bike.

"run bike and ride upto temp (ie. middle of gauge) without rad cap on.

keeping bike on side stand snap throttle to 4000 rpm and let return to idle, do this 3 - 5 times (expect some coolant loss)

stop engine and allow to fully ocol, refil to max of coolant and do same with bottle.

then shoudl be ok.

i know this is an old post now but im desperate for an answer, i have a similar problem, i will try thevrad cap replacement but also i have the question of what should i expect to see with the rad cap off, should the water be still but be sucked round or should it violently bubble or should the level gently rise and fall as it us sucked round?

Im supposed to be going to uni and taking this bike with me but this problem has reared its head. please help!

Vfr400KM wrote:i know this is an old post now but im desperate for an answer, i have a similar problem, i will try thevrad cap replacement but also i have the question of what should i expect to see with the rad cap off, should the water be still but be sucked round or should it violently bubble or should the level gently rise and fall as it us sucked round?

Im supposed to be going to uni and taking this bike with me but this problem has reared its head. please help!

it should bubble a little after inital filling as the air works out... if its bubbling a lot then the coolant is boiling (probably vent to expansion tank)

this is usually a blown head-gasket..

it could also be caused by the waterpump being knackered and not circulating the water..

id check the waterpumpfirst... becareful to remove only the cover and not to pull the waterpump out the engine else the oil will come out.

thankyou, i think the rad cap has fixed the issue. I am however worried i may just be hiding the issue or would boiling/otherwise bubbling coolant still push past the new cap?