Engine Coolant Boiling in my NT700VAB

The radiator cap may be the issue but you also need to fill the radiator itself, not just the overflow tank.
You have boiled away some coolant so that coolant left the radiator and went to the overflow tank. As the engine cools if everything is working and no leaks the coolant should suck by vacuum back into the radiator.
If everything is not correct then your radiator may be low on fluid and that compounds the problem.
When you get a new cap top off the radiator if needed before more troubleshooting.

Brad
 
firefox111 said:
Update: I added coolant in the reservoir then I let the engine run at above idle and the cooling fan turns when the temp reaches the middle (or above) mark of the temperature gauge. But then I heard the radiator boiling and then hot coolant was escaping in the drain so I killed the engine. Well, I ordered a radiator cap and we'll see what happens.
Click to expand...
With the engine running and near normal temperature but BEFORE the fan cuts in, feel the top and bottom radiator hoses (preferable), or left and right tanks.
They should be within a couple of degrees of each other.
More accurately, you can use an Infra-Red thermometer.
If the bottom hose is MUCH cooler than the top one the thermostat is not opening.
If the system is normal, the two hoses or tanks should have about a 5-10C (9-18F) difference with the fan running.

Macka
 
Additional note to above: be sure the thermostat is open when you add coolant to radiator. If not, you will only be filling the space between overflow tank and thermostat and not into radiator. Same process as when you change water pump on your car - run until you see hot coolant flowing into radiator and add until full. This way one insures all air is out of system.
 
Make sure you are not overfilling the reservoir tank with coolant. It can be hard to see the levels admittedly. The level should also be checked when engine is at working temperature not cold. Not easy due to amount of plastic you need to remove. Ideally have the level between the minimum and maximum levels. If cold level should be nearer minimum level. It is actually an expansion tank so it fills up when coolant is hot and sucked back in when engine is cold, if working correctly. Expelling coolant when hot could be because tank was initially overfilled.
You don't indicate mileage but unless you have more than 40,000 miles on the clock you should have no problems with coolant system except possible radiator damage.

I have just replaced thermostat and housing including radiator cap (cheaper than individual parts) while I was replacing the transfer pipe (tube between front and rear cylinders for coolant) rubber seals which fail (go hard) after 80,000 miles and dribble coolant out of bike, best seen when bike on side stand or when checking tappets (Valves. Check every 16,000 miles unless exceeding 5,000rpm on regular basis). My bike is now at 120,000 miles.
 
Update: I replaced the radiator cap and overheating issue is gone. Thanks to all who suggested replacing the radiator cap.
 
The relief pressure of the radiator cap on the NT is very low, around 2psi. The spring on a radiator cap weakens over time due to the heat, thus dropping the working pressure of the cooling system, lowering the boiling point, and well you get the picture.

I've replaced the cap with every coolant change, never had an overheating problem. It's cheap insurance for a hard to get at part on the NT.

Brad
 
I'll re-post this, on how to get a complete fill of the NT cooling system using a gravity funnel:

Coolant change using Lisle funnel | Maintenance

I recently did a coolant change on my NT700, and thought I'd pass along an easy method for filling the system. Lisle Tools make this handy fill funnel for car use, but it works just fine on the NT700. You will need two extra o-rings to get the fitting to seat tightly in the coolant neck before...
www.nt-owners.org
 
firefox111 said:
Update: I added coolant in the reservoir then I let the engine run at above idle and the cooling fan turns when the temp reaches the middle (or above) mark of the temperature gauge. But then I heard the radiator boiling and then hot coolant was escaping in the drain so I killed the engine. Well, I ordered a radiator cap and we'll see what happens.
Click to expand...
Wow....very odd....yes, if the bike idles the temp will go to slightly above the center mark on the gage and the fan comes on, then the gage should drop a bit....but nothing should be "boiling" although if you have a bad radiator cap, it could be opening ...to what? the Outside?
 
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