2000 Yamaha YFM400FWA(M) Factory Service workshop Manual, Page 86Get this manual
COOLANT REPLACEMENT 8.Remove: Radiator cap 1
INSP ADJ
WARNING
A hot radiator is under pressureTherefore, do not remove the radiator cap when the engine is hotScalding hot fluid and steam may be blown out, which could cause serious injuryWhen the engine has cooled, open the radiator cap as follows: Place thick rag or towel over the radiator cap and slowly turn the radiator cap counterclockwise toward the detent to allow any residual pressure to escapeWhen the hissing sound has stopped, turn the radiator cap counterclockwise while pressing down on it and then remove it