in an internal combustion engine for a vehicle having a drive clutch , a blowby control system includes a fresh air introducing channel , for introducing fresh air into a crankcase , and a blowby gas return channel for returning blowby gas from the crankcase to an air - intake system . a blowby control valve is provided in the fresh air introducing channel or the blowby gas return channel , for selectively restricting gas flow through the blowby system . a controller is also provided on the vehicle for controlling operation of the blowby control valve . the controller is programmed to switch the blowby control valve from the closed state to the open state , with the drive clutch connected , when the vehicle starts traveling . the blowby control system and method hereof can reduce variations in the engine speed occurring during engine idling and before clutch engagement , to improve ride quality of the vehicle .