A bit of theory
Sometimes unwanted fuel vapors appear in the fuel tank: in a heat wave, when the fuel splatters in the tank, and the surplus of fuel reappears from the fuel injection nozzles, or simply because of slight fuel evaporation. It's harmful to the fuel tank, to health and environment. For this reason, in recent decades fuel system in all vehicles has been fitted with a fuel tank venting system, which uses a carbon canister for absorption of excess fuel vapor from the tank.
The fuel pipe runs from the tank right to the carbon canister. Under the vapor canister, there is a hose that removes the water resulting from the venting process under the car's underside. On the other hand, the system is connected to the intake manifold via the bypass valve, which helps clean the carbon canister itself and facilitates more efficient usage of gasoline vapors. The bypass valve operation is controlled electronically.
When the car is in standstill, with the engine shut off, the bypass valve is closed and fuel vapors leak outside under their own pressure via the carbon canister. When the engine is running, the valve opens from time to time to intake air from outside for the system of absorber cleaning. In this case, the induction system receives extra air that is not taken into account by the air-flow meter. This opening of the valve also removes fuel vapors from the tank. For this reason, the bypass valve is configured to open on occurrence of certain conditions (revs) and can directly influence engine failures. In some vehicles, the bypass valve can be configured to perform air intake from outside and vapor intake from the tank one by one.
If the bypass valve is closed, it doesn't usually affect driving performance. However, sometimes, the internal diagnostic means of the vehicle may signal about engine fault. And the environment will be bitter about your car, because the absorber eventually gets clogged and removes fuel vapors from the tank without purifying them, which creates the stinky gasoline smell under the hood.
On the other hand, if the valve gets locked in the open position, the engine will constantly receive extra air that is not taken into account by electronics. This will create negative pressure in the fuel tank, which may affect the operation of weak fuel-oil pumps, misshape the tank or hinder tank cap opening. All this is not good, so you'd better completely choke up the pipe between the intake manifold and the carbon canister, or replace the broken-down bypass valve with a new one (they're pretty cheap, indeed).