My car has a solenoid valve as a purge valve, i suspect sometimes valve receive less than 12 voltage because of bad alternator regulator, and yes i will change my alternator after i am done with my exams, but i wanna know if receiving less voltage can make it not work sometimes? Because gas is still building up in my gas tank so it means maybe solenoid valve is not opening occasionally, but whatever i check it by giving 12 voltage externally it always work.
Automobile components such as relays, solenoids, electronic controls and other items are designed to operate properly with a range of applied voltage called the "voltage tolerance." Some items must operate properly when the starting motor is cranking the engine. Also, items must operate for a short time without the alternator to allow safe driving of the car if the alternator fails while driving. You may be able to find the typical voltage tolerance for automotive components by searching the internet. Voltage tolerance is an important part of the design of any electrical component. The techniques applied for that aspect of design are based on the principles of design for the component.
If an automotive component does not behave properly while driving, it may be failing in some manner, the load or mechanism that is supplies of operates may be failing, or the voltage of the alternator may be dropping below the expected minimum. If the alternator voltage is dropping, the battery should support the essential functions, but the non-essential functions like the cabin ventilating fan should be shut off while you move the vehicle to a safe location. If the battery does not support the essential functions, it is weak or the charging system is failing.
There is also the possibility of low voltage caused by failing load components or a wiring fault.
... gas is still building up in my gas tank so it means maybe solenoid valve is not opening occasionally...
I'm not sure you even have a problem. Pressure can and will build up in your fuel tank even with a properly functioning evap system.
As temperature rises (like from the heat of the day after a cool night) the vapor pressure of your fuel rises, causing the pressure in your fuel tank to rise. The evap canister and system will remain at that pressure until operating parameters are met and the computer triggers the purge cycle. You would need specific diagnostic equipment to actually "witness" the cycle happening.
If you are convinced you have a problem I would suggest checking for obstruction of the vent line, since you tested the purge valve and it worked.