Asking this for a friend. Their mechanic says the fuel tank ventilation valve is broken.
What is the fuel tank ventilation valve?
What does it do?
Is it an important part? If yes, Why?
EDIT: In general, could they replace it themselves? Would it be a difficult job?
Instead of rewriting this, I am stealing the following from this website:
Your tank has to breathe. In order to breathe, your tank must have a vent somewhere that will relieve both vacuum and pressure. Gasoline expands in volume as it warms up and shrinks in volume as it cools down. The fuel level of your tank changes throughout the day, even if you are not driving it.
You cannot put fuel into your tank, unless you can get the air out...and you cannot withdraw fuel from your tank unless you can let air in.
Up through the 1960's, most vehicles used vented gas caps. This is simply a gas cap with a hole in it. Unfortunately, this hole would allow the fuel to splash out when accelerating or turning a corner.
This was basically brought about due to emissions. The fuel tank is a closed system on cars since 1996 (for the most part). The valve allows for the equalization of air pressure.
EDIT: In general, could they replace it themselves? Would it be a difficult job?
This depends a bit on what model the vehicle is. On some vehicles, the valve and the tube from the tank form a complete assembly that is easy to access. On others, the tank would need to be dropped to gain access to its top (where the valve tube is connected).
If necessary, dropping a fuel tank is an involved process. You would need a vehicle hoist for the car, and a vertical jack to hold the tank while it is being lowered. Best practice would be to ensure the tank itself is empty or as near empty as possible, to reduce weight and associated risk.
Rusted connections can also be an issue. Beside needing to actually unfasten the old valve tube, you would need to take care and make sure no old mud or bits of rust fall into the tank and pollute the fuel.
In general, the old separated ladder chassis found on some 4x4 grants easier access and perhaps can make tank removal avoidable. Modern unibody car construction makes things more complex.
Valve is essential to filling and draining fuel from the tank. Replacing the fuel pump assembly where this valve is located is different on every type of car. Do some research about how to replace the fuel pump assembly on your friends car.
