The gasoline nozzle cuts off when the fuel tank is full, and this prevents overfilling. This can also become a very annoying cycle, when the nozzle continues to click off, several times in the process. Finding the causes are easier with a little understanding of the system.
When fuel flows into the fuel tank, an equal volume of air is displaced. If the air is not vented, pressure starts to build and prevents additional fuel from entering the tank. As in this simple demonstration, the liquid will not run into a sealed container, unless air is allowed to flow out. Open valve-b and the funnel will quickly empty; the same holds true for a fuel tank.
Expelling gasoline vapor into the atmosphere is not lawful. To vent the fuel tank and remove vapors, there is a system known as the evaporative emissions system. This system uses solenoids to open and close hoses and extract fumes and pressure from the fuel tank. A vapor canister is used to filter this flow and to contain emissions, until they can be burned in the engine. The canister is filled with charcoal pellets that hold gasoline vapors. When the purge cycle occurs, an engine vacuum draws the vapor out.
The evaporative emissions system also provides a vent when gasoline is being added. The air pressure and fuel vapors flow through the vent solenoid and into the canister.
Many vehicles also incorporate a vapor return hose in the fuel-tank filler neck. This hose allows excess vapor to vent at the top of the filler neck, above the fuel nozzle. This prevents gasoline from backing up in the neck and cutting the pump off prematurely.
When the vents become restricted, flow of fuel into the tank compresses the air within the tank. As the pressure rises, it overcomes the weight of the fuel flowing into the tank. The air pressure pushes fuel back up the filler neck. This causes the incoming fuel to rise against the nozzle. The nozzle will respond by cutting fuel flow.
Hoses can become blocked by deterioration of the rubber or physical damage. Charcoal canisters can also break down over time. Operating in a very dusty environment can plug the canister. Water may also be drawn into a canister, by driving on flooded streets. This breaks down the retaining elements in the canister and may allow charcoal to escape. These small pellets can quickly block the system and cause the vents to malfunction.
Continuing to fill a fuel tank after the nozzle cuts off, or "topping off" may cause fuel to enter the vapor canister
When fuel is forced in, the canister will be ruined. Fuel will cause deterioration of the canister which may allow charcoal pellets to escape. This charcoal will be drawn into the system by engine vacuum and cause damage. Often evaporative emission codes will be set, and the check engine light will come on.
When the fuel nozzle starts to cut off, several times when filling the tank, there is a problem. Having the problem corrected before additional damage occurs will often save a great deal of money. When the fuel nozzle keeps clicking off, call AGCO, it’s the place to go.