everytime i fill up my car, the pump always shuts off at just the right time to fill my tank. every tank is a different size, so how does the machine know?
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There’s a card mounted to the fuel pump that has 2 wires on it and a series of resistors.linked to those wires that has a wide range of electrical resistance. Attached to that card is a float that sits at the surface of the fuel in the tank. As the fuel depletes, the float gets lowered and that arm attached to it moves across the resistors and changes the voltage which the car interprets as a level of fluid in the tank.
Take a look next time, you’ll notice a little tube with a hole at the end of the gas nozzle.
The tube sucks in air as the gas pumps which is easy as long as the there is air at the tip of the nozzle.
Once the fuel level in the tank rises high enough it will block the tube, preventing it from sucking in air, and a sensor notices this immediately and stops the flow of gas.
It’s super smart, super simple, and super easy.
Then why does it always click to a stop when you first start filling a dreaded trayback hilux?
I’ve seen a few cars with clogged fuel evap hoses shut off the pump every squeeze due to the lack of air flow. So like someone else said, the small hose going to the nozzle is the key.
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U say right time? Every time? Hmm… speak for yourself. I sit there having to pull the trigger about every 5 seconds or so as it ‘thinks it’s full’ :/
Your fuel pump doesn’t know when to shut off the refueling.
Inside your tank there is a Fuel Level Vent Valve (FLVV). This valve works to allow the air to escape the tank to the carbon canister and EVAP lines during fueling. Once the fuel reaches a certain point it lifts the flat inside the FLVV cutting air off to your canister external EVAP lines. This sudden drop in pressure causes the nozzle to click off letting the user know the tank is full.
This valve is placed in specific area to ensure the tank design meets the required fueling point for advertised volume. Depending on the make or model it could be in Liters or US Gallons.
Older vehicles have an externally welded valve and newer ones have them completely internal for reduced emissions. Newer vehicles most likely have the lines coming out the top of the fuel pump giving the impression that it is the pump doing the work.
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