Why does my gas not fill all the way?

Question

Same as the title. I have filled twice, different stations, both fill to just over 3/4 and stop. I am not sure if this is the gauge or something with whatever magic tells the pump to stop. Is there any simple troubleshooting I can perform?

Answer 1

I have had this problem with one of my vehicles as well.

It usually has to do with the fillhose vent tube/breather tube being blocked or not working properly. This inhibits air pressure from being released from the tank as you are filling it. This will cause air to travel backwards through the fill tube and cause the pump to shut off as it senses fuel bubbling back up the hose.

I usually just filled the car at half fill speed and it worked. But if you want to fix it your are going to want to pull the breather tube off the gas tank and clean it out. Sometimes insects can get in it and clog it up. Depending on your vehicle it may be an easy fix. The breather tube usually runs right beside/ontop of the fill tube.

Hope you get it resolved!

Answer 2

Rest assured that gas pumps don't use magic to stop. When gasoline reaches the top of your tank all the way to the end of the pump nozzle, it covers a valve within the pump nozzel that shuts off the pump mechanically. So in all likelihood, gasoline has filled your tank by the time it trips the pump. The chances that both the gas stations you visited have faulty pumps seems less likely.

More likely, your fuel gauge is giving you a bad reading. It could be the gauge or it could be the sending unit in the tank itself.

Answer 3

It does depend on the car. As Hillsons says, it might be a faulty gauge or sender, but it might also be the nature of the filler neck, as some designs don't flow well, resulting in the neck filling with fuel before the tank has - I had a car recently where you had to fill it at half-speed otherwise the pump would be constantly clicking off. Others I've seen where you have to fill it until it clicks, wait a couple of seconds for it to drain down, then top it up, often two or three times.

You should be able to look up the tank capacity of the car, and compare that with the amount you are putting in to work out if it is actually full (in which case the gauge is mis-reading), or if you can really only fill it 3/4 of the way.