Beginning Mechanic's Guide: How To Test And Replace A Leaking Fuel Tank Filler Neck On A Car


A tiny leak in the fuel tank filler neck can cause the check engine light on the dashboard of your car to turn on. When you check what the error code reading is using a diagnostic tool that hooks up to the car's computer (something some auto parts stores will do for you for free) and it comes up P0440, it means there's something wrong in the Evaporative Emission Control System (EVAP) on the car. A common reason you'll get a P0440 code is because the fuel tank filler neck has a small leak in it. Here is how a beginning mechanic should test the fuel filler neck for leaks and how to replace it if necessary.

Smoke Test

The best way to test the fuel tank filler neck for leaks is by using a smoke leak detector machine made for testing the EVAP system on a car. You can usually rent a smoke leak detector machine at your local auto parts store. The machine comes with a cap that fits over the opening in the filler neck where the gas cap goes. The machine's cap will have a small tube sticking through it. You pour the smoking agent into the machine and turn it on. Once you see smoke being produced, you attach the hose on the machine to the tube on the cap on the filler neck. If you see smoke rising out of the top of the filler neck, or along the stem of the neck to the fuel tank, you know you need to replace the filler neck.

Removing the Fuel Tank Filler Neck

First, run the car until you are almost out of gas. This will prevent fuel from running out of the tank when you remove the filler neck from it.

The next step is to remove the gas cap. There will be two plastic rivets with torque screws in them to connect the filler neck to the bracket on the car at the top of the neck. Remove the screws with a torque wrench and pop out the rivets.

Now you have to climb under the car. The filler neck runs from the gas cap opening, bends around the bottom of the chassis, and connects to a rubber hose going into the fuel tank. The filler neck is bolted to the chassis. You should spray the bolt with some penetrating oil to eat through any rust (this will make it a lot easier to unscrew the bolt if it's rusted).

The filler neck is connected to the rubber hose going into the fuel tank with a hose clamp. Loosen the hose clamp and pull the neck out of the rubber hose. You can now pull the filler neck out from under the car.

Installing the New Fuel Tank Filler Neck

Slide the new filler neck into the place of the old one. Spray the inside of the rubber hose with a little penetrating oil. This will create a slippery surface and make it easier to slide the filler neck end into the rubber hose. Slide the filler neck into the rubber hose.

Bolt the filler neck back onto the chassis, but leave it a little loose until you attach the top of the filler neck to the bracket by the gas cap opening. Line up the rivet holes on the filler neck to the holes on the bracket and slide the rivets into the holes. Screw the torque screws back into the rivets. Now go back under the car and finish tightening the bolt holding the filler neck to the chassis.

Fill the car up with gas and go for a ride. The check engine light should go off after driving around for about 20 minutes or so. Talk to a center like Yearwood Performance Center for more information.

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