Same issue. Drive for a while....all is normal. Then; rapid decrease in the fuel level on the gauge down to "E" but the vehicle still drives normal. Open the fuel door and push on the metal flapper valve and release the built-up pressure in the fuel tank and the fuel gauge returns to normal operation. Gradually, the tank pressure builds up again and I go through the same sequence. It takes about 40 miles of driving for this to occur. Any ideas for me??? 2012 FX4 5.0L with 170,000 miles.

 

Codes? The evap system is supposed to keep the pressure stable. Sounds like it isn't.

 

My check-engine light never came on; even when the vacuum was intense enough to suck in the side of the fuel tank. I don't have a code reader, but am planning to bring it to my local parts dealer later today and will get them read at that time (if any). I need to release the damn pressure in the fuel tank first before I do anything. It's not drivable in its present condition.

 

Look up the procedure for testing the evap purge valve. It's in the engine bay but causes this type of suction your talking about. It's a simple test for suction but I can't remember the specifics. Worth a shot.

 

same here on my 2012 F 150 I stop at the pump & use the filler nozzle 3 or 4 times to bang the door & the light goes off I have no fuel cap so that is not an issue. so when I went to fill up today I stuck the fuel hose nozzle in and the back pressure scared the crap out of me any idea what may cause that ? Thanks