I'm trying to locate the air valve or vent that lets air in to the gas tank as gas is pumped out. Mine must be clogged because there is a loud hiss when i remove the gas cap and a croaking noise periodically when the truck is running coming from the gas tank area.

I believe these are indicators that a vacuum is forming in my gas tank because the mechanism that allows air into the tank as the amount of gas dwindles is clogged and needs to be cleaned.

I have a 1997 F150, its a 4.6, im sure that engine size is irrelevant in this case,

does anyone know where the valve or vent is on the tank, location and any visual information would be greatly appreciated i cant seem to find it. I was hoping it would be obvious and somewhere in plain sight

 

If you have no EVAP code set, there is no problem.
The EVAP system tests for a large leak and a small leak by drawing vacuum in the tank through very small test orfices .
This test takes a long time once it is iniatiated from scratch such as a code clear, otherwise it looks at the tank cap for being removed when the tank is filled, on a short term basis for just a leaky cap seal after the motor is restarted..

 

So its supposed to croake? That doesn't seem likes its functionin properly. What mechanism introduces air to keep the applied vacuum from becoming too great?

 

Why did you start a new thread?

https://www.f150online.com/forums/19...questions.html

 

to focus this and provide a better reference for future users, if you could the other one could be removed

 

It's more than a simple vent. On my '97, there is a big plastic (I think) box mounted under the bed (I think it's above the rear axle or in that general area) with some hoses running to the tank and towards the engine.

If you don't see any of those hoses, it could be that a previous owner decided to remove all that stuff (and maybe plug the connectors). I have no idea of what the ramifications of that is.

As to the evap codes mentioned by Bluegrass. I don't think my '97 works that way. My filler neck rusted right through creating plenty of room for air (and gas) to leak through. I didn't get any codes.

The fact that your building vacuum in the tank means that your tank and filler is holding up much better than mine.

 

thanks ill take a look for that and with the odd cap mechanism we have i dont think our systems are that sensitive to leaks