sdhunter

Okay, I'm new to the 4Runner family, with a 94 for my son.

Gotta question, when I go to fill it up, when removing the gas cap, you can hear the tank pressure equalize when you remove the cap. I've never had a vehicle do this....is this normal?

Thanks,
Mike

scope103

Yes. When the fuel warms up, it vaporizes, pressurizing the tank. The vented cap will let air in (as the tank drains), but it won't let fuel vapors (pollutants) out. So the tank will come up to a certain pressure when warm, and when the tank is closer to empty (more head space to fill with vapors) the vapors escaping is noticeable.

So where to the vapors go, between fillups? That's where the fuel-vapor canister comes in. It absorbs the excess vapors, and when the trucking is running faster than idle, the vapors are "disposed" of in the engine.

This set up has been required for many, many years to cut down fuel vapor pollutants. You've probably never heard it before because your other cars have better working vapor canisters, broken vented caps, or you're careful to only fill them when it is cool.