Pressurized hissing from gas cap removal

My 2000 V8, 4wd AC does it every time I fill up. Thought if there was an issue with the charcoal canister that it would have thrown a code by now, but who knows? What causes this on the Gen 1 and is it an issue?

Also, just curious how many other Gen 1 owners have pressurized hissing when they remove their gas cap?

 

I don't recall hearing a hiss. But if you're not tossing a code, I wouldn't be concerned. Trust me, EVAP checks will let you know if there's an over/under pressure issue.

That said, on earlier models the check engine light bulb can go out. So ... you may want to confirm yours is working, IIRC, it should show when you have the key at ON but not START/running.

 

Is it possible for the charcoal canister to be "full" and not throw any codes, just bad gas mileage?

 

How long does the "hissing" last, sustained or momentary?

I can remember hearing my 1st gen make a momentary hiss noise of air being sucked in when I first removed the cap if I ran tank real low.

Do you usually run it close to empty?

 

Mine does it too sometimes.

 

Define "bad gas mileage". At stock height and stock tire size, with the earlier-year 4spd transmission and stock configuration if you're getting 11-12mpg in the city and 16-17 highway on average that's pretty normal. ==> EDIT: Welcome to life with 4WD!

If the canister was full, you'd be throwing a code, I would expect.

 

Probably normal. My SC400 does it all the time, duration is how low my tank is almost like opening a brand new soda bottle. No codes and get about 18-20mpg on the highway. Assuming all Toyota gas systems are alike.

 

My 01 does it sometimes as well.

 

Hissing normally lasts about 1 or 2 seconds, so momentary. Strange, I thought it was air being pushed out?

I usually fill up right when the gas light comes on, or at about 5 gallons left in the tank (I've just learned where my gauge is when I'll need to fill up 21 gallons worth).

 

Maybe fill up at half tank and see if it does it... just for shits and giggles.

 

When my sister borrowed my truck she gave it back to me on below empty empty and I hear a noise when I opened the gas cap

 

Both of my Toyota vehicles do this. I assumed it was normal and a sign the gas cap was sealing properly.

 

The Land Cruiser 100 is known to have excessive fuel vapor pressure and my 1999 will blow all sorts of vapor out of the tank when I remove the gas cap. My 2004 DC Tundra doesn't let out any vapor when the cap is removed.

 

I thought it was a vacuum hiss & my tank vent might be plugged.

 

Perhaps a new gas cap is in order?

 

Mine does it too when the fuel level is run low. I try not to run it low often because fuel pumps work really hard and generate a fair amount of heat. When the pump is fully submerged in fuel, the fuel is helping to keep the pump cool. Regularly running your fuel tank to near empty means the pump is regularly running hot -- which can be detrimental to its longevity.

 

Wouldn't this be an indication that that the gas cap is properly sealing the tank?

 

Every vehicle I've ever driven, whether yota or not did this