Pressure build up in petrol tank

[cactusJack57]

Hi All,

I'm new to the site and I was hoping someone might be able to help me out. I have a 2007 toyota yaris t-sport and for the last few weeks a large amount of pressure is building up in my petrol tank, this pressure then vents through the petrol cap every 2 to 3 minutes so I assume there's a blockage somewhere. Does anyone have any idea what the blockage/problem could be? I was talking to a mechanic friend of mine and he's going to take a look at it, but he said it could be a number of things and it could be very costly so any ideas/suggestions would be appreciated.

[FROSTYBALLS]

Welcome to Toyota Owners Club.

Moved to the Yaris club.

[bathtub tom]

Petrol caps used to have a small hole to allow variations of pressure to vent in and out. Emission controls now don't permit this and as a consequence fuel systems have become complicated.

Petrol is a volatile liquid that readily vaporises, this results in an increase of pressure in the petrol tank as temperature rises. A negative pressure will result in the petrol being used and the volume in the tank reducing.

We now have complicated devices in the fuel system that mean a 'hiss' is often heard when removing the petrol cap. This could be vapour escaping or air being drawn in. MOT testers will use this as an indication the petrol cap is sealing adequately.

Diseasels are a different matter.

[cactusJack57]

Thanks for the reply, my mechanic friend had a look at it and it turned out to be a faulty vacuum valve and charcoal canister no.2 if anyone has the same issue.