89 Toyota Pickup Gas Cap Hissing

ADAMFABAREZ

MEMBER
1989 TOYOTA PICKUP

Recently everytime I take off the gas cap it release a lot of pressure(in the form of a hissing sound) and a clunk is heard (sounds like the tank is returning to it's shape or something). Even if I run the car for a minute there is a pressure relief. And when the car is off there is a pressure relief even after it had just been relieved a minute before. I replaced the gas cap thinking that it wasn't venting, but that didn't work. Is this normal? If not what needs to be replaced or checked? Please help. :D

Tuesday, March 6th, 2007 AT 8:31 PM

3 Replies

RASMATAZ

MECHANIC
75,992 POSTS

Have the Evaporative system check from the charcoal canister to the fuel tank

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Wednesday, March 7th, 2007 AT 6:02 AM

ADAMFABAREZ

MEMBER
4 POSTS

Thanks a lot, i'll try that.

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Wednesday, March 7th, 2007 AT 1:00 PM

JOEMECHANIC

MEMBER
28 POSTS

Most modern E.P.A. Compliant vehichles are euipped with an anti evaporative fuel system, which is meant to keep the fuel vapors out of the atmosphere, these systems reuire a gas cap that has a total seal up to 14 psi. The hissing you are hearing is not pressure, but rather vacuum;air sucking back into the tank, to compensate for the physical volume of fuel which has been consumed, and the crunkle sound is often the large flat portion of the tank(the part most sensitive to changes in atmospheric pressure)returning to normal, almost all modern cars do this to some extent.

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Tuesday, March 13th, 2007 AT 3:30 AM