loud swoosh sound in gas tank cap when opened

**spyghost** · April 9th, 2010 05:09 PM

Hi,

My car is 4g15 carb sohc and just for the purposes of maintenance i decided to dismantle my mech fuel pump and check on the lever actuation against the pipes.

I noticed that the fuel in the return line was "pressurized". odd because (1) its a return line to the tank, and (2) i am running a carb with mech fuel pump, and (3) the engine was not runing for more than a week already.

paghatak ko ng return line, e biglag bumulwak ng dirediretso ang gasolina... malakas ang agos. so i returned it back to the fuel pump. on the other hand, the other 2 lines, suction and out line do not spurt any fuel... return line lang.

so i decided to open my gas tank cap. to my surprise ang lakas ng swoosh sound. i just don't know if its pulling air or pushing air out of the tank.

after doing this, when i pulled out the fuel return line, no fuel spurting anymore.

is this normal? because i don't think it is. what can i do about this? i have these options to look at, but can you guys help me shortlist these things to look at:
- gas tank cap needs replacement?
- blockage in fuel lines (i doubt, but worth looking at)?
- dirty fuel tank? (i also doubt)
- evap system? (isn't it that the charcoal canister is zero maintenance?)

one more thing, it seems that my mech fuel pump isn't able to suck in fuel on extremely hot weather...

???

**aejhayl17** · April 9th, 2010 06:43 PM

i think normal lang "vacuum" sound pagbinubuksan yung tank cap.

whenever i have a full tank, pagnaubos ko na, pagkabukas pa lang ng gas boy, may maririnig na kong air na papasok after that gas vapors naman ang lumalabas. afaik, sealed ang fuel tank, so whenever gasoline level drops, may nababakanteng space, kaya pagkabukas ng gas cap, pumapasok ang hangin

**jick.cejoco** · April 9th, 2010 07:12 PM

the gas cap has a one-way valve, it allows atmospheric pressure to enter the gas tank to equalize pressure or the fuel system goes into vapor lock (the mechanical fuel pump will have a hard time pulling fuel because the line will be vacuum). when the car is on a hot soak, the tank pressurizes but to prevent the tank from rupturing, the system is equipped with the evaporative system that relieves the pressure to the charcoal cannister that stores the fuel fumes and releases the air to the atmosphere. in your case, the pressure release in inhibited. check the vapor line for obstruction. the older fuel plumbing has three steel tubing namely: fuel supply line, fuel return line and the vapor line.

**aejhayl17** · April 9th, 2010 07:14 PM

Originally Posted by jick.cejoco

the gas cap has a one-way valve, it allows atmospheric pressure to enter the gas tank to equalize pressure or the fuel system goes into vapor lock (the mechanical fuel pump will have a hard time pulling fuel because the line will be vacuum). when the car is on a hot soak, the tank pressurizes but to prevent the tank from rupturing, the system is equipped with the evaporative system that relieves the pressure to the charcoal cannister that stores the fuel fumes and releases the air to the atmosphere. in your case, the pressure release in inhibited. check the vapor line for obstruction. the older fuel plumbing has three steel tubing namely: fuel supply line, fuel return line and the vapor line.

eto yung gusto kong sabihin! nice one sir jc

**spyghost** · April 9th, 2010 08:26 PM

Originally Posted by jick.cejoco

- fuel supply line
- fuel return line
- vapor line.

in case of clog (which probably inhibits pressure release), how do you unclog these long pipes?

does the charcoal/evap canister get filled up? what's the maintenance procedure for this (testing, cleaning, etc..)?

how will i know if my gas cap needs replacement na?

**jick.cejoco** · April 9th, 2010 08:34 PM

Originally Posted by spyghost

in case of clog (which probably inhibits pressure release), how do you unclog these long pipes?

does the charcoal/evap canister get filled up? what's the maintenance procedure for this (testing, cleaning, etc..)?

how will i know if my gas cap needs replacement na?

to unclog, i always used compressed shop air going back to the tank with the gas cap removed. the charcoal canister does not require periodic maintenance. however, if the intake manifold is not able to purge it, it will accumulate vapors and get clogged up