I have No Pressure in Gas Tank when I open the gas cap on an 83 lg4. Is that a problem? I have a new canister. How often does the Canister purge valve go bad?

thanks,
phil

 

and theres supposed to be no or very little pressure as the fumes from teh tank are diverted to the charcol canister. If you hear, as one person decribed it, '10 seconds of hissing and a loud thump' as the tank restores itself to its former shape when the gascap is opened, then you ahve a problem due to the tank not venting.

 

that's why i love this board.

for a year now i've been going through exactly what demented described

i just never asked the question.

is there a way to get rid of that? new gas cap or something.

i also removed the charcol canister but it's the same way with or without it.

 

That is precisely, extremely, exactly, wrong.

There is supposed to be pressure. There is a whole control system, complete with a safety check / pressure valve, to ensure that there is pressure.

A thing to look at might be the little plastic check valve, located in the rear axle tunnel just in front of the left rear brake, on the end of one of the lines coming out of the gas tank.

 

hmmm... ive never had much noticable pressure in any of my gm cars, even in the middle of summer. To the post above RB's, if your car was building up enough pressure to need to bypass the vvaporized gas through the reliefe valve, and build up a good head of pressure in the process, then the system wasnt venting properly to the canister. When i did away with teh charcoal canister like you, there was no where for the vapors to go except through the valve so the tank was always pressureized. So much so that it would squirt fuel back up through the lines when i took the fuel system apart. i took the pressure relief valve apart and replaced teh valve assy with some gas resistant open cell foam to prevent dirt from getting in. you could also put a breather on teh end of the line. its not environmentally friendly as teh fuel isnt being recaputured but it will solve your pressure problem.

 

I guess I have a dumb question. What generates the pressure? Where does it come from? Almost everything is the car runs on vacuum. What would put pressure into the tank?

phil

 

The evaporation of gasoline....

 

ok so right now i have a vaccum cap on the line under the hood from the charcol canister. if i take it off would that stop the massive amount of pressure build up?

or get a breather for it if i can find one small enough. or should i get under the car and mess with that valve thingy you were talking about?

 

Pressure is supposed to build up. That's the way the system is designed. In fact, many new cars will light their Check Engine light and set a code if there's insufficient pressure in the gas tank; the usual cause of that is a loose gas cap. But, in case of a leak or a failure of some part, the CE light alerts the owner that something is wrong, which could have emissions consequences.

So if you tank builds up pressure, it's perfectly normal and expected, and designed that way. There's nothing you need to fix because it's working right.

Incidentally, my 83 has been doing that (pressure release when I remove the gas cap) for about 18 years so far that I know of. I assume that it probably also did it for the couple of years it existed before I got it.

 

see if you can find a breather first. Otehrwise, just pull apart the plastic relief valve and take the stuff out. It clips together so its pretty easy to take apart. Make sure you put some sort of filter on the system at teh very least so you dont contaminate the fuel system with dirt.

 

how come none of my other cars have built up enough pressure to cause noticable hissing? Even in my moms **** burner. I know theres supposed to be some pressure to get the fuel to be absorbed by the EVAP equipment but ive never noticed so much that there is hissing when i open the tank. I donno, maybe its just cooler here then it is in ohio.

 

When my car stopped having pressure in the tank, I also smelled gas. Turned out the gas cap went bad. Got a new gas cap and no more gas smell, cheap fix too.