Gas tank pressure?

[sewebster]

Another question, sorry

 

So, when I pulled my engine I just left the fuel lines in the engine compartment. I noticed a couple days later that fuel leaked out sometimes, and I think it was during the day when the garage heated up. I'm pretty sure the lines were above the level of the gas tank. I solved the problem by pulling the lines up as high as I could get them, above the hood level.

 

I am wondering if I have a clogged line from the tank to the canister though... shouldn't that be relieving excess pressure? The line on the canister side looks clean, no carbon leaking out of canister. Maybe the gas tank is just higher than I realize and I was siphoning? But the lines really weren't all that low, they were at like the same height as the battery or something...

 

When you open your gas tank and you hear the air flowing, is that pressure coming out, or air going in to relieve a vacuum? If it's gas coming out, then why doens't that go to the canister too?

[Tungsten]

When you open the cap, the air rushes into the tank.

[sewebster]

When you open the cap, the air rushes into the tank.

 

Ok, cool. So I guess there is a one-way valve somewhere in the evap system that prevents air from flowing back in. So my original question makes sense... why would my gas tank develop enough overpressure to leak fuel out the lines? Either there is a clog, or I somehow had the lines low enough to siphon, those are the two possibilities I see.

[XSrcing]

Air flows in? I thought all EVAP systems worked on positive fuel tank pressure. There is a solenoid that allows the fuel vapor to flow in to the charcoal canister.

[Tungsten]

Ok, cool. So I guess there is a one-way valve somewhere in the evap system that prevents air from flowing back in. So my original question makes sense... why would my gas tank develop enough overpressure to leak fuel out the lines? Either there is a clog, or I somehow had the lines low enough to siphon, those are the two possibilities I see.

 

You had a broken hose and it is normal for fuel to squeeze out of it. If you just disconnect the lines at the fuel sender and leave the gas cap closed, in a perfectly sealed tank, fuel will come out of the feed nipple.

 

I know this because when I went to replace the seal on the gas tank between the sender and the tank, I forgot to leave the gas tank open so when I tightened it up, fuel just started gushing out of the sender. Fuel vapors inside will pressurize the fuel out of the fuel tank.

Edited October 5, 2010 by Tungsten

[OldSlowReliable]

...

 

When I took my tank out of the truck to weld the frame, I had the cap on properly, but the outgoing lines open to the air, and fuel leaked out (and promptly stripped the tank of all paint/coating)

 

It did it constantly throughout the day, until I made sure the cap was off, when it didn't do it

[slade420]

im having similar problems with my d21, the fuel check valve on the line coming out of the tank was clogged, and a second line, what i think is vacuum line is clogged and broken around the front passenger side tire... is this second line a vacuum line? It T's off and goes down to the top of the front diff, i think the other end goes to the top of my auto trans. im guessing these are fairly important for shifting, are they not?

[CrazyLarry]

im having similar problems with my d21, the fuel check valve on the line coming out of the tank was clogged, and a second line, what i think is vacuum line is clogged and broken around the front passenger side tire... is this second line a vacuum line? It T's off and goes down to the top of the front diff, i think the other end goes to the top of my auto trans. im guessing these are fairly important for shifting, are they not?

slade, That "second line" is just a vent line for the front diff and trany.

Edited October 6, 2010 by CrazyLarry

[MY1PATH]

"air rushing in" is a misinformation.

Gasoline is constantly vaporizing, the emptier your tank the more room there is for vapor and the vapor likes to expand. Thats why, if you fill up after a constant 300 mile drive you hear a woosh, because all that vapor has not been able to get thru the canister yet so it RUSHES OUT. this is also why some emissions stations test your gass cap; to be sure it is no constantly venting to atmosphere.

the canister does vent but the vapor has to get thru a pile of carbon first which slows it down and stores some of it which gets sucked up thru a vaccum line when the engine is running.

All fuel systems are postive preassure or neutral preassue.

 

if your open lines are leaking remove the gas cap, they will stop quick.

[sewebster]

"air rushing in" is a misinformation.

Gasoline is constantly vaporizing, the emptier your tank the more room there is for vapor and the vapor likes to expand. Thats why, if you fill up after a constant 300 mile drive you hear a woosh, because all that vapor has not been able to get thru the canister yet so it RUSHES OUT. this is also why some emissions stations test your gass cap; to be sure it is no constantly venting to atmosphere.

the canister does vent but the vapor has to get thru a pile of carbon first which slows it down and stores some of it which gets sucked up thru a vaccum line when the engine is running.

All fuel systems are postive preassure or neutral preassue.

 

if your open lines are leaking remove the gas cap, they will stop quick.

 

 

Ok, so maybe I don't even have a problem after all. That would be nice

[slade420]

slade, That "second line" is just a vent line for the front diff and trany.

 

thanks man, that is great news. im glad its not vacuum related.

 

oh ya, cuz my vent line was clogged, the tank would pressurize and when the cap was removed gas vapour came rushing out, enough to blow the gas cap a good 10 feet.

[Dowser]

"air rushing in" is a misinformation.

Gasoline is constantly vaporizing, the emptier your tank the more room there is for vapor and the vapor likes to expand. Thats why, if you fill up after a constant 300 mile drive you hear a woosh, because all that vapor has not been able to get thru the canister yet so it RUSHES OUT. this is also why some emissions stations test your gass cap; to be sure it is no constantly venting to atmosphere.

the canister does vent but the vapor has to get thru a pile of carbon first which slows it down and stores some of it which gets sucked up thru a vaccum line when the engine is running.

All fuel systems are postive preassure or neutral preassue.

 

if your open lines are leaking remove the gas cap, they will stop quick.

 

 

That's what I was taught as well. Its not air rushing in as many believe it to be. In fact, If you take a long ass drive and empty your tank, then fill up and don't hear that WHOOOOOSH.... Then that's more of a problem.