cause of pressurized fuel tank?

[TDS-MN]

Hey All,

Ok, so if it's not one thing, it's several at once it seems with this truck.

But, the one that I'm chasing right now is, what could be the cause ( and remedy) if the fuel tank seems to 'pressurize' while driving? If I keep the gas cap loose, I don't have the problem. If I get it maybe half-a-turn too tight, fuel ends up pushing past the sending unit on top of the tank, and running down all over the place. Could this just be a vent hose somewhere that isn't allowing pressure to release? But curious how I'm getting any pressure in there anyway, it's got teh mechanical fuel pump on the engine, and I thought that the whole thing was gravity fed from tank to pump anyway.

Tank was replaced by a local shop a few years ago. This issue actually didn't even show up right after that, but was actually a few months afterward that I first noticed it. I have typically been just leaving the gas cap ( my second one, after replacing it thinking maybe it had clogged venting itself or something) loose, but mostly screwed on. Stock 305 w/ quadrajet, by the way.

Took the truck in to the exhaust shop this morning to fix an undehood exhaust leak, probably either donut gasket or manifld gasket is my guess. The noise that is making really makes driving the truck somewhat unpleasant currently. Hopefully I will at least be able to get THAT solved today. . . .

[willett]

what year is the truck? TBI? Carburated?


you probably have a clogged breather. On my '78 the breather is integrated onto the filler mouth. There is a 3" filler line and there is a 3/8" vent line that also clamps to the underside of the assembly. I'm not sure about newer years though.

as for the pressurization it can be return from your primary fuel filter (many filters have return to prevent vapour lock) or it could be return from your intake.

Either way is is perfectly normal for fuel to be flowing back down to the tank in most cases and you most likely have a clogged breather.



you will have to drop your tank and probably pull the sending unit off and then try blasting compressed air through the breather line. don't stick your face anywhere near the exhausting end though!

[Pyrotechnic]

Depends on the truck. Non emissions trucks had a small breather on the tank sending unit. It's the same kind of breather used for a rear axle.

Emissions trucks had the whole charcoal canister setup, I couldn't even begin to tell you where to start on that.

[tucsonjwt]

3 possibilities:
1) Clogged filler neck vent hose. If so, replace with special fuel line, not vacuum or heater type hose.
2) If you have vapor recovery system, then check hoses to/from the recovery canister and check if the canister itself is clogged.
3) If you don't have vapor recovery system, then you should have a short 3" long hose coming from the sending unit with a one way plastic vent valve in the end. This vent allows fuel vapors to vent to the air, which was allowed through early 80s on "heavy duty" 3/4 ton trucks and up. Let me know if you have this setup and need pics.

[motornut]

some caps are vented but only "in"
duno if a non vented would cause that
sounds maybe a line/tank vent plugged yup

[TDS-MN]

Thanks everyone. This is my '85, which does have all of the emmisions stuff for the day. Do have the charcoal canister under the hood, driver side right behind the headlight. the fact that this started independently from the tank replacement, i think that i will start there, and at the vent line near the filler to verify if there are any obstructions I can notice.

I really don't want to have to fight with dropping the tank, if i can avoid it at all.

It's wierd, as I only put enough gas to fill to about half-tank, in my 20 gallon tank. That SHOULD leave a lot of room inside, but the fact that gas is pushing past the sending unit and dripping at a fairly decent rate to make a puddle, there is obviously pressure in there. When I did loosen the gas cap at noon, there was a rush of air out the filler neck, which I sort of expected.

I'm guessing that, now that I have the gas cap loose, I won't have any further dripping the rest of the day. It's a work-around, but the bottom line is, there's something wrong with the whole system. Hopefully some of the things suggested will get me going in the right direction.

It's sad, because back when I was driving this truck regularly, I always remembered to leave the cap loose. This morning on the way to the shop, I just tightened it too much, being out of practice. By the way, the guys at the shop were not interested in working on the exhaust with the gas pouring out, so no fix on that at all today either. Hopefully next week.

[tucsonjwt]

Try disconnecting the lines to/from the evaporative canister. If the canister is clogged up this should solve the pressure problem in the gas tank. You might have to loop the two ends of the hoses together if your emissions system requires a closed loop to operate. It seems to me that if you have fuel coming out of the sending unit nipples, then the clamps on the hoses at those nipples must be loose or the hoses worn at that location. You can still buy replacement evaporative canisters if that is your problem.
You can lie under your back with a flashlight and inspection mirror and get a pretty good look at what the sending unit plumbing looks like. There is room to get your hand up there and feel around, if you are so inclined.

[lunchbox99]

I had the same issue on mine, if you are running a mech fuel pump which i believe you said, check to see if it has 3 ports on it. if it does get one with 2 ports, plug the 3rd line(emmission line from canister i believe). hook up the other 2 and fixed my problem...napa fuel pump was $12.

[cdowns]

lots of people disconnect the evap cannisters and seal the port off at the gas tank causing the pressurized problem// i've always disconnect and used the tubing as tthe vent

[TDS-MN]

well, the emissions system on the truck is pretty passive. No computer, so it just has cat converter and charcoal canister, and AIR pump. Hopefully I'll get a chance to look at it this weekend if the rain holds off. Should be something I can easily bypass for testing purposes at least.

As predicted, now that I have the gas cap loose, I was able to drive the 18 miles home, and no dripping when I parked.

Interesting mention of the fuel pump. I did replace that last year, but don't recall how many hose ports it might have had. Will take a look, for what it's worth. I THINK that I had this issue even before that time, but am not 100% sure anymore.

Thanks for the other ideas though.