Gas tank pressure

[Jolby]

I am wondering what is the best fix for excessive gas tank pressure. It actually floods the carb and leaks down onto intake manifold. I hooked up my under bed tank and capped off unused lines. Can I solve problem with a different gas cap and if so which one? Do I need some type of tank vent or canister? The filler that I have uses the same cap as the stock tank but with that cap the pressure is high, large whoosh when uncapping.

[tincan1966]

Unless you've got an in-tank fuel pump, there's nothing in the tank, itself that will cause pressure.
if the tank isn't vented properly, normally you will have a vacuum situation, which will cause fuel starvation, not flooding. And the "whooshing" sound when removing the cap is the vacuum being broken, or air entering the tank.
From the picture, it appears you have the correct vent connected.

The carb flooding would indicate to me a couple of things:
1) fuel pressure is too high-need to check output of fuel pump.
what pump do you have?
What carb do you have? If you have an Edelbrock carb, for example, they really do not like pressure over about 5-5.5 psi. even a GM carb(2 or 4 bbl) shouldn't be over 6.5-7 psi.
2) it is possible that the needle and seat are either dirty or worn and not sealing off the incoming fuel to the carb.

Does it food at idle or while driving, or does it flood after you shut it off? All of these can be different issues with different diagnosis and repairs.

[Jolby]

I didn't have any problems until connecting the tank to the fuel pump. Everything was fine using a gas can sitting beneath the engine. I have an Olds 403 engine with a quadrajet on top. The pressure builds up in the tank while it is sitting in the garage dormant. First it force gas out of the connection between the hose on the tank and the line in the frame rail. Replaced the line with a better connection, then truck ran and idled but then ran rough and started to sputter and then would not restart. Took the carb off to check the float and needles. Left the line in an upright position, went to dinner and a large amount of gas had escaped. All of this with the truck still on jack stands. I haven't driven the truck since the complete rebuild from the frame up.

[racecougar]

The only vent I can make out in that pic is the filler neck vent line, which just keeps fuel from "coughing" back at you when filling the tank at the gas station. From what I can see, the other nipples on the tank are capped off. It sounds like you either need to run one of those to a vent, or switch to a vented cap.

[tincan1966]

Is your pump a 2 line or 3 line?

There is nothing in a stock tank to create fuel pressure. Maybe your lines are connected incorrectly like having the return routed to carb and supply back to tank(if you have a 3 line fuel pump) If it didn't do it before, then it's apparently in how you connected the tank to your fuel system.
I cannot tell in the picture- what is the small hose on the passenger side behind the sending unit connected to?. Looks like you have a later model tank with a return line provision. The vent is the hose running up with the filler neck.

What is the actual fuel pressure?

What's it do if you remove the gas cap?

[Jolby]

It is a 3 line fuel pump with the return capped off, been that way all along. I am going to go and pick up a Stant 10623 gas cap, it is vented but I am not sure if it works both ways like some caps to either equalize the vacuum or pressure in the tank. I was wondering if hooking up the return from the fuel pump would help with the pressure but didn't really think so. I couldn't find a charcoal vent readily online, need something like that as long as it is parked in the garage.

[TJ's Chevy]

Is your pump stuck in the on position???? Probably a stupid question...but just wondering...

[Rich 5150 69]

If his gas tank is full or even just close, and no vented cap the ambient heat will expand the gas causing it to take the path of least resistance, thus the carb, there have been several times I have filled my truck up in the spring mornings when the gas was cool and the truck sit out at work in the sun and the gas had expanded pushing out thru the factory filler cap, think I would put either a vented cap or a pigtail vent on the tank, the more gas you have the larger the expansion rate.

[tincan1966]

Originally Posted by T.J. MCCAULEY

Is your pump stuck in the on position???? Probably a stupid question...but just wondering...

It's a mechanical pump.

[tincan1966]

If the tank had no vent, and no return line connected, you would experience a fuel starvation issue, NOT an over pressure. There is nothing pushing fuel back to the tank.

It's possible the pump is defective, but with only 1 line connected, how would fuel get back to the tank while it was running?

Still hard to tell without an accurate fuel pressure reading- that's going to be the only way to know for sure if it's really too much pressure, or some other problem.

[pdxhall]

Originally Posted by Rich 5150 69

If his gas tank is full or even just close, and no vented cap the ambient heat will expand the gas causing it to take the path of least resistance, thus the carb, there have been several times I have filled my truck up in the spring mornings when the gas was cool and the truck sit out at work in the sun and the gas had expanded pushing out thru the factory filler cap, think I would put either a vented cap or a pigtail vent on the tank, the more gas you have the larger the expansion rate.

I agree. I've had plastic gas cans bulge out from pressure build up from sitting out in the sun. Temp difference will cause pressure changes in the tank if it is not vented in some way.

[luvbowties]

Originally Posted by Jolby

It is a 3 line fuel pump with the return capped off, been that way all along. I am going to go and pick up a Stant 10623 gas cap, it is vented but I am not sure if it works both ways like some caps to either equalize the vacuum or pressure in the tank. I was wondering if hooking up the return from the fuel pump would help with the pressure but didn't really think so. I couldn't find a charcoal vent readily online, need something like that as long as it is parked in the garage.

My system is quite similar to yours, and you might want to copy it. I put an in-line filter on that line you have capped off on the passenger's side. Ran a longer fuel hose from that outlet, routed it higher, and installed the filter with the other end open to allow for BOTH expansion/pressure AND contraction/vacuum.
HTH,
Sam

[51 okie]

Originally Posted by luvbowties

My system is quite similar to yours, and you might want to copy it. I put an in-line filter on that line you have capped off on the passenger's side. Ran a longer fuel hose from that outlet, routed it higher, and installed the filter with the other end open to allow for BOTH expansion/pressure AND contraction/vacuum.
HTH,
Sam

Sam I am looking at doing what your talking about. Where did you route the longer hose from the extra port on the tank to in order to not get debris and water from road sucked in?
Did you just use a regular fuel filter or something else? Thanks

[AcampoDave]

Sorry to but in, but I got an anti roll over check valve vent from Tanks Inc. and ran a hose to it up the rear stake pocket. The pocket retains the check valve in the nessessary vertical position and also keeps it high enough and out of sight. Works perfectly.

[luvbowties]

Hi 51okie. I used a regular inline filter and ran it up into the wheel-house. You may want to cap the open end with some screen/mesh to keep out dirt dobbers. I did not, and have had no issues at all. As bad as those wasps are around here, the odor of gasoline may shun them. As far as debris is concerned, that has been a non-issue too--the filter works BOTH ways anyhow, "going and coming".
Good luck.
Sam