Pressure In Fuel Tank When Removing Gas Cap

[LuvNLife]

I sure could use some advice on this one...

'87 GMC square, 305/700R converted to carb. Running a Holley Red with a filter inline before the pump and a regulator at the carb. Because of the low fuel pressure of the pump, it's a returnless system. I just got it back from the body shop today - the paint looks great, for what it's worth.

While it was in the body shop, and the bed was off, I had them install a between-the-rails 19 gallon fuel tank, with the fill now in the bed. It starts and runs great, but I only drove it about 1 mile to the shop of a friend of mine who'll tuck it inside for me. On the way there, I stopped for gas because the body shop mechanic forgot to install the new fuel gauge. When I unscrewed the gas cap (threaded, with an O-ring seal), I smelled raw gas, and when I got it almost completely out, a real blast of air came out; no fuel, just a hell of a lot of air. It's almost like the tank is pressurized.

Any thoughts on where I could start troubleshooting this, or is it nothing to be concerned about?

Jerry

[IADC43]

I'd get a new vented gas cap. If that isn't possible (because it sounds like a custom cap) then the shop should install a vent line.

[donut]

Head back to the shop and have them finish (you did pay for it correct?) Sounds like they forgot to vent the tank somehow.

[LuvNLife]

Originally Posted by donut

Head back to the shop and have them finish (you did pay for it correct?) Sounds like they forgot to vent the tank somehow.

The shop is someone I've worked with in the past, and stands behind his work 100%, and no - the last payment is outstanding. Like Reagan said - trust with verification! His guy still needs to install the fuel gauge, anyway.

The tank is a POL piece, and I do know there's a bung in a corner that could be a vent or return line. Given the horrible access to the tank once it's in, I'm finding chopper (motorcycle) tanks seem to run the diameter threaded and vented cap, so I may give him the option to switch to one of those. I have a bed cover on it already, so the visual wouldn't phase me.

Jerry

[LuvNLife]

Originally Posted by IADC43

I'd get a new vented gas cap. If that isn't possible (because it sounds like a custom cap) then the shop should install a vent line.

What threw me is the pressure rushing out. In my (far from expert) way of thinking, a bad or missing vent line would create a vacuum, with the fuel pump constantly pulling the fuel out, and opening the cap would have sucked in air, rather than push it out. So, maybe what I'm thinking was a rush of air coming out, was actually air rushing in, then back out?

I know the shop will check the vent line, but I like to be able to talk to them somewhat intelligently...

Jerry

[Slammed84]

temperature change inside the tank could cause pressure to build

[LuvNLife]

Originally Posted by Slammed84

temperature change inside the tank could cause pressure to build

It's been a balmy 60-some degrees here today, and before I got the truck, it was stored indoors at about the same temp. No tailpipes close to the tank, either - they're on the to-do list for next week.

When I was a teenager, I pumped one heck of a lot of gas, and remember this would sometimes happen with the old style gas tanks and caps, but being young and dumb, never bothered to ask "why"...

Jerry

[donut]

The temp change could be the result of the sun. It doesn't take long. Lost a couple gallons with a venting issue of my own. Disconnect the fuel line, and just the sunlight was enough to push fuel out the open line. (yes, dumb of me. I was just going to the parts store...)

[Rich84]

I had the same issue with mine after changing to TPI..I drilled 2 small holes in my gas cap so it can vent. No more issues.

[LuvNLife]

Originally Posted by Rich84

I had the same issue with mine after changing to TPI..I drilled 2 small holes in my gas cap so it can vent. No more issues.

I'm glad someone else mentioned this.

I spent a ton of time trying to research the issue, and noticed a lot of the aftermarket vented caps had a pretty simple hole in them. I've never been one to fill up my fuel tank all the way into the neck, so I may go with this as a fix...

Jerry

[rich weyand]

If the weather is nice, they are always going to push air. When you filled it, the gasoline coming out of the ground was at 55*. Warm it up, it will expand.

A couple of 1/16" holes is plenty. I fill mine to the top, and the only time I ever have any leakage is if I stop with very full tanks pointed uphill on my 13* driveway slope.

[LuvNLife]

Originally Posted by rich weyand

If the weather is nice, they are always going to push air. When you filled it, the gasoline coming out of the ground was at 55*. Warm it up, it will expand.

A couple of 1/16" holes is plenty. I fill mine to the top, and the only time I ever have any leakage is if I stop with very full tanks pointed uphill on my 13* driveway slope.

Great to know about the size; I was leery of going over something like 1/64th. Thanks very much for the reply.

Jerry