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#1 |
Registered User
Join Date: May 2010
Location: Crossville, TN
Posts: 142
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Pressure In Fuel Tank When Removing Gas Cap
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 |
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#2 |
Registered User
Join Date: Mar 2014
Location: Nevada, IA
Posts: 92
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Re: Pressure In Fuel Tank When Removing Gas Cap
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.
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Matt 43 |
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#3 |
Registered User
Join Date: Aug 2009
Location: S.C.
Posts: 1,593
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Re: Pressure In Fuel Tank When Removing Gas Cap
Head back to the shop and have them finish (you did pay for it correct?) Sounds like they forgot to vent the tank somehow.
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'86 C-30 dually, 454/tbh400 '73 K-20 350/350/205 (sold ![]() I'm kinda like duct tape- no real purpose, but handy to have around. |
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#4 | |
Registered User
Join Date: May 2010
Location: Crossville, TN
Posts: 142
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Re: Pressure In Fuel Tank When Removing Gas Cap
Quote:
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 |
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#5 | |
Registered User
Join Date: May 2010
Location: Crossville, TN
Posts: 142
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Re: Pressure In Fuel Tank When Removing Gas Cap
Quote:
I know the shop will check the vent line, but I like to be able to talk to them somewhat intelligently... Jerry |
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#6 |
Registered User
Join Date: Mar 2014
Location: Tulsa, OK
Posts: 281
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Re: Pressure In Fuel Tank When Removing Gas Cap
temperature change inside the tank could cause pressure to build
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84' C-10 Custom Deluxe 97' & 98' C-1500 Silverados |
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#7 | |
Registered User
Join Date: May 2010
Location: Crossville, TN
Posts: 142
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Re: Pressure In Fuel Tank When Removing Gas Cap
Quote:
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 |
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#8 |
Registered User
Join Date: Aug 2009
Location: S.C.
Posts: 1,593
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Re: Pressure In Fuel Tank When Removing Gas Cap
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...)
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'86 C-30 dually, 454/tbh400 '73 K-20 350/350/205 (sold ![]() I'm kinda like duct tape- no real purpose, but handy to have around. |
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#9 |
Registered User
Join Date: Mar 2012
Location: Woodbury, Ct.
Posts: 1,680
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Re: Pressure In Fuel Tank When Removing Gas Cap
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.
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#10 | |
Registered User
Join Date: May 2010
Location: Crossville, TN
Posts: 142
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Re: Pressure In Fuel Tank When Removing Gas Cap
Quote:
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 |
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#11 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Pressure In Fuel Tank When Removing Gas Cap
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.
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Rich Weyand 1978 K10 RCSB DD. |
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#12 | |
Registered User
Join Date: May 2010
Location: Crossville, TN
Posts: 142
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Re: Pressure In Fuel Tank When Removing Gas Cap
Quote:
Jerry |
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