high pressure buildup in fuel tank - help!

[hairlesshobo]

I have an '84 C10 that builds up a lot of pressure in the fuel tank. If I fill the tank up and drive it until the tank is almost empty, when i remove the gas cap there is a lot of built up pressure. I know it is pressure and not vacuum because if I loosen the cap quickly, it will literally blow the cap off the fuel neck.

In case it's relevant, this truck was a dual tank setup. The original owner bypassed the secondary tank and selector valve and ran just hoses straight to the driver side tank. I am running a 350 with a 4 barrel, engine driven fuel pump w/ return line to tank.

Anybody have any idea why my tank would be building up pressure?

Thanks in advance

-Steve

[beamn7]

Check the vent line to the vapor cannister and also verify you have the correct fuel cap....

[cory d]

i get this too. im curious to find out what causes it

[hairlesshobo]

Originally Posted by beamn7

Check the vent line to the vapor cannister and also verify you have the correct fuel cap....

I have tried both gas caps (when I say both, I mean the one that was already on the driver side and the one that was on the passenger side.) I will check the lines running to the vapor canister. Who knows what the previous owner did to the thing.

[donut]

Originally Posted by beamn7

Check the vent line to the vapor cannister and also verify you have the correct fuel cap....

What he said. My old truck didn't have the canister and I had to use a vented cap, or in my case, hook up a vent to the tank. (1973)

[bucannon]

get a vented cap for a low budget fix or as everyone else is saying trace your vent line and see where its been plugged by the previous owner. but why would you want to not have dual tanks????

[76stepsidechevy]

Originally Posted by bucannon

get a vented cap for a low budget fix or as everyone else is saying trace your vent line and see where its been plugged by the previous owner. but why would you want to not have dual tanks????

Usually when one or both are rotten and someones too cheap to fix it the right way. 16 gallons doesn't get me very far, im looking to add a second tank. Hurts a little less at the pump but its getting kinda old standing there in the cold every other day.

[hairlesshobo]

the passenger tank is in rough shape and one of the mounts are mostly rusted out. If it were me, I would have just stopped using it instead of disconnecting it. I never cease to be amazed by the things the previous owner did...
Posted via Mobile Device

[cory d]

Originally Posted by hairlesshobo

. I never cease to be amazed by the things the previous owner did...

i bet most of us can say that

[bucannon]

Originally Posted by cory d

i bet most of us can say that

the previous owner of my truck cut the emergency brake lines because of a bad switch on the pedal making the "brake" light stay on. so now the light is still on(unless i twist tie the pedal up) and the E-brakes don't exist.

[cory d]

oh wow, he dousnt sound like that great of a problem solver. the previous owner of my truck put bucket seats in it and they were so hookied in there that when i turned they would sway and the mounts started to bend unger my weigth (170 lbs). i swaped them for a benhc shortly after though

[sethv]

Originally Posted by bucannon

but why would you want to not have dual tanks????

I think it's less of a hassle to have 1 tank.. less wiring, and you don't have to worry about the switch going bad, etc. I'm a believer in less is more, lol.

[tucsonjwt]

My dual tanks have 3 outlets on the sending units - one of the outlets has just a short piece of hose with a one way vent cap to relieve pressure. In 1983, 3/4 ton trucks were considered heavy duty trucks and did not require vapor canisters, so you might check your sending unit to see if you have that short vent hose and replace the vent cap. I replaced that short hose with a long hose and ran it back near the rear bumper to reduce the gas fumes I was getting from the short vent into the cab.

[hairlesshobo]

I am going to get under the truck this weekend and take a look and see what I can find. I hope to get this taken care of cause the pressure buildup is annoying and it seems like it doesn't run quite as well once the tank is built up with pressure (who knows, that might be my imagination.)

-Steve

[RastaRobb]

My 84 K20 has the dual tanks. If you look in the gap between the bed and the cab, you can see the lines going into the tanks. There is also a breather valve that is only about 2 inches long, and terminates. I got a big lump of mud over the whole valve once, and it caused me problems. Check those breathers.