Gas tank under pressure

[k20]

On my 87 K20 I have converted to carb and am running the electric fuel pumps with a bypass regulator. I have been noticing lately that when I go to fuel up, when I open the caps I have a large amount of air coming out of the tank. Doesnt do it everytime, just sometimes. Any ideas? I would think a plugged vent but then I would think it would do it everytime. Also seems to do it more on the driver side tank than the pass side tank. Thanks

 

[dendeanda]

I put in an 89 5.7 that was converted to carb, using a manual fuel pump, and noticed that I was getting a weezing noise from the gas cap when running and when filling up. I replaced the original gas cap with a new generic vented one and never had that problem again.

 

[chevyuser74]

or u could check to see if you just plugged your vent line, u took the charcoal canister out of the truck?

 

[4xcrazy]

mine does this mostly when the tank is near empty, also alot of gas caps will even state to open slowly, due to pressure.

 

[sandracer799]

On my 87 K20 I have converted to carb and am running the electric fuel pumps with a bypass regulator. I have been noticing lately that when I go to fuel up, when I open the caps I have a large amount of air coming out of the tank. Doesnt do it everytime, just sometimes. Any ideas? I would think a plugged vent but then I would think it would do it everytime. Also seems to do it more on the driver side tank than the pass side tank. Thanks

My 87 has always done this. Did it when it still had the stock TBI. And still does it now that it is carburated.

 

[dyeager535]

There should be little pressure in the tank if the vent system is working properly. EVAP cannister is open on the bottom, any pressure the tank builds up should be vented at low pressure through there.

All later caps vent around 16PSI, which means they aren't designed to vent under normal conditions.

 

[k20]

Maybe its normal, just aint never really noticed it, so who knows. Canister is still in the truck, mostly cause I relocated it under the batt tray when I still had to pass a visual test, and havent gotten around to takin the batt tray out to remove it.

 

[Chevy305]

I have just noticed this on my tuck too since I bought a new gas cap. I ordered it from Classic Parts and it says its supposed to fit my truck. It doesn't make sence because my charcoal canister has been missing since I bough the truck so the vent is located high up on my core support with a sintered bronze filter.