[360Rocket]

1970 Pontiac GTO. 455, Edelbrock Performer RPM, stock fuel lines, in line filter, stock fuel pump. stock fuel tank. Holley Ultra Avenger 750 CFM carb electric choke and vacuum secondaries.

So this weekend, I took @50 mile round trip and put the car back in the garage, I shut off the motor, and heard what sounded like percolating from the gas tank. I remove the gas cap and and its under high pressure and the percolating stops after a huge "woosh" out of the filler neck. I put the cap back on and the pressure builds again. I slide under the rear of the car and put my finger over the breather hose on the tank and feel pressure escaping from the hose. I have 2 other hoses leaving from the tank to the front of the car, one on the drivers side leading to the fuel pump and another leading down the passenger side to who knows where? I see where there is a crushed section in the steel line on the passenger side near the front of the car but that has been that way for many years. The tank bottom was warm but not near enough to boil fuel, the lines were a bit warmer than the tank but not by much. Now in front of the engine block between the fuel pump and carb is the steel factory line that leads to the fuel filter and the carb. THAT line was hot enough to have you removing your hand within a few seconds of touching it. The closest ANY fuel lines get to the exhaust is maybe 6 inches when the exhaust goes over the rear axle but the lines didn't feel that hot. After about 5-10 minutes of resting in the garage the pressure seems to dissapate from the tank. What do I have going on here? and better how do I fix it? What am I looking for?

[gto4evr]

The line on the passenger side went to the charcoal can that's mounted on the core support. it took fumes from the tank and sends a line up to the carb to burn up (an emissions thing) but it wouldn't be causing your issue if it's not connected unless maybe you've got the line plugged instead of just hanging there and your tank vent is plugged too.. Are you sure it's pressure outward from the tank and not vacuum sucking in when you pop the cap?

[360Rocket]

Originally Posted by gto4evr

The line on the passenger side went to the charcoal can that's mounted on the core support. it took fumes from the tank and sends a line up to the carb to burn up (an emissions thing) but it wouldn't be causing your issue if it's not connected unless maybe you've got the line plugged instead of just hanging there and your tank vent is plugged too.. Are you sure it's pressure outward from the tank and not vacuum sucking in when you pop the cap?

So the line on the passenger side isn't the return line coming from the fuel pump? I haven't seen a canister on the radiator support but I will check again. As far as the pressure, On the top of the passenger side of the gas tank there is a vent hose that when I slid under the rear of the car, I held my finger over the vent hose and had pressure releasing from that vent hose. There is also a hose up near the neck of the fuel filler and it too had pressure.

[76TA462]

Not sure if this is much help, but here goes. I have a 76 TA. I have (currently changing out) the exact same set-up as you - same intake, carb etc. The 3/8 line from tank to fuel pump on driver side is the fuel line. On the passenger side there are two lines. One goes to the fuel pump and is the return. The other originally was a vent line to a charcoal cannister that was mounted on the passenger side rad support. Decades ago I removed the charcoal and it’s line, and have only a short vent line (a piece of the original) that I capped with a small glass filter. The OEM cap that I retained also has venting capabilities. Absolutely never a pressure build-up problem. That said, you mention a pinched line. Maybe a cheap easy process of elimination might be to cut the pinch out and clamp in a rubber line – its all low pressure so no worries there. On my car the return line goes up the passenger side, crosses over under the engine cross-member then up between engine and cross- member to fuel pump. All OEM routing.

[johnta1]

Is the gas cap a vented type?
Check the vents to make sure they are clear.


A few pics from service manual:

[Forrest]

You obviously have a pressure build up..I assume you do not have the original lines set up as from factory...I would just run a return line and a vented cap.. good read on pressure build up.... only it's a Chevy!.. but very similar to ours....check it out.. https://www.corvetteforum.com/forums...-cap-lids.html

[360Rocket]

So the pinched line on the passenger side IS the return line from the fuel pump. The tank has a vent hose on the top of the front of the tank on the passenger side and it is clear of obstruction and working properly. I also have a vent hose near the top of the filler neck and it is clear as well. I haven't been able to find a canister anywhere on the car nor a line going to it. I don't belive my gas cap is vented but wouldn't think I would need it with 2 vents on the tank? Gonna fix the return line crush and see if things change. Thank you for the illustrations, they greatly help.