Pressure in Gas tank

[Tex63]

How much pressure should build up in gas tank before cap vents? I was smelling raw fuel last week coming from my gas cap, looked old and the cork seal was gone, so replaced with new vented cap from local autozone part # 6040. No longer smell fuel from the tank area, but created a different problem, pressure built up in the tank enough to force through the lines and drip from the block to the garage floor before I noticed a few days later. I have been checking daily and opening gas cap to relieve the pressure. Could I have another bad gas cap, or weak fuel pump membrane? What to check next ?

 

[1958 delivery]

Sounds like the cap is not vented.

 

[64ss409]

They are designed to hold pressure and keep fumes in. They are one-way valved to prevent tank collapse while the engine is running and pulling gas from the tank.

Here is copy-paste from Ecklers.

• • Replacement Cap
  • Vented/Anti-Surge
  Chevy vented gas caps are an integral part of every automobile. When it comes to the gas tank, security and integrity are paramount. The gas cap is vital in preventing corrosion, delivering pressurized gas to the fuel pump, and keeping harmful carcinogens inside the tank and impurities out. Eckler’s Late Great Chevy offers gas caps to fit your classic Chevy. Ensure peace of mind by providing your classic Chevy with a vented gas cap. The chrome finish affords authenticity and class for your 1958, 1959, 1960, 1961, 1962, and 1963. Available at Eckler’s Late Great Chevy today!
  

 

[blkblk63ss]

Vented Gas Cap Function
The vented gas cap is designed to vent small amounts of air into the car gas tank line. The vented gas cap has a one-way release valve that is pressure-activated. When the pressure reaches a certain point (defined by so many pounds per square inch) the valve opens by a very small amount, relieving some of the pressure. In this case, the pressure is formed on the outside of the tank, due to the vacuum formed from the displacement of fuel on the inside. After the pressure inside the line equalizes, the pressure valve closes. The valve is set to balance the pressure with the surrounding atmosphere. Air is allowed in, but no fumes are allowed out of the gas cap, because of environmental concerns.

 

[Fathead Racing]

In the summer heat my 60 impala, I'm assuming I have the correct cap, always has pressure when I remove the cap. I don't think it would be an issue if your needle and seat were up to spec.

 

[models916]

Pressure or vacuum?

 

[blkblk63ss]

You can test your cap by taking and sucking on the little valve portion of the inner side. You can suck air ,but you will not be able to blow outward. And yes i am sure there will some smart a## remark's on this!!!! Vacuum can be mistaken for pressure on a tank.

 

[Fathead Racing]

Pressure or vacuum?

Pressure.

 

[jdk971]

this is a education for me. put a ls in a vette. burnt up the fuel pump in the tank the first year. with a new fuel pump the engine
acts like it is starving for fuel when you get on it, now and then. all i did was drill out the old vented fuel cap. i think i will get
a new one. jim

 

[Fathead Racing]

this is a education for me. put a ls in a vette. burnt up the fuel pump in the tank the first year. with a new fuel pump the engine
acts like it is starving for fuel when you get on it, now and then. all i did was drill out the old vented fuel cap. i think i will get
a new one. jim

I don't mean to infer that you have not done your homework, this is for my own information. Is there a return line on this system?

 

[jdk971]

no return, it has the newer vette type canister filter in the rear outside the tank. jim

 

[Tex63]

Ordered another vented fuel cap, hope it's just an easy fix. Odd that enough pressure would build up and push all the way through the carbs, shouldn't the fuel pump membrane stop the flow ?

 

[Don Jacks]

The membrane wouldn't have anything to do with it,but a weak inlet check valve could.There are 2 check valves in a mechanical pump,one for suction from the tank[inlet].and one for pressure to the carb [outlet] When the membrane moves one way,fuel is drawn into the pump and the suction keeps the outlet closed,when it goes the other way,the pressure keeps the inlet valve closed. It sounds like if you have fuel being forced into the carbs,that either you have a lot of pressure being built up in the tank,or a weak check valve assembly.Either way,the needle and seat assemblies should prevent flooding and the gas smell.

 

[Phil Reed]

The membrane wouldn't have anything to do with it,but a weak inlet check valve could.There are 2 check valves in a mechanical pump,one for suction from the tank[inlet].and one for pressure to the carb [outlet] When the membrane moves one way,fuel is drawn into the pump and the suction keeps the outlet closed,when it goes the other way,the pressure keeps the inlet valve closed. It sounds like if you have fuel being forced into the carbs,that either you have a lot of pressure being built up in the tank,or a weak check valve assembly.Either way,the needle and seat assemblies should prevent flooding and the gas smell.

What happened Don.......can just barely read your post?????????????? This isn't government suppression, is it????????????

 

[Fathead Racing]

What happened Don.......can just barely read your post?????????????? This isn't government suppression, is it????????????

Yeh, I thought I was seeing things, I switched to ice tea, still looked the same Phil

 

[Don Jacks]

Damned computer issues,guys!

 

[skipxt4]

Damned computer issues,guys!

Must be a Mac.