Gas tank pressure buildup

[dkmerth]

Hello All!

Lately, I've begun to notice a hissing sound as I take off my gas cap. Today, I removed the cap a bit faster and suddenly the cap popped off and gas shot out of the filler tube. It was an outward pressure rather than a suction pressure. I literally only has 1/4 tank of gas in the car and some of that managed to shoot out and onto the trunk lid. Few things to mention:
This is a 1977 124 Spider.
1) A few weeks ago I replaced a the vapor check valve in the trunk. The white plastic "flying saucer" looking check valve had broken. I couldn't locate a new one so I replaced it with a new models "all metal" version that has a directional arrow on it. I admit that then I installed the metal one that I had no idea what direction to connect the hoses regarding that arrow. I suspect it is possible that the check valve was installed backwards. Anyone know the correct direction for the arrow (what hose connects where?)?
2) The gas cap on the car is solid metal with a rubber gasket seal. I know on modern cars gas caps are supposed to permit pressure relief right? Perhaps replacing the gas cap with a modern gas cap could allow pressure to release as needed.
Should I be concerned that the pressure in my gas tank could blow a gas line or cause the tank to fail?

Any thoughts? Appreciate all the help for this novice!

[TheBender]

I recently had the same exact problem on mine. I think the air got sucked in so fast the gas shot up and out. It ended up being a clogged tank vent. The corrosion/gunk had to be drilled out. I believe our cars do need slight vacuum build up in the tank, but not like that. If you do a search, I think I remember reading some old posts on here where people drilled the smallest of holes through the cap.

[18Fiatsandcounting]

As Bender mentioned above, one acceptable solution is a small hole drilled in the gas cap, or you could even just leave the gas cap slightly loose instead of fully tight.

That "flying saucer" check valve does 3 things:
1. Allows gas vapors from the tank to make their way up to the charcoal canister in the engine compartment.
2. Admits air into the gas tank if it starts building up a vacuum.
3. Vents the tank if it become severely overpressurized.

So, it's not just a one way valve, but is actually rather complicated. I doubt installing a simple check valve would do all three things above, so you might have to accept some compromises. My guess is that the arrow on an aftermarket valve should point towards the line that goes to the front of the car.

A small hole in the gas cap, or a slightly loose gas cap, or an aftermarket vented cap, would accomplish both #2 and #3 above, so that's the easiest fix.

-Bryan

[dkmerth]

As Bender mentioned above, one acceptable solution is a small hole drilled in the gas cap, or you could even just leave the gas cap slightly loose instead of fully tight.

That "flying saucer" check valve does 3 things:
1. Allows gas vapors from the tank to make their way up to the charcoal canister in the engine compartment.
2. Admits air into the gas tank if it starts building up a vacuum.
3. Vents the tank if it become severely overpressurized.

So, it's not just a one way valve, but is actually rather complicated. I doubt installing a simple check valve would do all three things above, so you might have to accept some compromises. My guess is that the arrow on an aftermarket valve should point towards the line that goes to the front of the car.

A small hole in the gas cap, or a slightly loose gas cap, or an aftermarket vented cap, would accomplish both #2 and #3 above, so that's the easiest fix.

-Bryan

Thanks for the response. I will check tomorrow to see the direction of the arrow and will see if I can find a "low profile" vented gas cap (not much room for thick gas cap with the fuel door). Otherwise I will wrestle with the idea of drilling a hole in the gas cap then probably just do it. Insert face palm emoji here.

[redcars]

I have removed the vent valve and cleaned it with good results.

[TheBender]

That’s great!

[Redwolf]

This post was timely - I had the exact same thing happen to me two days ago. Never had it happen before and have had the car 25 yrs.

[SanJoseSteve]

It could be an easy fix.... at least it was for me. When I had the car painted, I re-installed that part upside down! Not only did I get a volcano of gas erupting when removing the gas cap as you describe, but when I disconnected the fuel filter to replace it and then went out to lunch, I came back to find a pool of raw gas on the floor under the engine. Summer sun warms up the back of the car while the garage door is open and pressurizes the tank. Fuel gets pushed up to the disconnected filter and pools on the floor not far from my water heater!

The foot of that valve is not indexed so its quite easy to install backwards. There is a black collar on the right orifice that tells you which end is which. It only works though if the valve is clean enough for you to notice it. Sometimes they slide off the valve and stick inside the fuel line too when you remove them. I'd post a picture for you but after several years of trying on this forum, I gave up. Ha! -Steve

[SanJoseSteve]

Oh.... and it goes without saying, Be careful mounting and then sliding the hoses onto it. Those things can be very brittle. Fortunately, almost every junkyard 124 I've come across has had its original one intact. Dirt cheap too..... -SF