The Stovebolt Site Forums The Shop Area General Truck Talk Gas Tank Pressure

I just recently acquired a 1950 Chevy 3100 and I replaced the gas cap with a new one from LMC because the one on the truck was dented and the rubber seal was disintegrating.

However, now when I remove the gas cap, there is a loud pressurized sound - not sure if it sucking air in or pushing air out. I could understand there being a little bit of pressure in the tank, but I am concerned that it is a lot of pressure. Is there supposed to be some sort of tank vent? The old gas cap didn't have anything built into it, but the new one looks like it has some sort plastic vent thing in the middle inside.

There should be no pressure and no vacuum.

The new gas caps, like just about everything else you buy today, are absolute crap. The valve inside, meant to vent the tank doesn't work. You end up having vacuum pressure build up as the fuel pump sucks out the gas because no air can enter to fill the void.

Take the cap apart and remove the little valve and spring.

Bend those two ears at 12:00 and 6:00 and remove the valve assembly. Behind the tan plastic part with 5 holes is a rubber valve and spring. Pull it out and put the rest back together.

It will work as it should after that.

I have simply drilled a 1/8" hole in the plastic near the edge where the letters are in Otto's pic and solved this problem on 4 different trucks.

I did as Otto suggested and all is good now. Thanks!

Isn't that pathetic that a brand new part with a simple job to do won't work from the get go? This cap design is identical to the one on my 1964 Galaxie 500. It's 60 years old and still works perfectly

The only visible difference is that the plastic part with the holes is made out of metal on the Galaxie cap. I pushed a small screwdriver through the hole and the valve and spring work perfectly.

It seems like gas caps should be working like a manifold thermostat except the spring allows it to open/close based on pressure versus heat. I’m guessing the spring specs for the cheap copycat caps aren’t being adhered to or are wrong. Removing the springs and drilling holes is akin to a wide open (well, narrowly open) gas tank in your garage.

There aren't any "good" caps available. The cheap copycats are all you can buy. The only two brands I've ever seen are Motorad and Gates and its obvious that they are identical and made by the same manufacturer. You have to make do with the junk being offered.

I don't necessarily think that the spring specs are wrong. They just get stuck and don't work. In my case, the cap worked for a while and then didn't.

Mine works fine w/o a vent. In fact all three do

I got my ‘57 cap from LMC Truck about 5 years ago. Works fine.

That's a locking cap completely different from the failing ones. I live in the crime ridden capital of the Deep South and I've never had anyone try to steal my gas.

The original caps did not have any fancy spring loaded valves in them. They incorporated Precision Engineered Leakage into the design of the cap. They did the same type of engineering to the caps on the brake master cylinder.
I chose to drill a small hole in the underside of the plastic as opposed to removing the valve to lessen the amount of Engineered Leakage.

Is This a better cap? It looks just like the original on my truck.
Samantha

Otto, LMC Truck sells the same cap without lock in stainless.

Samantha - Yes! Those have a correctly designed and functioning vent (you can see when looking at the underside pic viewed in Carters ad). I have one purchased about a year ago and have no complaints.

Thank you, Dan! I will be needing to replace mine and was planning on getting this one.
Samantha

Engineered leakage. I'm stealing that term!

Yes, that LMC one does look like the original. It's made out of metal!

They could have saved a lot of hassle and just done the trucks like they did the early 50s cars. The vent is built in to the filler pipe rather than the cap. It's difficult to see in the picture but there's a stamped divot in the rim at the 12 o'clock position.

sure there are. Don`t buy cheap things.

Add a charcoal canister under the truck. Put a Tee in the filler vent tube near top run a loop above the cap and back down thru the floor and add a charcoal cannister under. Cap stays sealed so you don't have smell gas when stopped and blowing in the window that is down on a nice day for a drive. and if you have to replace it because you forgot to put it back on you don't have to modify the new cap to actually vent.
You can garage park it and not smell up the garage.

Do you have any pictures or diagrams on how to put the tee in the filler vent tube and hooking up the hoses properly? Mine is currently smelling up the garage and I want to eliminate that problem. I have no visible gas leaks or seeps.