Help! Pressure Build-up In Gas Tank

[pjmoreland]

I have a 67 Chevy truck that has a problem where sometimes I get a massive build-up of pressure in the gas tank. When I take the gas cap off, air and gas come spraying out. The biggest problem with this is that sometimes the pressure forces the float valve in my carburetor fuel bowl to get forced open, causing gas to dump into the carburetor, making my truck die. This problem usually occurs on days when the morning is cold, and then it heats up quite a bit in the afternoon.

The problem seems to be caused by the gas cap. The gas cap has a check valve in it that lets air in as the gas tank empties during normal operation, but the check valve does not let pressure out. I got so fed up with this, that I drilled the check valve out, but now if I go around a corner, gas leaks down the side of my cab. At least this is better than having the engine die because it is getting massively flooded. It's not a very good solution though.

When I bought the truck 12 years ago, the original owner told me that the truck required a vented gas cap. The gas cap that came with the truck wore out several years ago, and I have not been able to find a vented gas cap anywhere since then. I'm wondering if he EPA has banned vented gas caps, or something like that.

Has anybody run into this problem? If so, what have you done to fix it? Does anybody know if vented gas caps are available anywhere?

Thanks for the help.

[Tx Firefighter]

You can buy one at any parts store. Stant brand is the most popular. They keep them in boxes on the shelf, so the parts guy has to look it up like a thermostat or any other Stant part.

I bought one last summer for five dollars and some change.

[pjmoreland]

The gas cap I have on my truck is a Stant 11623. It's the one that has (had) the one-way check valve in it. It was not letting pressure out of my tank. If I remember correctly, the Stant catalog they have at the auto parts store doesn't even list a gas cap for 1967. I think the earliest they listed was 1969 or something like that.

Here's what is said on the partsamerica website about a Fram gas cap for 67 Chevy:

"Keeps fuel from evaporating. Keeps contaminants out. Keeps fumes in (EPA regulated). Slowly releases pressure from the filler neck prior to cap's removal to alleviate the problem of fuel expulsion."

I don't see how the pressure can get out if the fumes can't get out. I can see why the 72 trucks have the evaporative emissions canister connected to the extra line going to the gas tank.

[Tx Firefighter]

Mine was cataloged for a 1970 truck, but the 67 should be the same shouldn't it? Neither of these trucks had the evap system like they had in 72 so they were both vented to the atmosphere to relieve the built-up pressure.

[pjmoreland]

I'll bet that if you blow on the hole that is on the inside of your gas cap, in the center, you won't be able to force any air through it. If you suck on it, air will pass through. That's the check valve in there that is not letting pressure get out.

I suppose there could be something wrong with my fuel pump that is causing it to pump pressure back into the gas tank.

[ddsmith]

I ripped my gas cap apart to cut the spring down some on the relief valve to make it open at a lower pressure. You could cut the spring down to the point that 1/2 psi would open it. If 1/2 psi is overwhelming your floats you've got problems because the pump puts out at least 4-5 psi. Check and see how hot your carb is after running the truck. If you do not have a spacer between the carb and the intake the gas getting into the carb may be boiling out of the bowls.

[pjmoreland]

About a week ago I got in my truck and started driving it. The engine was cold at that point. I drove about a mile and came to a stop light. As I slowed down to the sop light, my engine died, so I pulled off to the side of the road and opened the hood. There was gas streaming out of my carburetor onto the top of the intake manifold. I'm just asking for an engine fire here. I had my tools with me, so I popped the top of the carburetor off, and noticed that there were air bubbles blowing out of the fuel bowl float valve. I removed my gas cap, and a HUGE rush of air came out of the gas tank. I checked the carburetor again, and there were no longer any bubbles coming out of the carburetor. I did notice that the little bent wire piece that the float valve needle hangs on (Quadrajet) was missing, so the valve needle might not be seating properly. It didn't look like there was any dirt in the valve. I put the carburetor back together, and left the gas cap off. My truck started back up just fine as long as the gas cap was off. Once I put the gas cap back on, the problem occurred again. That's why I drilled a hole in the gas cap valve.

This afternoon the flooding problem occurred again, so that tells me pressure in the gas tank is not the problem since I still have a hole in my gas cap.

Tonight I will rebuild my carburetor. I'll see if that helps. I'm guessing that the fuel bowl float valve is just worn out. If that doesn't fix the problem, I'll replace the fuel pump. I suppose I should buy a fuel pressure gauge so I can see what the fuel pressure is.

Thanks for the ideas.

[Robert1970C20rstbukt]

Any gas cap listed for a 67-71 truck will work. '72 is when the EEC crap came out. I'm having a problem finding a vented cap for the tank I have, which came out of a '72 C10 that had the damn EEC hoses and the vent up in the cab. I really wanted to keep the original '70 tank and get it cleaned and sealed, but that idea didn't fly with Dad.

[pjmoreland]

I just got done rebuilding my carburetor, and at least for now my engine isn't flooding. The fuel bowl float was sitting a bit high, so it wasn't putting adequate pressure on the needle valve. We'll se how it goes.

The nice thing about the EEC setup on 72 trucks is that if you park your truck in a garage, the entire garage doesn't stink of fuel vapors all the time. My 67 really stinks up the place.

I don't know this for certain, but I suspect that the gas caps that are sold now for the 67-71 trucks do not function the same as the gas caps originally installed from the factory. I suspect that the EPA put an end to vented gas caps, and so I get to deal with major pressure build-up in my gas tank.

[flip66]

I have run into the same problem with the pressure in the tank but not the carb. I have tried all kinds of caps none will solve the problem they all say they are vented but I still have the same problem they do not vent out. I can leave the truck setting in the sun on a warm day and remove cap and it will push the cap off and spray gas out. The heat from the sun will make mine build up pressure. Is anyone else have this problem. The gas cap for the 71-72 are different then 67-70 the 72 is smaller I have a 71 and a 72 tank and the cap that fits my 69 will not fit them.

[longnlow72]

excuse my ignorance, are 67 tanks not vented?

[pjmoreland]

I believe they are supposed to be vented, but I can't find a gas cap anywhere that will let pressure out of the tank. All the gas caps I can find will only let air into the tank as the gas is pumped out by the fuel pump. There are only two ways out of the gas tank: one is the opening that is sealed by the gas cap, and the other is the fuel line that goes to the fuel pump.

[ddsmith]

Maybe a little help for you guys needing to stop the pressure build up without spilling fuel all over your truck. The pictures in this thread are being hosted and I strongly recommend you save them if you need them in the future because if the hosting place goes away the pictures will be lost. I don't save them because I don't have the space on my computer for all the pictures I've taken. This is a picture of the gas cap which should be very similar to what your looking at.



The two metal tabs hold the valve assembly in place. The two tabs get bent up to allow the valve assembly to come out.



The valve assembly removed from the cap looks like this.



This is what the bottom of the valve assembly looks like.



The rubber gasket gets removed to allow removal of the plastic ring which allows taking the assembly apart.



The plastic ring gets removed and the assembly looks like this.



Now down to the modification part. The large spring is the one which controls venting the tank and it's tension controls the pressure that the tank can build before it opens to relieve. The spring holds down this metal valve that has a gasket on the end. One end of the spring rides in the metal valve part and the other end pushes against the plastic hold down ring.



You'll also notice a white platic valve looking thing in the center. This is the vacuum breaker to allow air to enter the tank. No need to mess with this valve.
This is a picture of the original spring.



This is a picture of the modified spring.



Then you put it all back together and you can test it by blowing into the plastic piece and see if you like the pressure it relieves at. Just remember the spring must have a little tension to prevent gas from spilling on your paint. Good luck and I hope this helps with the gas tank pressurization thing.

Don Smith

[flip66]

Thanks I am going to try it. It looks like you left about a coil and a half is that right.

[ddsmith]

It's a measurement thing. You need to make sure that there is a little tension on the spring to seat the valve when no pressure is there. You can check this by trying to blow through the valve before you do anything to it. I'm sure you'll find it hard if not impossible to unseat the valve. Most people are not designed to develop much pressure. There are some exceptions. When you are modifying the spring remember that the distance between the plastic retaining ring and the metal valve is the distance that is the minimum the spring height must be. You really want to have some compression of the spring but not much. When you put it all back together again. Check the valve by blowing into the holes on the tank side. You should hear and feel the pressure relieve. If you suck on the valve the vacuum breaker will allow air to flow into your mouth. Just remember gas doesn't taste very good. You might want to wipe it down before doing this.

Don

[454_72_4X4]

Nice write-up ddsmith!
I've had the pressure problem a few times myself,until I accidently left my cap on the bedrail at the station and then run over it when it fell off!
I think I might get a new cap and mod it like you show.

Questions;
My truck is a 3/4 ton 72. I think it has the vent pipe that goes up the passenger side of the cab, (it been along time since I've had that area uncovered).
Where does this pipe go?
I think somebody mentions a canister of some kind. What/where is it?
My truck has never had any type of canister that I'm aware off.

[pjmoreland]

Thanks a lot for the highly detailed gas cap modification description. Excellent idea. I'll give it a try.

[SLS]

ddsmith,

That was an excellent illustrated tuitoral on the gas cap mod!

Thanks for taking the time to create it.

Helpful people like you are what makes this the finest Chevy truck site on the planet!

[franko72]

This website rules! very informative .answered my problem

[Rollie396]

454_72_4X4,here's where the carbon canister was located in my 72 c10.(passenger side fender beside bat. box.
oops doesn't wanna show up.That's where it is anyhow

[Robert1970C20rstbukt]

I still have the original gas cap from my truck, here's two pics of it. If you look close in the first pic is says VENTED on the inside of the cap. It worked great, the original tank never had any pressure build up.

[flip66]

Thanks ddsmith I just finished the mode it was really easy I can tell the difference. Still have to let it sit in the sun I will let you know how well it worked.