My 72 convertible builds up pressure in the gas tank. After a long hot day, I can remove the cap and it will almost blow the cap outwords with a rush of fumes. The cap says it is vented, which I think is a load of crap based on the pressure build up. I guess it is vented to allow air in under suction. The tank is a new re-prod piece and only has ONE tube exiting the top. That tube is the fuel to the car.

So how do I vent the buylt up pressure, but not allow fuel to spill out of the filler neck every time I accelerate?!? Why is this tank even offered as a product? :rolleyes:

I did some searching and there is no REAL solution to this problem. Aside from ordering a certain locking gas cap from Ebay and drilling holes in it, hoping the rubber booty keeps fuel from splashing out. I race, often, so this cap will see a lot of fuel pressing against it.


My other chevelle, with an original tank, has 3 lines coming out of the tank.

Should I just drop the tank and weld in a vent?
Can I fix this on the gas cap side?

I had the same problem after adding the reproed tank. What I did was just drill a small hole in the top of the cap. By top, I mean when you screw the cap on, it will always be on the top (I have mine at the 12 o'clock position). No more fume build up, and have no noticable loss of fuel.

I'll give that a try when I get home.

I still dont' get why gas comes out the back already. I guess the cap on it is just very very very cheap. Keeps pressure in, but lets gas out. Sounds like a conundrum. :D

Mike, James Hinshaw showed me how to drill 2 each, 1/16 inch holes, under the rubber seal but not thru. If I remember will take pic tonight. Solved the pressure and suction issues on my 70 and 67. No gas can spill out either.

Mike,
A common problem with the stock design on those rear fill tanks. Been discussed here many times.
My '70 used to leave gas at the starting line everytime I raced it. :o
Until the fuel cell was installed...

I drilled that 1/16 inch hole in my cap. It still built up pressure like Mike discribed, I noticed that after I drilled the hole, when I installed it on the tank, that a (brass?) collar inside the cap was spinning and not letting the hole line up all the way thru the cap. Guess I'll have to get a different cap cause I've tried rotating it several times and I can't get the original hole, (in the brass collar) I drilled to line back up.

I bought a new cap at Autzone for $6. It has an interesting dual pressure mechanism in it. The one I've been running, that has problems, has a kind of check-valve in it much like a PCV valve on your motor. This new one has two things going on. One is a spring loaded plunger that allowed air to be sucked into the tank. I even put my lips on it and verifying that part. :) But it had anotehr spring loaded diaphram, that when pushed against hard enough, would let air exit OUT of the tank. Onloy problem was I had to really push on it to get that spring to compress. So, I bent two metal tabs outward, took it apart, and trimmed the spring in half. Put it back together and now I can pushe air out of it much easier, but I think not easy enough to allow gas to slash out because it has kind of diffuser (serveral tiny holes) that should prevent the gas from hitting up against this inner membrane.

So it's not a full on atmospheric vent. more like a +3psi of positive pressure vent. This way, when the tank equalizes, I won't have a garage that smell like gas. I've seen mention of loosing gas due to vapors exiting the tank. Trivial detail, but relavent none the less.

I'll try this for a few days and see what the "woosh" and "splash" factors are. :D

The line coming out of the tank is on the sending unit, they can be changed for one with EEC (Evaporative Emissions Control) commonly found on 71-72 gas tanks which has 3 tubes coming out of it. Also, 1969 tanks have vent lines built into the tank and all they use is a pair of rubber hoses going up to a little "breather". My 70 has the same problem with building up pressure, I just never did anything about it yet. It tends to mess with the fuel gauge a little bit at times also.

Mike, I'm glad you made that comment about your garage smelling like gas. That is exactly what has been happening to mine after I drive it and put it back up. I've been in full panic mode, thinking something is leaking or something worse. I'm going for a new cap this week. this week is dedicated to fuel issues anyway, making a fuel log for it at work, looking cool so far.

Mike,
Why not fix it correctly and get the right tank and hook up all the evap canister hoses etc ? There should be a vapor seperator behind your back seat that is designed to eliminate the exact problem you are having. It works in conjunction with the charcoal canister under the hood to eliminate vapors.

I will Bill. I'm just very busy these days. I took a lot of time away from the world to build and install the motor. Now I just want to drive it everyday. :)

I have the right everything and mine builds up pressure

Are you talking about pressure, or about PRESSURE? There is obviously some threshold somewhere. For myself, I believe that SOME pressure is always going to be there, low/high or in/out. The whole move to seal up the mobile point source of evaporative loss inherent in cars had to involve limits on gas tank pressure, in and out, or the fuel system would not work. So it's going to occur. But is it outside of tolerences and likely to cause performance trouble or parts failure? Probably not. Here's a link to the thread where I gave the part numbers of the double versus single vented Stant gas caps, and my comments on them. http://www.chevelles.com/forums/showthread.php?t=186510&highlight=gas+cap Basically, I have contented myself with the idea that Stant has some logical and engineering grip on the acceptable limits on tank pressure, and my use of the dual vented cap on a post '70 car stripped of its vapor recovery systrem solves my problems. I still have a (smaller) whoose of incoming air being sucked in when I pull the cap, but nothing that would cause me to think that I was suffering some kind of problem.

I Tried the STANT 11646 cap from ROCK AUTO (www.rockauto.com) and NO MORE PRESSURE, NO MORE GAS LEAKS OR GAS SMELL. Also, the return line from my sender is caped off and clamped. Thanks for the tip.

Thanks for the part number Joey, I do appreciate it. :thumbsup: