The Stovebolt Site Forums The Shop Area General Truck Talk Gas leaking out filler neck after fill up

Hey guys,

Need a little help here! I just filled up my 52 truck and drove it back home only about 4 miles from station. Parked it and came back out a little while later to see gas running down the outside from cap/filler neck. Seems it built up pressure but I thought my gas cap was supposed to vent to keep this from happening. The cap does have a vent hole on inside. What am I missing? Is something else clogged up? Certainly you don’t have to keep the tank low enough to keep this from happening do you?

Some info on this forum points to a possible new cap that will not vent pressure but will vent vacuum, loosen the cap and feel or listen for pressure escaping vs. vacuum.

What good is that?

I had a problem with a new cap that the inside vent got stuck. It created a vacuum in the tank and wouldn't allow air to enter to replace the fuel used by the engine.

In your case it seems to be going the opposite direction.

Did you fill the tank up all the way full up the filler neck? If so, the fuel probably expanded in the heat of the day and forced its way out the vent hole.

I have a problem alright!

Here is the inside of the cap. I can blow air through it from inside to outside using a compressor. Shouldn’t that mean the cap is venting????

What Otto said about pressure building up in the tank. Our old trucks are not a modern design to capture fuel vapors and leaks. I bought a new Spectra Premium (original style) tank, filler hoses and gas cap. Filled it up with non-Ethanol. Everything was running great. A few days later I pulled it out of the garage and parked it in the driveway. The sun came up, the cab got hot inside, and it leaked gas at the cap. Thank goodness I had parked it on a slight slope. The gas didn't drip down the side of the cab and ruin the paint. You have to remember not to fill the tank too full. Many gas stations have an underground tank, and the gas is at ground temperature. Gas expands as it warms up.

But isn’t the cap supposed to vent and account for that if it is working properly?

Here is a picture of the original cap. New caps come for places in Asia. Construction is unknown and fit is problematic. The original AD fuel system must vent when the vacuum inside the tank becomes too great. I believe there is a check valve in the cap, but I really don't know. A fuel pump pumps gas to the carburetor at about ~3.5 to 4 lbs. If you didn't have a vent, the truck would starve for gas just like a lawnmower with a plugged-up vent hole. Pressure (not vacuum) build-up inside the tank is another matter and for the AD trucks, this is a weakness. The gas cap needs to prevent gas from leaking out but if the pressure is too great it overcomes the seal(s) and leaks. The solution is to allow an area for the gasoline to expand by not filling the tank too full. Another issue I heard about is that some Asian gas caps don't fit tightly and allow gas to leak regardless of pressure.

If a vendor sells you a part, that was made for your truck, that doesn't work well or doesn't fit, contact the vendor and explain.

PastorMatt, what is happening is that the gas is expanding in volume to the point is extends up the filler tube and gas pours out through the vent in the gas cap. There is no pressure build up per se.

I had the same thing happen to me with my '51 and it occured inside a very hot Arizona garage. Right next to the electric water heater! I never top off the tank unless I am on a road trip.

Kent

Simple fix. Drill an 1/8th" hole in the center recess of the blue plastic. I always go off center in case there is a valve in the center.
Works like a champ and makes the cap function exactly like the original caps did.
Whoever is making reproduction caps must have hired an overly enthusiastic engineer who wanted to be environmentally conscious and designed a cap with modern features.
Cheers,
Carl

Carl, That will work if his cap isn't venting, to an extent, but doesn't address if the tank is too full!

Matt, Several other folks above have mentioned that "topping off" in-cab mounted tanks to the point it leaves little to no room for expansion will result in the gas overflow problem you described. The fill tubes attached to these tanks leave very little room for expansion to begin with and yes, gas does expand A LOT when it's temp increases between in-ground cooler storage tanks to sitting in a hot sun enclosed cab and tank.

I will admit to making the very same mistake when I first got my Grandpa's truck. Filled it up, then gave the handle an extra squeeze after it self-shut-off like I have always done on all my other vehicles. An hour later sitting in the hot sun and I had gas dribbling down the side of the truck cab, leaking through my properly vented gas cap. Best rule of thumb when filling these tanks is at a minimum, don't give that handle an extra squeeze when it auto shuts off. Better yet, if your truck is going to be parked anywhere in direct sun or inside a hot garage, etc., leave it a little short when filling. These efforts will help you from a repeat overflow performance.

When I fill mine, I listen to the gas entering the tank by keeping the pump hose end pulled slightly away from the filler neck. Towards the end of the fill, you can hear the fuel starting to come up towards the filler neck. I stop then. Yes, when filling, my ear is close to the filler neck. People look at me like "What is he listening too?" Again, when it sounds like the gas is almost high enough to start coming up the filler neck, I stop. Have yet to have a problem with gas spitting out at me.

Be realistic about your old truck. It’s nothing like the nice Honda CRV or modern pickup, or whatever you and your family drive every day- in terms of technology it’s closer to a lawn mower , an old lawn mower. Overfill it , get a mess.
If you fill the gas tank right to the top , into the fillet neck , you are asking for trouble. As previously stated , gasoline expands when heated. The underground tank at the gas station is cooler than summer air temperatures. Fill it up to the top , park it in the sun , the results are what you picture depicted.

Don’t challenge all those connections above the gas tank in your old truck by having standing fuel in the neck. Keep the fuel level down in the tank and you should be fine.

I never top mine off unless I am about to go on a long enough trip to burn a couple of gallons right away.
More often, it seems, I am continuously verifying the accuracy of the empty line on the gas gauge...
The trick is noticing at the very moment when the needle stops jiggling, knowing that I have 30 miles to find a gas station.
Win some lose some.
On the bright side, with these cool old trucks, people seem to be waiting in line to pull over to help you.

Replaced my lost loose fitting gas cap on my 1952 1/2 ton pickup with the inexpensive gas cap available from our vendors. Immediately noticed large amount of pressure build up in tank when removing cap after driving. Also started noticing a regular booming/banging sound from behind. Checked frame, springs, etc. Nothing help in eliminating the sound. From this forum thread I started thinking about my pressure build-up. Ordered a more expensive Brothers Original Cap for my '52. Solved all problems with pressure build up and booming sound.
Dan

Yep sounds like your new cap is not venting both directions properly

and the newer one fixed it

I had to remove the valve from my cap due to the same problem. New stuff is all crap from China. Meanwhile, the original 85 year old cap on my Packard works perfectly. It's identical in design to the original Chevy cap.

I think gmc caps are different than chev caps if I recall correct I need to find a cap for my 54 gmc I tried two locking caps from rock auto and they don’t latch on properly.

Looks like gas is leaking from underneath the old rubber gasket. Maybe replace the fuel fill gasket and fill neck rubber hose coupling.

Also, I would fill up with the truck door open; and I would tap on the tank to figure out how full the tank was. I always left a good bit of space in the tank.

Here's a question.....Should the vent valve in the cap run with equal pressure going in and out?

Original caps didn't have a valve. New ones do and need to be disabled. For some reason the aftermarket put valves in their caps unnecessarily which causes the development of a vacuum which restricts flow to the fuel pump, and/or causes high pressure in the tank as the day grows warmer causing fuel to puke out once the valve release.

It's an easy fix. The bottom of the new caps are usually plastic. The valve is in the center. Drill a 1/8" hole just to the outside edge of the valve. That will allow the tank to vent just like the originals did. You still want to avoid filling the tank all the way to full unless you are on your way on trip where you will use a gallon or two of gas right away.

When that happened to me, I disassembled the valve mechanism and just left the cap with a small vent hole where the valve was.

It would seem like gas caps need two valves to maintain proper pressure. One to relieve over pressurization do to expansion and the other to eliminate negative pressurization when engine fuel pump is running. Just a hole in the cap allows vapors to escape, although minimal, still a dangerous combustible in a confined garage.

Cars used vented caps for 100 years venting fumes directly to the atmosphere. They aren't dangerous. Every car I own has the same style cap and they're all in the garage.

That is why I recommended only a 1/8" hole.
These new caps are not designed correctly and do not allow the tank to adequately breathe in or out.
The original caps were designed to breathe.
Do I know that a 1/8" hole is the perfect amount of air, duplicating the design of the original cap? No.
After I drilled 4 different caps on 4 different trucks with a 1/8" hole, upon removing the cap, I no longer hear the sound of significant vacuum, nor the sound of pressure release.
If one was more comfortable with a 1/16" hole rather than 1/8" hole to ease their mind concerning the possibility of too much venting, I have no argument against that.
Two of my trucks, which are kept inside, with 1/8" vent holes drilled in their new caps do not cause any discernible odor of gas inside of my garage.

Could you post a pic of one of your caps with this hole? Are these hole drilled all the way through the cap?

Modern vehicles have a non-vented gas cap, instead of venting the fumes directly to the atmosphere, the tank is vented to a charcoal canister to absorb the gas fumes. The canister is then in charge of keeping the tank in a neutral pressure. I think the gas cap on my '57 might be original or at least from that era of vented caps, it has a leather gasket and a simple hole in the center of the bottom. I have no odor from the cap and no issues with pressure or vacuum.

Go to Rock Auto and look at the pictures of the Gates gas cap. You'll see that the underside of the cap has a plastic vent on it with 5 holes.

It's' held in place with two metal tabs . Those can be bent up to remove the plastic assembly. Inside you'll find a valve and springs that are supposed to allow the cap to equalize pressure inside the tank.

As usual, the replacement manufacturers haven't figured out how to make simple things function properly so the valve ends up sticking closed and creating a vacuum inside the tank.

Remove the little valve from inside the plastic and reassemble the cap. Works perfectly after that.

Using the coefficient for gasoline expansion 0.00053 per degree Fahrenheit, a 10° rise in temperature equates to 10.9 ounces. A 30° rise would equate to over a quart!

EDIT: Adding to this equation, the average ground temperture at 5’ deep is 55. On a 90° day the cab sitting in the sun could easily go over 130° or more. 130°-55°=75°. The would be well over a half of gallon (0.636 gallon).

Reference:
BSI Engineering

Pretty certain I bought my locking cap from LMC Truck a few years back. Their current 2025 vol 1 CA 118 has locking vented gas cap on page 75. I attached pictures of mine and attempted to hold seal down to captures how it vents. Not real clear though.

You only need to drill through the thin plastic disc.

Thanks to Otto's suggestion above, I did just that. Very easy to remove the valve.