Gas cap vent hole

A

AlanD

Senior Member
I'm working on the car out in the sun and I hear a quiet whistling sound and as I twist off the gas cap it makes that "chu" pressure release sound. I know the gas cap has to vent and looking at the bottom of the cap I see the tiny vent hole. Tell me if I'm thinking wrong, but the ring on the gasket is the lip of the filler hole, so the vent hole is on the inside of that seal.
This cap has been on the car for years and I haven't experienced any problems in this area, but should this fixed or modified?

What's your gas cap like?
 

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Alan, based on my experience i'd say your close to having the engine just stop because of a plugged vent hole. The cap is sort of a double wall construction. Try removing the gasket and blow air through the vent hole to clean it out. I couldn't believe the crud that came out of mine. I eventually went with a new cap. Hope this helps.
 
What you have is a standard poor quality repo gas cap that is prone to plugging of the vent hole. The small hole above the single rivet is the vent hole. That hole goes through three things, the metal plate that grabs the tank boss, the red fiber washer, and a brass wafer that is under the red washer. All three of the holes must stay in alignment in order to vent the tank. What happens is the center rivet loosens up with the on & off action of the cap. The red washer drags on the tank boss and all three items with the holes rotate out of position and the tank no longer vents. The whoosh sound you hear when you remove the cap is air rushing into the tank.

There is a way to modify the cap. Drill the existing hole out very carefully so as not to go through the roof of the cap. That will restore the vent hole. Drill a second hole 180 degrees from the first hole. Drill for a tap size of a 6-32 screw. Make sure you drill through all three items, but don't go through the roof of the cap. Tap the hole and screw in a 6-32 screw. Coat the screw with either lock-tight or JB weld. This will prevent the screw from falling out into the tank and will prevent the three items from rotating on the center rivet and keep the three vent holes in alignment.

An alternative is to order an Eaton cap from Snyder. The cap has two rivets and two vent holes and though not identical to an original cap, it is close, and the vent holes will stay in alignment.

Tom Endy
 
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Thanks for the responses. I wish I could remember from where I bought this gas cap but its been too many years ago. I'll try your fix before I order a new one.
 
Can you post a pic of yours so I can see it. They say in the south a pic is worth beaucoup words.
 
Thanks for this post. On my first road trip with my 31 PU, it stalled twice after a 30 minute run. The problem was no venting in the gas cap. Ford barn to the rescue again.
 
Thanks for this post guys - saw it the other night and sure enough tonight, pitch black out with the family at some friends away from home it wouldn't start - opened the cap and "woosh", started right up - will perform the screw fix tomorrow.
 
Outstanding response/explanation by Tom Endy!
I experienced this vent hole misalignment issue with the cheapo
Repro gas cap. But it took me a long time to find out why my engine was occasionally dying, because of a blocked vent in the cap. After I discovered the problem, which was a misalignment of all the pieces, I replaced the cap with a repro Eaton style cap, but the venting was too good and let gasoline slosh/splash around and come out and spill all over the top of my gas tank, even when the tank was only 3/4 full. So I replaced that with another cheapo repro cap, and will keep my eye on that one for eventual misalignment or maybe drill and secure all three rotating pieces as Tom suggests. I really like the looks of the Eaton style cap, but I don't want gasoline spilling out from the two vents. I even tried a new leather gasket for the Eaton style cap, but the venting again was too great and let too much gasoline out. This is just a warning to beware of gas spilling out of the newer Eaton style caps; hopefully your experience will be better.
 
Pat, do you have a picture of the spill over cap?
I've not heard of this problem before, but have an idea for a fix if I can see the cap.
 
picture.php


Get an Eaton Style (on the right)... worked wonders for me. Mine was from Snyders...
 
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Pat, if your problem cap looks like Mikes on the right, I can't see how it would leak gas from sloshing.

BTW, I have the filler screen in my cars, so there is almost no sloshing at the cap area.
 
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Yes, I have the same Eaton style cap, and it allows gasoline to spill out on top of the outside of my tank, even after changing the leather washer to another one from A&L. The other style gas tap does not leak, therefore I am using it instead; just need to be aware that the vent hole can get blocked again by the 3 rotating pieces.
 
Tom Endy said:
What you have is a standard poor quality repo gas cap that is prone to plugging of the vent hole. The small hole above the single rivet is the vent hole. That hole goes through three things, the metal plate that grabs the tank boss, the red fiber washer, and a brass wafer that is under the red washer. All three of the holes must stay in alignment in order to vent the tank. What happens is the center rivet loosens up with the on & off action of the cap. The red washer drags on the tank boss and all three items with the holes rotate out of position and the tank no longer vents. The whoosh sound you hear when you remove the cap is air rushing into the tank.

There is a way to modify the cap. Drill the existing hole out very carefully so as not to go through the roof of the cap. That will restore the vent hole. Drill a second hole 180 degrees from the first hole. Drill for a tap size of a 6-32 screw. Make sure you drill through all three items, but don't go through the roof of the cap. Tap the hole and screw in a 6-32 screw. Coat the screw with either lock-tight or JB weld. This will prevent the screw from falling out into the tank and will prevent the three items from rotating on the center rivet and keep the three vent holes in alignment.

An alternative is to order an Eaton cap from Snyder. The cap has two rivets and two vent holes and though not identical to an original cap, it is close, and the vent holes will stay in alignment.

Tom Endy
Click to expand...
Hi Tom,

I have a 28 Model A that has the original cap and cleaned out the holes inside the car using compressed air, but I can't see any vent holes on the perimeter of the cap, when the cap is screwed down is the leakage between the threads sufficient? Or should I just buy a new cap. What is happening is when I drive the car for more than 20 minutes the fuel stops flowing, I have drained and cleaned the tank and with the cap off the flow is strong, with cap on strong then weakens. This is the picture of my gas cap

GAS CAP.jpg
 
The vent holes are in the lip of the cap. I can see the vents in your picture, look closely and you will see them also..

Here is a picture of my cap showing the vent holes...
 

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I had the same kind of original cap on my 28 Leatherback. It was clogged. I placed the cap (less the gasket) in an ultra-sonic cleaned with enough solvent, like carburetor cleaner, to completely submerge the cap and let it run for an hour or two. I periodically tested to see that each of the perimeter Vent holes was clear. Once they were I sprayed brake carb cleaner through each Vent hole to ensure that all were clear. I have no problems since then. It isn’t a quick process but it worked well for me, I am lucky to have a nice original cap. Supergnat
 
1955cj5 said:
The vent holes are in the lip of the cap. I can see the vents in your picture, look closely and you will see them also..

Here is a picture of my cap showing the vent holes...
Click to expand...

Thank you so much! I didn't see the holes and they are definitely clogged. Will clean and then all will be well. :)
 
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