ESGWheel
Petty Officer 1st Class
- Joined
- Aug 29, 2015
- Messages
- 228
This is about the gas cap for the Perko Combination Fill and Vent Necks. Like in Picture 1. This is not about the newer EPA compliant gas caps or with tanks have a separate vent.
There was a recent post regarding these and the question came up: How do they work? I was curious too so took my spare cap apart. It’s not obvious by looking at it (no vent hole!) but it does work with a cleaver design. Here’s how:
The cap screws down and seats onto the neck with an O-Ring. This keeps out water and other elements. Note the neck has both the fill and vent hose sharing the same opening, again look at Picture 1.
What is clever is that the venting happens above the O-Ring and below the metal cap. Between these two items are 4 slots that are the vents. Air can enter via these slots and thru the “Surge Plug”. The Surge Plug is the bridge between the Neck that is sealed by the O-Ring and the small space between the O-Ring and the metal cap. See Mockup and the other pictures.
Note that the design of the Surge Plug acts as a one-way valve allowing air to enter but if there is pressure in the tank, it more or less seals by moving against the plastic housing. It’s not a perfect seal by any means and I am sure that is on purpose. In other words, as the boat sits in the hot sun and there is expansion of both the gas and the vapors in the tank, it will vent out but with more difficulty than allowing air to enter. So, my evaluation is that if just a little pressure builds up, it contains it but if its more, it will leak by.
My trailering cover comes down below the gas cap and this is why I smell gas on occasion when I uncover the boat and never any other time. Its also why, if the tank is very full and sits in the hot sun, some people experience gas spills > literally the liquid gas expanding in the tank and fill/vent hoses to the point where the only escape for the larger volume of liquid gas is the vented gas cap.
If this Purge Plug gets stuck closed or plugged up or the screen gets contaminated with an insect nest, it could lead to improper venting and thus fuel starvation stalling the engine.
Which leads me to two items I encourage folks to do and does not matter if your gas cap is this type or not. Carry a spare gas cap. If you lose you cap while filling it will be no fun trying to stuff a rag or something into the fill neck to keep out the water / debris until you get a new one. A marina gas attendant once told me there a number of caps below the dock from their chains breaking. Perhaps meant as a joke, but… Also consider installing a “Vent Whistle”. This device is spliced into the vent line where it starts to go vertical and whistles like heck when filling. When get a ‘gurgling whistle’, tank is full. When I used to trailer my boat, this was a rock-solid indicator to get a full tank with no spilling, ever. And no liquid gas spilling out due to too full and hot sun. Now that I dock it, it’s still useful at the waterfront gas stations but due to the rocking of the boat it will gurgle sooner depending on how much the boat is rocking. Regardless, I still get a good fill (tank is 35 gal and I always need to fill it full) and the guys at the dock love it. One tip is to tell the attendant about it and encourage them to keep the gas flowing and trust the whistle. Some are used to slowing down the feed as it fills to preclude spillage and if too slow the escaping air is not enough to get a strong whistle. Here is one on Amazon link.
There was a recent post regarding these and the question came up: How do they work? I was curious too so took my spare cap apart. It’s not obvious by looking at it (no vent hole!) but it does work with a cleaver design. Here’s how:
The cap screws down and seats onto the neck with an O-Ring. This keeps out water and other elements. Note the neck has both the fill and vent hose sharing the same opening, again look at Picture 1.
What is clever is that the venting happens above the O-Ring and below the metal cap. Between these two items are 4 slots that are the vents. Air can enter via these slots and thru the “Surge Plug”. The Surge Plug is the bridge between the Neck that is sealed by the O-Ring and the small space between the O-Ring and the metal cap. See Mockup and the other pictures.
Note that the design of the Surge Plug acts as a one-way valve allowing air to enter but if there is pressure in the tank, it more or less seals by moving against the plastic housing. It’s not a perfect seal by any means and I am sure that is on purpose. In other words, as the boat sits in the hot sun and there is expansion of both the gas and the vapors in the tank, it will vent out but with more difficulty than allowing air to enter. So, my evaluation is that if just a little pressure builds up, it contains it but if its more, it will leak by.
My trailering cover comes down below the gas cap and this is why I smell gas on occasion when I uncover the boat and never any other time. Its also why, if the tank is very full and sits in the hot sun, some people experience gas spills > literally the liquid gas expanding in the tank and fill/vent hoses to the point where the only escape for the larger volume of liquid gas is the vented gas cap.
If this Purge Plug gets stuck closed or plugged up or the screen gets contaminated with an insect nest, it could lead to improper venting and thus fuel starvation stalling the engine.
Which leads me to two items I encourage folks to do and does not matter if your gas cap is this type or not. Carry a spare gas cap. If you lose you cap while filling it will be no fun trying to stuff a rag or something into the fill neck to keep out the water / debris until you get a new one. A marina gas attendant once told me there a number of caps below the dock from their chains breaking. Perhaps meant as a joke, but… Also consider installing a “Vent Whistle”. This device is spliced into the vent line where it starts to go vertical and whistles like heck when filling. When get a ‘gurgling whistle’, tank is full. When I used to trailer my boat, this was a rock-solid indicator to get a full tank with no spilling, ever. And no liquid gas spilling out due to too full and hot sun. Now that I dock it, it’s still useful at the waterfront gas stations but due to the rocking of the boat it will gurgle sooner depending on how much the boat is rocking. Regardless, I still get a good fill (tank is 35 gal and I always need to fill it full) and the guys at the dock love it. One tip is to tell the attendant about it and encourage them to keep the gas flowing and trust the whistle. Some are used to slowing down the feed as it fills to preclude spillage and if too slow the escaping air is not enough to get a strong whistle. Here is one on Amazon link.
