Mercedes Mullins
Fuel tank ventilation is essential to allow fuel to be pumped without creating a vacuum. This can be achieved through the gas cap or a breather tube. The latter is typically routed higher than the tank and exits below to prevent spillage when overfilled. Drilling a hole in the gas cap can also create a vent, but this method may not always be effective. Another option is to use a charcoal canister, which can help with fuel overflow. Some vents, like the Newton TPV6 tank vent, are designed to let air in and out but not fuel. When designing a fuel tank vent, it is important to consider the type of fuel line used as modern fuel can seep through micropores in certain materials, causing a strong gas smell.
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What You'll Learn
Venting through the gas cap or a breather tube
Fuel tanks need ventilation to allow fuel to continue to pump without causing a vacuum. This can be achieved through the gas cap or a breather tube.
Venting Through the Gas Cap
Up through the 1960s, most vehicles used vented gas caps, which are simply gas caps with a hole in them. However, this hole would allow fuel to splash out when accelerating or turning a corner. To avoid this issue, you can drill a hole on the inside of the gas cap for venting. This method has been tried and tested by some users, who have reported no issues with fumes or gas running out.
Venting Through a Breather Tube
Breather tubes are another option for venting fuel tanks. These tubes are usually routed up higher than the tank and then exit below it to prevent spillage if the tank is overfilled or possible siphoning. It is important to ensure that the vent line is higher than the highest point on the tank, including the fuel filler neck. The vent line should also not have any dips where fuel or condensation can get trapped.
Some users have recommended using a Newton TPV6 tank vent, which allows air in and out but not fuel. This vent requires creating a loop that is high enough to be above the fill. One suggestion is to loop it up to the roof, while another option is to loop it over the fill pipe inside the trunk and then back down and out of the trunk using a sintered filter with a bulkhead fitting.
Another option for the breather tube is to use a rollover valve, such as the TPV8 valve, which can be installed upright and out of the way. The vent line can then be routed back under the car to a small filter.
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Using a rollover valve
A rollover vent valve is an essential component of a fuel tank, as it allows the tank to breathe and prevents pressure or a vacuum from building up during normal operation. As fuel is added to a tank, the air inside needs to escape, and as fuel is drawn out, air needs to return to the tank.
When installing a rollover vent valve, it is important to avoid common mistakes. Firstly, ensure the vent line is the highest point in the tank system, even higher than the fuel filler neck. This is to ensure that the tank can breathe properly. Secondly, the vent valve must be mounted vertically, as mounting it at an angle or horizontally will cause the vent to shut, and the tank will not be vented. Thirdly, avoid creating a trap within the vent line by running it down and then back up again, as this will cause issues with venting.
There are two types of rollover vent valves: in-tank valves and remote-mount valves. An in-tank valve is installed inside the fuel tank, while a remote-mount valve is installed outside the tank, and is used when there is no room for an in-tank valve. Both types of valves perform the same function of preventing fuel from draining out during accidents or rollovers. This is achieved through a check ball inside the valve that closes when the valve is on its side or upside down, stopping fuel from escaping.
Some considerations when using a rollover valve include the potential for slower filling times, as well as the need to ensure proper orientation during installation. It is also important to be mindful of venting fumes, as they can pose a safety hazard if they escape into the car's interior or come into contact with electrical components.
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Drilling a hole in the gas cap
However, one drawback of this method is that fuel can splash out when the vehicle accelerates or turns a corner. To mitigate this issue, it is recommended that the vent line be positioned higher than the highest point on the tank, including the fuel filler neck. Additionally, the vent line should be straight and avoid dips to prevent fuel or condensation from becoming trapped, which can lead to a build-up of pressure or a vacuum in the tank.
When drilling a hole in the gas cap, it is important to consider the size and number of holes. Some sources suggest drilling three 1/8" holes in the gas cap, which has been found to prevent fuel spillage. These holes allow the tank to vent, reducing the risk of vapor locking the engine. It is crucial to ensure that any holes drilled are small enough to prevent fuel from splashing out, especially onto other vehicles or people nearby.
Additionally, the angle of the gas cap should be considered when drilling the hole(s). Drilling a small hole in the lower area of the cap, on the gas side, may allow gas to return to the tank without pooling inside the cap. This design consideration can help prevent fuel accumulation and potential leakage. It is also important to note that drilling holes in the gas cap may not be necessary if the engine has a fuel vapor capture system, which includes an additional hose and valve system to vent the tank and route escaping fumes.
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Using a fuel vapour separator
Fuel tank venting is a crucial process that prevents pressure build-up in your tank, allowing it to function properly. When you fill up your tank, air needs to escape, and when fuel leaves the tank, air has to enter to avoid creating a vacuum. If the vent is blocked, pressure in the tank can build up, risking damage to the tank, fuel leakage, or even an explosion.
A fuel vapour separator is not necessary for your car to run, but it is important to prevent vapour lock or over-pressurization of your fuel tank. The vapour separator allows the vapours to be recovered in the charcoal canister and then pulled into the carb and burned, while also preventing liquid fuel from escaping into the vapour recovery system.
To create a fuel vapour separator, you will need a stainless steel drinking bottle, a bulkhead adapter, activated charcoal, and fine steel wool. The vapours enter via a 3/8" barb fitting secured into the bulkhead adapter. Pack each end with about 3 inches of firmly packed steel wool and place about 4 ounces of activated charcoal in between. The vent line is 3/8" with about 70 1/8" holes on the far side of the canister, drilled completely around the bottle. The cap is secured using a cotter pin. By removing the cotter pin, the canister can be serviced with new steel wool and activated charcoal.
It is important to note that the activated charcoal will eventually get saturated and need to be replaced. Additionally, the separator should be mounted vertically to prevent it from shutting off.
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Using a charcoal canister
A fuel tank vent is necessary to allow air in to replace the fuel volume pumped out. One option for venting a fuel tank is to use a charcoal canister. This method can be used to prevent fuel odours and is considered more environmentally friendly.
To make a fuel tank vent using a charcoal canister, you will need a canister with an inlet and an outlet. The canister must be mounted higher than the tank. The inlet of the canister should be connected to the tank vent line, while the outlet should be connected to a vacuum line that runs to the air cleaner. This setup will pull air and vapours from the gas cap, tank, separator, and charcoal canister into the motor.
It is important to note that the charcoal in the canister will eventually become saturated and need to be replaced. To prolong the life of the charcoal, it is recommended to have a purge valve located away from the canister to run fresh air through it and desorb the fuel vapour. Without a purge circuit, the canister may quickly saturate and become ineffective at containing fuel vapour.
One user reported making a homemade charcoal canister for their fuel tank vent using a stainless steel drinking bottle, a bulkhead adapter, activated charcoal, and fine steel wool. They secured the vapour inlet using a cotter pin, which can be removed to service the canister with new steel wool and activated charcoal.
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Frequently asked questions
A fuel tank vent is necessary to relieve vacuum and pressure. Gasoline expands in volume as it warms up and shrinks in volume as it cools down, so the tank must be able to breathe.
One way is to use a gas cap with a hole in it, though this may cause fuel to splash out. Another way is to use a breather tube, which is usually routed up higher than the tank and exits below it to prevent spillage.
Some products that can be used include a Newton TPV6 tank vent, a TPV8 valve, and Eddie Motorsports stainless steel marine vents.
