Aardwolf

I have been curious about the evap system since I learned that the gas cap vents. From what I have learned, when the car sits the evap system cannot vent the gas tank pressure so to prevent gas tank damage, gas caps are made to vent. One place I read sited 1 PSI for the cap to vent. This means that a car that sits a lot will be venting from the cap. I believe the canister also has a vent.

I also have been curious about removing the evap system from my track car. Searching I found that some members remove the system and plug the lines. This would leave the cap to vent. If the cap vent fails there could be tank damage. Others write that they leave the lines open to vent by the canister. This sounds dangerous to me as fuel would spray out of there in a roll over.

Options for removing the evap system:

1. Run new line off the tank vent fitting, put a loop or two in the line, and a PCV valve at the end. This is very inexpensive but the PCV can seep.

2. Run new line off the tank vent vetting, put a loop or two in the line, and add in a fuel cell tip valve. These close tight and will not leak gas in a roll over. They are more expensive but I would not want to burn to death.

3. Loop a new line between the tee tank vent then put a dead end line on the fuel pump assembly vent line. This totally plugs the system and relies on the gas cap to do all the venting.

I thought I'd post this to help clear up the topic. If you look at the underside of your gas cap you can see one of the vents and push it to see that there is a spring.

Here is some more data:

The interior of the cap is much more complex than most would realize, with a couple flap type check valves. Take it apart some winter just for fun.

http://www.patentstorm.us/patents/54...scription.html

Gas Cap- Most people don't realize the gas cap is an emission control device, but it is. In precontrol days, the gas caps main job was to keep gasoline from sloshing out of the tank, and dirt from getting in the tank. It was equipped with a small vent hole so that the tank could breathe. Air entered through the cap to make up for fuel as it was used, and fuel vapors exited through the cap as internal pressure rose on warm days.

Today's emission control gas caps are considerably different. They are either of a solid construction (venting is provided by other means) or they contain a pressure/vacuum valve. The valve type cap will vent tank pressure if it exceeds 1 psi. It will also allow air to enter the tank if a vacuum exists within the tank. In other words, the valve type cap can vent pressure or relieve vacuum as the situation warrants without allowing gasoline vapors to pollute the environment.

The valve itself is a simple double spring arrangement similar to a radiator cap. One spring reacts to internal pressure while the other reacts to external pressure. A plate or diaphragm between the two springs opens and closes to allow air to pass through the valve in the direction needed.

Internal fuel tank pressure can be also vented by means of a three way valve in a vapor line to the charcoal canister. Some ford vehicles do not have a pressure/vacuum relief gas cap. Instead they use a three way valve in the fuel tank vent line to control internal tank pressure. The valve vents tank pressure to the charcoal canister. When there's a vacuum in the tank, the upper diaphragm will allow air to be drawn into the vent line to the tank. The lower diaphragm serves as a safety vent for excessive tank pressure in case the main vent line becomes clogged.

If a gas cap has to be replaced on a vehicle, the replacement must be the same type as the original (sealed or vented).

http://smogcarsuck.tripod.com/id41.html

AGENT 86

It's my understanding that the C4 cap is NON vented and all venting is sent to the cannister.

Aardwolf

Do you still have your stock one? If so look under it for a part that is spring loaded. If the cap cannot vent, then the canister would have to vent. Or there would be damage to the tank from nothing venting while the car sat.

dan0617

I could be wrong but from what I've read and found by searching, the system vents to the canister while the car is sitting. When the car is started the fumes are pulled from the canister to the engine to be burned. The gas cap only vents at a very very high pressure, just to prevent damage to the tank should the canister or line going to it get plugged. Under normal proper operation and car sitting in the sun, the gas cap doesn't vent. I know mine builds alot of pressure, way more than I would think as normal and way more than any other car I've had, but everything seems to be working as it should. When I open my cap it hisses hard for about 5 seconds. BTW, I laughed my *** off when I saw your subject line!!!!!!

Mr. Peabody

My 95 gas cap does not vent. In fact this is one of the things that is tested for when we go for our by-annual emissons test here in WA. If the cap vents to the atmosphere you fail.

Don't know if this is different for older C4 however.

cmontel727

My vent line got plugged and my tank would hiss for like ten seconds in the sun. Once the line was cleared no more hissing. If you elect to do the canisterectomy you would have to replace the cap with a vented one.

coupeguy2001

I went through emissions testing today.
Arizona is following federal guidelines, and they do the sniffer test twice. (They put the car in gear, and on the rollers, they run the car to 55 MPH, and then back to idle 2X).
Then you move the car forward, and they take the cap off the gas tank, and pressurize the gas tank. THen they put some sort of sensor on the tank vent line, and pressurize the tank again with the sensor hooked up.
Then while the computer is checking that, the attendant puts the gas cap on a fixture, and pressurizes it, then cuts off the line pressure, and it has to hold pressure for 3 minutes. Sooo....
I am guessing by that, the cap does not vent except at a higher pressure.
I passed by the way. ultra clean

Aardwolf

Since there is some confusion in the other thread about gas tank venting I thought I would bump this to the top. You need only turn your gas cap over and look at the valve there to know it's vented.

--------

Motor vehicles are subjected to wide variations in temperature and air pressure, both while in use and when parked. Elevated temperatures and reduced air pressures, in particular, result in the generation of hydrocarbon vapors within the vapor space of the vehicle's fuel tank. Modern motor vehicles include a gas cap adapted to seal the open end of the tank filler neck, to prevent atmospheric venting of these polluting vapors. Such vehicles also are equipped with a charcoal canister, having an inlet interconnected to the vapor space in the upper portion of the fuel tank, by means of a vapor vent line. The outlet of the canister is interconnected to the engine's air/fuel intake system, such as the intake manifold, or the like.

The function of the charcoal canister is to absorb excessive gasoline vapors generated during high temperature and/or low ambient pressure conditions, while simultaneously avoiding a dangerous vapor pressure buildup in the fuel tank. Thus, in a proper functioning evaporative system, when pressure in the vapor space exceeds atmospheric pressure, fuel vapors migrate through the vapor vent line into the vehicle's canister, where the hydrocarbons are absorbed by the charcoal. If vapor pressure is sufficient, the filtered vapor is safely exhausted to the atmosphere through an air vent in bottom of the canister.

Then, during driving, the vacuum existing in the intake manifold draws fresh air in through the same vent, vaporizing the hydrocarbons. The gas vapor is drawn by vacuum through the purge line, and introduced into the intake manifold for combustion. In this manner, the gasoline is fully utilized, and the charcoal canister is fully purged of vapors, restoring its hydrocarbon storage capacity for the next cycle.

If the charcoal canister is not regularly purged while the vehicle is driven, the charcoal will eventually become saturated and the trapped hydrocarbons will escape directly into the atmosphere through the canister's air vent. When this occurs, the major advantages provided by the evaporative system are defeated. The negative environmental impact of uncontrolled vapor discharge through the canister rivals that of the vehicle's exhaust emissions. Moreover, this vapor emission also decreases the overall fuel economy of the vehicle, by venting hydrocarbons which would otherwise be burned in the engine. Consequently, proper purging of the canister has significance both for the environment and for the conservation of energy.

In recognition of these facts, the Federal Environmental Agency (EPA), has mandated the testing of this canister purging function, in the context of a more comprehensive vehicle inspection and maintenance testing procedure, known as the "I/M 240" test. In the course of this 240 second test, a vehicle is put through a predetermined driving cycle on a dynamometer, simulating vehicle performance at various speeds and during acceleration/deceleration conditions. While being so tested, the purge line leading from the canister to the engine is constantly monitored, using a sensor and a recording instrument. This confirms that at some point during the predetermined driving test cycle, an adequate purging event has occurred.

---------

Vented Gas Cap Function

# The vented gas cap is designed to vent small amounts of air into the car gas tank line. The vented gas cap has a one-way release valve that is pressure-activated. When the pressure reaches a certain point (defined by so many pounds per square inch) the valve opens by a very small amount, relieving some of the pressure. In this case, the pressure is formed on the outside of the tank, due to the vacuum formed from the displacement of fuel on the inside. After the pressure inside the line equalizes, the pressure valve closes. The valve is set to balance the pressure with the surrounding atmosphere. Air is allowed in, but no fumes are allowed out of the gas cap, because of environmental concerns.
Inner Parts
# The vented gas cap has a chamber under the plastic cap handle that inserts into the fuel fill line. The cap screws shut, and this chamber fits relatively close inside the intake tube of the fuel line running to the tank. On each side of the chamber are small openings where air is able to enter. These chambers lead to the pressure valve, which, as the level in the tank decreases, allows air inside to remove any possibility of a vacuum forming.

---------

Fuel cap venting

Actual fuel cap venting is (+/- 1psia) - 1psia = 2"HG vacuum, so your fuel cap will vent at (rounding Barometric pressure to 15psia) 16psia overpressure, also at 14psia under pressure.

When tank is full of fuel, little free-air volume remains between the surface of the fuel and the top of the tank, so any hiss will be small - as fuel level drops, free-air volume above the fuel increases, so the hiss will be greater - this indicates greater volume of air in the tank, not greater pressure or vacuum.

The tank and cap was designed to also maintain ~2"HG vacuum, intended to reduce aerated fuel, which is foaming you cannot see, the bubbles being too small to break threw fuel surface tension - since each bubble below the surface of the fuel is a tiny microcosm of atmospheric pressure, any reduction in BP above the surface will reduce surface tension, allowing the smaller bubbles to break threw, thereby reducing aeration.

The fuel cap should be checked regularly to ensure it meets correct pressure\vacuum venting specs - use a radiator-pressure hand pump or a vacuum hand pump.

Aardwolf

Adding a pic:



If suction is applied to the bottom of the cap it very easily sucks in fresh air. There has to be some feature for this so a vacuum isn't created as the fuel level goes down. I didn't have a good way to test what it would hold for pressure the other way.

Just an update, I have had the system capped as in option three for around two years and its been working well.

Deepa

I know whenever I go to refuel, i get a really long wooosh when I take the cap off...

Is something plugged somewhere???

Mike

Aardwolf

Some pressure is normal.