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Old 08-26-2011, 01:56 AM   #1
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Fuel tank vent valve VS. Canister purge valve

Seems they do somewhat the same thing, eliminate fuel tank pressure...no? Not wanting to eliminate either, but what is more detrimental for fuel economy? Do they work in conjunction with each other or completely and totally separate?

I made a post regarding bad mpg on a trip, fuel boiling, gas smell, and found that the hoses to my charcoal canister weren't even connected and the canister purge valve hoses were broken and gunked up.

So that led me to this question. If the threads are united I understand, but I wasn't getting anywhere with that other thread.

Thanks.
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Old 08-26-2011, 09:38 AM   #2
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Re: Fuel tank vent valve VS. Canister purge valve

Your fuel tank cap is a one way valve allows air in but not out. So as fuel in tank gets hot and builds pressure a relief valve lets the fumes vent into the charcoal canister. Charcoal absorbs the fumes and stores them. So there is no odor escaping that can be smelled. The relief valve only opens at about 1-2 lbs of pressure so at times it will be normal to hear a slight hiss when opening the gas cap on a hot day. Excessive hiss or pressure in the tank indicates a defective relief valve or plugged line to the canister, or completely fouled canister (rare). Also could be EVAP solenoid or line from canister to evap solenoid is plugged with charcoal particles (common). Now when you are driving at normal speeds (RPM and MAP) the PCM opens a solenoid (EVAP Solenoid) and it will create a path from the charcoal canister to the intake manifold and suck the fumes stored in the canister out. The fumes are burnt off and there is no effect on performance or mileage. The system prevents fluorocarbons from entering atmosphere (emissions) and eliminates gas odors. Has been in uses since the 1980's and is usually very reliable.
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Old 08-26-2011, 12:37 PM   #3
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Re: Fuel tank vent valve VS. Canister purge valve

The charcoal adsorbs the fuel vapor - different that absorption.

The system prevents hydrocarbons from entering the atmosphere - no fluorocarbons in the fuel system.

When the EVAP solenoid opens, the vacuum in the line pulls fresh air through the charcoal to desorb the hydrocarbon that was collected.

The solenoid does not control pressure in the fuel tank. A plugged line will simply prevent the hydrocarbon vapor from being removed from the surface of the charcoal. As a result, the vapor will vent out of the system through the canister, with no reduction in hydrocarbons if the charcoal is fully covered.

http://shbox.com/1/evap_system.jpg
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Old 08-26-2011, 06:56 PM   #4
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Re: Fuel tank vent valve VS. Canister purge valve

So if the canister wasn't connected to the purge valve, wouldn't the purge valve just let the fumes and pressure out into the air....therefor causing the gas fumes around and in the car correct? So the EVAP solonoid would just draw in fresh air, correct? So would that point to Fuel Tank Vent valve as tthe fuel boiling/evaporation/bad mpg problem?
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Old 08-27-2011, 11:37 AM   #5
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Re: Fuel tank vent valve VS. Canister purge valve

The fuel is boiling because there is heat input. Could be a dying fuel pump, could be an oversize pump, could be missing heat shields. The high fuel temp causes the pressure to rise. Cruising in O/D requires little fuel. A lot of the fuel that gets sent to the engine gets sent back in the return line, picking up heat on both legs of it's journey, and in the engine compartment. Can get pretty hot by the time it gets back to the tank. In very high HP cars, with very large fuel pumps, we've used fuel coolers to try and minimize the problem of high pumping rates and high return rates during mild street driving.

Pressure in the vent space of the tank will reduce the boiling, since it counters the vapor pressure of the fuel. IF the fuel is hot enough, when you open the cap, the pressure is released and the fuel starts to boil even more, venting even more vapor.

With the heat problems you identifed in your post on the Advanced Tech forum, high ambients in AZ it explains part of it. What is the eleveation above sea level of the area where you were driving? Higher you go, lower the barometric pressure. Fuel boils easier, unless it was purchased at a high elevation where the blend is adjusted to lower the Reid vapor pressure.
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Old 08-28-2011, 01:01 AM   #6
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Re: Fuel tank vent valve VS. Canister purge valve

I want to say its less than 1200ft. Gas was filled here in San Diego and re-filled in Gila Bend, 1hr southwest of Phoenix.
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Old 08-31-2011, 09:59 AM   #7
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Re: Fuel tank vent valve VS. Canister purge valve

There are two valves in the system, the EVAP solenoid and the vacuum valve mounted behind the drivers side rear seat (big white valve).

The vacuum valve keeps a vacuum from being built up in the gas tank as fuel is depleted, ie, it allows fresh air IN to the tank. It does NOT allow pressure from inside the tank out.

The EVAP system is a closed system. ie, for it to work the input is the fuel vapors from the gas tank and the output is the vacuum in the intake, thus there is a vacuum switch that allows for vapors to flow from the tank to the switch, then from the switch to the charcoal canister and then from the canister to the intake. If there is no vacuum present, there will be no venting of pressure from the tank, period. Since our engines are in vacuum much more than they are near atmospheric, it is a non-issue. The computer still controls the EVAP solenoid to be active on high-vacuum/best for emissions conditions to maximize performance. The charcoal canister also has a fresh air inlet on it, but it is metered and controlled.

The idea behind the EVAP system, which also includes the vacuum switch I mentioned at the beginning (it allows fresh air into the tank to help pull through the fuel vapors in low-tank pressure conditions) is that fuel vapors never go into the atmosphere and the non-burnable vapor byproducts are filtered and the burnable ones are sent through the EVAP lines into the intake and burned. This is all timed to peak efficiency and done to reduce emissions.

This is also an elegant solution to tank pressurization and tank vacuum issues. The EVAP system bleeds off any pressure built up (and thermal expansion of gas due to heat picked up circulating through the fuel system is much greater than the rate of fuel useage, thus you will always build vapor pressure in the gas tank) and the vacuum-prevention switch allows fresh air in before thermal expansion occurs (such as cold starts and in cold climates), thus preventing vacuum being pulled on the tank.

Again, fact of the matter is that the system never, I repeat, NEVER vents fuel vapors to the atmosphere. I know for FACT that the vacuum valve holds to over 60psi when pressure in the tank is present.

Key is, without a properly working EVAP system and a functional vacuum valve, you WILL build either pressure or vacuum in the tank. To vent fuel vapor to air is the most irresponsible thing anyone can do. It is dangerous and stupid. Those systems NEED to remain in place on any vehicle driven on the street.

I am really tired of reading all the mis-information about how these systems work. Someone please make this post a sticky.
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Old 09-01-2011, 11:27 PM   #8
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Re: Fuel tank vent valve VS. Canister purge valve

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Originally Posted by 95 TA The Beast View Post
There are two valves in the system, the EVAP solenoid and the vacuum valve mounted behind the drivers side rear seat (big white valve).

The vacuum valve keeps a vacuum from being built up in the gas tank as fuel is depleted, ie, it allows fresh air IN to the tank. It does NOT allow pressure from inside the tank out.

The EVAP system is a closed system. ie, for it to work the input is the fuel vapors from the gas tank and the output is the vacuum in the intake, thus there is a vacuum switch that allows for vapors to flow from the tank to the switch, then from the switch to the charcoal canister and then from the canister to the intake. If there is no vacuum present, there will be no venting of pressure from the tank, period. Since our engines are in vacuum much more than they are near atmospheric, it is a non-issue. The computer still controls the EVAP solenoid to be active on high-vacuum/best for emissions conditions to maximize performance. The charcoal canister also has a fresh air inlet on it, but it is metered and controlled.

The idea behind the EVAP system, which also includes the vacuum switch I mentioned at the beginning (it allows fresh air into the tank to help pull through the fuel vapors in low-tank pressure conditions) is that fuel vapors never go into the atmosphere and the non-burnable vapor byproducts are filtered and the burnable ones are sent through the EVAP lines into the intake and burned. This is all timed to peak efficiency and done to reduce emissions.

This is also an elegant solution to tank pressurization and tank vacuum issues. The EVAP system bleeds off any pressure built up (and thermal expansion of gas due to heat picked up circulating through the fuel system is much greater than the rate of fuel useage, thus you will always build vapor pressure in the gas tank) and the vacuum-prevention switch allows fresh air in before thermal expansion occurs (such as cold starts and in cold climates), thus preventing vacuum being pulled on the tank.

Again, fact of the matter is that the system never, I repeat, NEVER vents fuel vapors to the atmosphere. I know for FACT that the vacuum valve holds to over 60psi when pressure in the tank is present.

Key is, without a properly working EVAP system and a functional vacuum valve, you WILL build either pressure or vacuum in the tank. To vent fuel vapor to air is the most irresponsible thing anyone can do. It is dangerous and stupid. Those systems NEED to remain in place on any vehicle driven on the street.

I am really tired of reading all the mis-information about how these systems work. Someone please make this post a sticky.
While we're on the subject of mis-information,

Per my 1995 Service Manual:

"The fuel tank vent valve, located in the rear of the vehicle by the fuel tank, is a pressure/vacuum relief valve. When fuel tank pressure exceeds a specified pressure the valve will open allowing tank pressure to bleed off. When the fuel tank is in a vacuum condition, the vent valve will open when the vacuum is within a specified range allowing fresh are to be pulled in."
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Old 09-02-2011, 06:50 AM   #9
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Re: Fuel tank vent valve VS. Canister purge valve

And the manual is WRONG!

Grab one of those valve and see what pressure it releases at. It does not.

It is federal emissions law, you cannot vent fuel tank vapor to the atmosphere, EVER!

No vehicle manufacturer would ever put a device on that would vent to atmosphere. Think about it, if it vents pressure and there is a spark/fire present you would have an explosion. Venting of tank vapors/pressure is the job of the EVAP system.

Again, the service manual is obviously wrong.
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Old 09-02-2011, 06:53 AM   #10
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Re: Fuel tank vent valve VS. Canister purge valve

Fact of the matter is, if you don't have a properly working EVAP system on the car, the tank will build pressure, which will, eventually, burn out pumps and can vapor lock fuel system. Been there, done that, seen it first hand.
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Old 09-02-2011, 03:15 PM   #11
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Re: Fuel tank vent valve VS. Canister purge valve

Designing a system that does not provide full vacuum and pressure relief in the event of a system malfunction would be totally irresponsible. Over-pressuring the tank can cause it to rupture, creating a slightly larger problem than that caused by venting the pressure to the atmosphere.

Chiltons also indicates it is both a pressure and vacuum relief valve.
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Old 09-02-2011, 05:49 PM   #12
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Re: Fuel tank vent valve VS. Canister purge valve

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Originally Posted by Injuneer View Post
Designing a system that does not provide full vacuum and pressure relief in the event of a system malfunction would be totally irresponsible. Over-pressuring the tank can cause it to rupture, creating a slightly larger problem than that caused by venting the pressure to the atmosphere.

Chiltons also indicates it is both a pressure and vacuum relief valve.
Like I said, I pressure tested brand new out of the box vacuum valves as well as the one on both my cars and they all held past 60psi of pressure.

It may vent pressure at 100psi or something silly, but that is well beyond the vapor-lock point of stopping fuel flow due to too much pressure in the tank.

Fact of the matter is, you cannot expect it to relieve everyday pressure seen in the gas tank. That is the only justification some of these asshats use to remove the EVAP system. Fact of the matter is both systems have to be in place.
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Old 09-02-2011, 05:49 PM
 
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