EVAP System

Purge Valve

  • Purge valve is normally closed.


Vent Valve (aka Drain Valve)

  • Vent valve is normally open. It is commanded to be closed for leak detection testing.
Will fuel vapors go into air when vent valve is normally open?
  • The EVAP canister, which is between the tank and the vent solenoid, will store the vapors. The vent solenoid will allow atmospheric pressure to push vapors from the canister to the engine when both it and the purge solenoid are open.


Fuel Cut Valve (aka Rollover Valve)

Rollover vent valves perform two essential functions.
  • Used in conjunction with your vent line, a rollover valve will allow your tank to breathe and not build up pressure or vacuum during normal operation.
    • Remember, as fuel is added to your tank, air in the tank is displaced and has to go somewhere. Vent lines and vent valves allow air to be released so pressure doesn’t build. Likewise, as fuel is drawn from the tank, the vent lines and valves allow air to return to the tank so a vacuum situation is avoided.
  • The second function of the rollover vent valve (and what separates it from regular vent valves and lines) is to prevent gas from running out of your vent line during an accident.
    • When the rollover vent valve is on its side or upside down, a check ball inside the valve will close to prevent gas from running out.
  • If it sticks (after overfilling), it might trigger a “gross leak” code,
    • Since  the vacuum applied to the tank during the evap leak test might not get applied to the tank (the air is drawn from the tank via the rollover valve), PCM will not see a change in FTP (no tank depressurization).


Shutoff Valve

  • Located at the top of the fuel filler pipe.
  • When a filler gun is inserted into the filler pipe, the shut off valve closes the evaporation line.


Pressure Control Solenoid Valve

  • When the fuel tank pressure becomes higher than atmospheric pressure, the valve is opened allowing fuel vapors to be introduced into the canister.
  • When the fuel tank pressure becomes lower than atmospheric pressure, external air is being taken from the drain valve into the canister.


Switching Valve (two tanks)

  • The switching valve is activated by pressure from the active in-tank fuel pump.
  • When working properly it will switch both the feed and return lines to the proper tank. It isn’t uncommon for internal leaks to develop causing the return fuel to go to the wrong tank.


Reference Orifice

  • The purpose of reference orifice is to simulate the effect of a leak having exactly the same size as the reference orifice.
    • When the system is evacuated through a reference orifice, the resulting vacuum level represents the level that can be achieved with a leak having the size of the reference orifice in the system. Thus, if the maximum allowable orifice size for a given regulatory jurisdiction is 0.020″, then evacuating the system through the reference orifice will establish a reference vacuum level.
    • Cost. As noted, frequent changes in EVAP leak regulations lead to costly modifications to standard testing procedures. Here, the orifice holds maximum potential for a change.
  • https://www.google.com/patents/US20150082866


Vapor Return Line (aka Vent Tube, aka Breather Hose)

  • During refueling of an automobile, liquid fuel entering the fuel tank displaces gases therein, such as air and fuel vapor.
  • Without the vent hose,
    • There would be a lot of bubbling and gurgling, as air tries to get out past the incoming gas, which would slow down the refueling process (similar effect to draining a bottle of water).
    • Gasoline could back up in the filler neck, slosh through the opening, and cut the pump off prematurely.
  • Full Tank. When the liquid fuel level covers the vent pipe opening, gases can no longer vent, thus causing a pressure increase which shuts off the fuel pump.
  • On older designs a vent pipe carries the gases to the tank exterior,  venting directly to atmosphere.
    • Environmental problem. Gasoline doesn’t just magically disappear. Once it gets cold, it will condensate and get mixed in with water in dew, or rain.


Fuel Tank Cap

  • Has relief valve that prevents development of a vacuum in the fuel tank in the event of a problem with a fuel vapor line.


EVAP Canister

  • Stores fuel vapors, with help of carbon pellets.
    • If the canister is not purged, the carbon pellets can become saturated, after which hydrocarbons will escape to the atmosphere, causing pollution.



EVAP Monitoring Criteria

  • Vehicle Speed
  • Engine Load
    • The tests require the engine to be running, at an idle, with near a zero load.
      • This means there will be a steady vacuum, and fuel will not be sloshing in the tank.  Ambient temperature is also considered and must be above 40 degrees Fahrenheit.  A high limit, of 100 degrees Fahrenheit, is set for most models.  A few vehicles have the upper limit at 86 degrees Fahrenheit.  If the temperature is outside these ranges, the tests will not execute.
  • Ambient Temperature
    • Fuel heated begins to produce pressure.  A rise in temperature occurs as the fuel pump gives off heat.  Ambient temperature is also a concern.  The GM and Ford systems do not use a fuel temperature sensor.  Instead, they check intake air temperature (IAT) and compare it with engine coolant temperature (ECT) reading.  If they are near each other, the vehicle has not been running.  Engineers assume the vehicle has unheated fuel, when meeting these conditions.
  • Fuel Level
    • The fuel level must be between 15% (1/4 tank) and 85% (3/4 tank) and steady.
      • If the fuel level is changing, as with sloshing, the tests will not run.
  • Barometric Pressure
  • No DTC

calcul  A few EVAP Codes  d level  fail  P0446  pass  level  P0442  P0455  calculated time  limit  agcoauto.com

  • P0446… Clogged Vent Valve
  • P0442…Small Leak
  • P0455… Large Leak




EVAP Failure Symptoms

Rough / low idle, stalling (purge solenoid stuck open)
  • When the canister purge valve is failing the engine often begin to miss out. The fuel vapors are building up inside the charcoal canister and pushing into the engine cylinders, causing the engine to choke. Because the purge valve is not releasing the fuel vapors back into the emissions system, the vapors are not recycled.
Huge vapors pressure release from the fuel tank you open fuel cap. (purge solenoid stuck closed, etc).
  • Due to restricted vapors flow into intake manifold.
Damaged Spark Plugs (purge solenoid stuck open)
  • A good sign that the purge valve is not working is when the spark plugs go bad. Fuel is allowed into the intake system without being mixed properly. The fuel will be too rich, which causes the spark plugs to burn and wear prematurely. Since the valve is operated by a vacuum and releasing the build up of the fuel vapors is accomplished by the purge valve, a clogged or damaged valve does not allow the proper fuel mixture to enter the engine.
Failed Emissions Test
  • The canister purge valve will clog and prevent the recycling of the fuel vapors. These fuel vapors do not get sent back into the emissions system, causing the automobile to fail a state sanctioned emissions test. The release of harmful carbons are being released through the exhaust, causing the vehicle to fail the emissions test.
Blown Gaskets
  • Once the purge valve fails, pressure builds up in the emissions systems causing the gaskets to blow open. Damaged gaskets will leak oil or, at worst, spray oil out onto the engine and engine compartment. This pressure is so intense that it breaks the tight seal the gaskets make and pushes through, creating oil leaks.

Why The Gas Nozzle Keeps Clicking Off?

  • EVAP canister is plugged
  • Vent Valve
  • Rollover Valve not opening, so when refueling, fuel tank air doesn’t have anywhere else to go.


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