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Evaporative emission systems perform a few simple functions.  Venting the fuel tank when needed and disposing of the fumes are the primary purposes.  They use an incredible amount of complexity to address these tasks, largely due to Government regulation.  This leads to many check engine lights, perhaps more  than any other system on the vehicle.

The purpose of the EVAP system

To avoid a vacuum, when we pump anything from a sealed container, something must take its place.  This is true for the fuel tank on a vehicle.  Removing fuel, requires the entry of air.  The same is true when filling the tank.  As fuel enters, air and vapors exit.  Without a vent, filling the tank would be almost impossible.

For many years, a small hole in the fuel cap served to vent the tank.  Fuel vapors exited through the opening.  This brought up a question of fuel vapors in the atmosphere.  When fuel is warmed or agitated, evaporation begins and this produces fumes.

Early evaporative emissions systems

diagram of non enhance evaporative emissions system

Some think these fumes may harm the environment, so government regulations require a means of containment.  Early systems employ a charcoal canister to catch these emissions.  Fuel tank pressure pushed the fumes into a container, and the vacuum drew them out, as the engine accelerated. The system is mechanical and relies on physics.  This works well, but provides no method to verify things are working as designed.

For instance, if the fuel cap were left off or replaced with a vented model, we defeated the system.  They would soon mandate and eventually evolve self-testing  systems into the  evaporative emissions systems of today.

A self-testing system

Diagram of the enhanced, self testing evaporative emissions system

A 2000 California mandate requires the use of enhanced emissions systems on all vehicles sold in the State.  Almost immediately, all vehicles sold in the US met the new standard.  These systems monitor and control purge flow, and detect leakage as small as twenty-thousandths of an inch or half a millimeter.  

Enhanced systems require careful service to avoid leaks and check engine lights.  They are also easier to diagnose, with information supplied by the power control module or PCM.

Two primary methods of testing

Countless designs exist, but most use one of two methods for testing.  GM, Ford and many Asian systems use vacuum tests.  Chrysler, many European, and some Asian vehicles use pressure tests.  Vacuum testing is simpler but pressure tests work under more situations.

For example, drawing a vacuum on the tank, and watching to see if it holds, will check for leakage.  To do this, a vent valve is closed, the purge valve opens and the engine draws a vacuum.  A sensor in the fuel tank monitors this.  The purge valve closes when we have achieved the proper level.  Testing consists of watching the rate at which vacuum decays.  

Unfortunately, a vehicle in motion jounces the fuel.  This increase in pressure could appear as a leak.  With vacuum testing, a pressure rise, from fuel, causes the tests to abort.  Elevated fuel temperature, as with a vehicle that has been running also stops the tests.  Before vacuum-testing will commence, we must meet several criteria.  A series of pretests verifies this.   


Conditions needed to run vacuum EVAP tests

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.

Before the tests run, the PCM also checks the vehicle speed and 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.

Other factors include barometric pressure and 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.  The PCM must also be clear of diagnostic trouble codes, to enable the EVAP testing.  Because of these specific requirements, the vehicle may not test the EVAP system on every drive-cycle.  This can cause the check engine light to turn off temporarily, although a code is still present in history.

Specific tests for different purposes

Large EVAP leaks

Diagram of the enhanced, evaporative emissions system testing

The PCM monitors how long it takes to achieve a specified vacuum level.  Time required for the level to decay is also checked.  For instance, if the system cannot achieve a vacuum and fuel level has increased (the tank is filled,) it will trigger a check-engine, or check-fuel-cap light.  These are pending codes and will clear if the next large-leak test passes.  If the fuel level has not increased (no fuel has been added,) it will set a hard code, such as P0455,  for a gross EVAP leak.

With Ford and GM, when we achieve the vacuum, but it will not hold, a pending code will be set.  Although a code is present, the check engine light will not come on.  Instead, they run a second test and if it fails, the PCM turns on the light.

GM pulse modulated purge solenoid

The purge valve is a pulse-modulated solenoid, meaning the opening is infinitely variable through the PCM.  Rather than an on/off signal, the computer specifies a duty cycle, based on calculations.  After commanding the purge valve to close, the PCM expects to see no additional rise in the vacuum.  Should it continue to rise, a leaking purge valve is suspected, and a pending code is set.  Failing two consecutive tests will set a check engine light and a hard code such as P0441, for un-commanded purge flow.

A vacuum that builds too quickly, on two consecutive tests, could mean a plugged vent or bad vent-solenoid.  A hard code, such as P0446, for vent failure, is set, and they turn on the check engine light.  Failure of the vacuum to decay, in the proper time, after testing, sets the same code. Of course, other conditions can account for these situations.  That is why we can never take codes literally.  For example, they may misrepresent an off-range tank-pressure sensor as a vent or purge valve code.

The small leak test

The EVAP system runs a more precise test, for small leaks, over a longer period.  This test may run after we switch off the engine and may take several minutes. The PCM monitors how long the vacuum takes to decay to a preset level.  Calculations for vehicle speed, a sloshing of fuel and ambient temperature are added.  Should any of these conditions exceed limits, the test will abort.  The next time the vehicle meets all conditions, the PCM will attempt the test again.  Many vehicles may only attempt this test twice in one day.

a few enhanced EVAP codes that set

When the vacuum falls in less time than it should, a pending code is set, and they run the test again.  Failing two consecutive tests sets a code P0442 and turns on the check engine light.  

If the test maintains a vacuum over too long a period, they assume a plugged vent, or the purge valve may be leaking.  This failure also sets a pending code.  If the next test passes, the code is cleared.  Two consecutive failures will set a check engine light.

A vacuum that maintains, over the correct amount of time, passes the small leak test.  When two consecutive tests are successful, they change the readiness-monitor status to complete.  State inspection requires all monitors to be complete, before they approve a vehicle.  Clearing any codes also sets the monitors as incomplete.  This is to prevent tampering with the system.

As mentioned earlier, we may also use pressure, rather than a vacuum, to test for leaks.  Systems that use pressure testing are the next Detailed Topic.

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