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Purge/Seal EVAP System

What is the purge/seal EVAP system?

The most common EVAP system used

What’s it used on?

Most Ford and GM vehicles, many newer Asian vehicles and some Euro vehicles

Purge/seal EVAP components

  • Fuel tank
  • Vent Solenoid
  • Fuel Filler Neck
  • Purge Solenoid
  • Fuel Tank Pressure Sensor
  • Lines/Hoses
  • Gas Cap
  • Charcoal Canister

How does a purge/seal EVAP system work?

Fuel vapors are stored in the fuel tank and the vapor canister. When conditions are correct, the engine control module opens the purge solenoid allowing engine vacuum to draw stored fuel vapor from the EVAP system into the engine and burns them with the air/fuel mixture.

How does the ECM test for EVAP leaks?

Understanding how the vehicle tests for leaks can aid in the accurate and efficient diagnosis of the purge/seal EVAP system.
Vehicle conditions must be correct for the EVAP system to be leak tested by the ECM
These conditions include, but are not limited to:

  • No related diagnostic trouble codes
  • Proper fuel level

Tip : This is usually between 15-85%
  • Proper coolant temperature
  • Proper ambient air temperature

Tip : Most EVAP systems won’t test under 32°F
  • Specific engine rpm and load Upon meeting these conditions, a typical ECM controlled EVAP leak test goes as follows:
  • The ECM opens the purge solenoid allowing intake manifold vacuum begin to draw on the EVAP system.
  • The ECM closes the vent solenoid. This seals the purge/seal EVAP system.
  • The ECM monitors the fuel tank pressure sensor (FTP) output. It should be reporting an increase in vacuum.

Tip : If no vacuum change is noted, this is a gross leak and a P0455 will commonly set.
  • Upon the FTP vacuum reading reaching a pre-determined threshold within a set amount of time, the ECM shuts off the purge.
  • Now the purge/seal EVAP system is sealed and sitting in vacuum
  • The ECM watches the FTP for vacuum decay. The vacuum level must remain below the ECMs threshold for a predetermined amount of timing that will vary with conditions.

Tip : Depending on the rate of vacuum decay, a small or large leak code may set here. A quicker decay, a large leak; A smaller decay, a small leak.

Tip : No EVAP system is 100% sealed. The system will always display SOME vacuum decay when sitting. This is normal. The vacuum reading threshold will be specific to the vehicle and should be found in service information.
  • If the vacuum reading on the FTP stayed below the threshold the leak portion of the test is complete.
  • Upon leak test completion the ECM may chose to open the purge or vent solenoids and close them again. This is done to ensure there’s no restriction.

Tip : The vacuum reading will jump quickly and hold again if the ECM cycles the purge or vent quickly. If it doesn’t decay fast enough a flow restriction is present.

What is Engine Off Natural Vacuum Leak Detection?

A new way to test for very small leaks without the vehicle running while still using the same components of the purge/seal system.


Tip : An EONV system will still do many of the same tests as the purge/seal system. The benefit of EONV is that if the very small leak test is passed, an intrusive vehicle running large leak test isn’t required.

How does EONV work?

To understand how an EVAP system can be tested without the vehicle running, we first need to talk about fuel, gasoline, and what it does naturally; evaporate.

Fuel evaporates at different rates at different temperatures. This leads to a change in pressure in a sealed container. Look at gas cans stored in the garage. On an unusually cold day the can may contract slightly. Opening the nozzle reveals a small amount of vacuum in the can. The opposite is true on a warm day. A gas can will expand slightly, relieving pressure when the nozzle is opened.

This same phenomenon happens on our cars fuel tank. After a warm car has been shut off the ECM begins a timer. After a predetermined amount of time the ECM will close the vent solenoid and watch the fuel tank pressure sensor. This seals the system. If the fuel is getting warmer, from hot exhaust or pavement, the pressure in the fuel tank will increase. If it cools, it will decrease or show vacuum. This subtle increase or decrease needs to pass a similar threshold to a leak test, though not as dramatic of a pressure/vacuum reading. If the EONV EVAP system can build pressure or vacuum, it passes the test. No leak is present. If it doesn’t pass this test the ECM will preform a leak test the next time the vehicle is running, and conditions are correct.


Tip : A leaking purge solenoid can set a small leak code on an EONV system

Diagnostic Checks

  • Inspect for loose gas cap
  • Visual inspection of hoses for cracks, leaks and proper routing and attachment
  • Using a scan tool or multi-meter, check key-on, engine-off (KOEO) FTP sensor voltage for proper reading (See chart)
  • Using a full function scan tool, perform an EVAP system leak test to verify failure is present

Tip : Many EVAP faults are intermittent. Temperature can play a huge role in leaks being present or not. Ensure your testing conditions match the trouble code setting conditions
  • Verify proper purge solenoid functionality. Click here for more information
  • Verify proper vent solenoid functionality. Click here for more information
  • Perform a system leak test using a smoke machine. Click here for more information