PCM Will Help Test GM's Enhanced Evaporative System
By Jim Watson
The purpose of these tests is to check the canister purge solenoid and vent solenoid for operation and functionality, and the integrity of the system for leaks. The tests will occur based on the parameters stored in the PCM memory.
To ensure proper operation of the General Motors (GM) enhanced evaporative system, the power control module (PCM) will perform up to six tests: power-up vacuum test, excessive vacuum test, purge solenoid leak test, loaded canister test, weak vacuum test and small leak test.
The purpose of these tests is to check the canister purge solenoid and vent solenoid for operation and functionality, and the integrity of the system for leaks. The tests will occur based on the parameters stored in the PCM memory. Here is a brief description of each test:
- Power-up Vacuum Test: This test is run when the vehicle is in a cold key on/engine off (KOEO) mode when the fuel level sensor indicates more than 15 percent and less than 85 percent fuel volume in the tank. No vacuum or pressure should be in the system because the vent solenoid is in the de-energized state or open to atmosphere. The fuel tank pressure sensor should indicate 0.0 inches of H2O or 1.5 volts. Failure of this test indicates a blockage between the vent solenoid and the fuel tank via the charcoal canister or a faulty fuel tank pressure sensor.
- Excess Vacuum Test: This test is run during a normal purge operation of the vehicle. The PCM monitors the vacuum or lack of pressure in the fuel tank. If the PCM sees an increase in the voltage (beyond programmed values) from the fuel tank pressure sensor, it has determined the vent circuit between the charcoal canister and the vent solenoid is restricted or blocked.
- Purge Solenoid Leak Test: This test is run to check for a leaking purge solenoid. When commanded off, the solenoid should block all vacuum. To perform the test, the PCM will turn off the purge solenoid by releasing the ground, and then ground the vent solenoid, closing it. The PCM will then monitor the fuel tank pressure sensor. An increase in voltage indicates the purge solenoid is not closing off tightly.
- Load Canister Test: This test is run during normal purge operation of the vehicle. Since the charcoal canister is supposed to collect and store fuel vapor, when purge is occurring, the assumption is that extra fuel will be introduced to the engine. As a result, the heated oxygen sensors (HO2S) should indicate a richer-than-normal situation. The fuel trim values should then come down due to the fuel coming from the charcoal canister. This would be considered a "pass" for the test.
- Weak Vacuum Test: This test is only run following a "failed" loaded canister test. If the loaded canister test failed, it indicated no fuel was added to the engine from the charcoal canister, and therefore a leak in the system must be present. To perform the test, the PCM commands the vent solenoid closed by grounding it while duty cycling the canister purge solenoid to ground. The result should be a voltage increase on the fuel tank pressure sensor, indicating a vacuum increase in the tank.
- Small Leak Test: This test is run next, but only after a "passed" weak vacuum test. After a sufficient amount of vacuum is developed in the fuel tank during the weak vacuum test, the PCM knows the system does not have any hoses off or gapping leaks and it will now look for a small leak. The PCM will turn off the canister purge solenoid, which will (or should) trap the vacuum in the evaporative system. The PCM will now calculate for leaks as small as 0.04 inches. The size of the leak is based on the amount of fuel in the tank, the amount of vacuum in the tank and the amount of time it takes to bleed a specific amount of vacuum.
Note: During the small leak test, if the customer should stop to fuel the vehicle with the engine running, as soon as the fuel tank cap is removed all vacuum will be lost. Since the PCM assumes a leak, it will set the DTC and illuminate the MIL. In some cases, the customer may also experience difficulty fueling the vehicle. Remember, the vent solenoid has been commanded closed, therefore the only method of venting is past the pump nozzle. This will cause pressure to build and trip the pump handle off.
Jim Watson is an IDENTIFIX GM specialist. He is ASE master and L1 certified. He is a member of the Service Technicians Society and teaches third-year automotive students at a technical college in Minnesota.
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