Evaporative Emission Control: Notes

WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.

The Evaporative (EVAP) emission control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. See Figure . Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the Powertrain Control Module (PCM) will command the EVAP purge solenoid ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid OFF, open, fresh air will be drawn through the solenoid and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion.

The system is required to detect the evaporative fuel system leaks as small as 0.04" (1 mm) between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative system integrity by applying a vacuum signal, ported or manifold, to the fuel tank to create a small vacuum. The PCM then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts sub-tests to detect the fault conditions. If the diagnostic fails a sub-test, the PCM stores a diagnostic Trouble Code (DTC) to indicate the type of fault detected. See EVAP SYSTEM RELATED DTCS table.

EVAP SYSTEM RELATED DTCS

Diagnostic Test	DTC	Possible Causes
Weak Vacuum Test (Large Leak Test)	P0440	A large leak above 0.06" (0.05 cm) or blockage somewhere in the system
Vacuum Decay Test (Small Leak Test)	P0442	A small leak of 0.02-0.06" (0.05-0.15 cm) somewhere in the system
Canister Vent Restriction Test	P0446	A restriction in the EVAP vent system
Purge Valve Leak Test	P1441	A stuck open or partially open purge valve

The EVAP system consists of the following components: