Evaporative Emissions Control (EVAP) system
Description
1. The Evaporative Emissions Control (EVAP) system prevents fuel system vapors (which contain unburned hydrocar- bons) from escaping into the atmosphere. On warm days, vapors trapped inside the fuel tank expand until the pressure reaches a certain threshold, at which point the fuel vapors are routed from the fuel tank through the fuel vapor vent valve and the fuel vapor control valve to the EVAP canister, where they’re stored temporarily, until they can be consumed by the engine during normal operation. When the conditions are right (engine warmed up, vehicle up to speed, moderate or heavy load on the engine, etc.) the Powertrain Control Module (PCM) opens the canister purge solenoid, which allows the fuel vapors to be drawn from the canister into the intake manifold, where they mix with the air/fuel mixture before being consumed in the combustion chambers. This system is complex and virtually impossible to troubleshoot without the right tools and training. However, the following description should give you a good idea of how it works:
2. The EVAP canister is located under the vehicle, in front of or to the side of the fuel tank, depending on the model. The EVAP canister, which contains activated charcoal, is the repository for storing the fuel vapors, and is designed to be maintenance-free (it should last the life of the vehicle).
3. The fuel tank pressure sensor, which is located on top of the mounting flange for the in-tank fuel pump/fuel level sending unit module, monitors the pressure inside the tank, and transmits its measurement to the PCM during an OBD-II leak test.
4. The EVAP canister vent solenoid is normally open. But it seals off the EVAP system for inspection and maintenance (I/M 240) testing and for OBD-II leak and pressure tests.
5. The EVAP canister purge solenoid, which is under the control of the Powertrain Control Module (PCM), regulates the flow of vapors being purged from the EVAP canister into the intake manifold. The canister purge solenoid is normally closed. It opens only when directed to do so by the PCM, which uses the availability of intake manifold vacuum and data from various information sensor inputs to determine when and how long to open the valve. The interval of time during which the purge valve is opened by the PCM is known as its duty cycle.
General system checks
6. The most common symptom of a faulty EVAP system is a strong fuel odor (particularly during hot weather). If you smell fuel while driving or (more likely) right after you park the vehicle and turn off the engine, check the fuel filler cap first. Make sure that it’s screwed onto the fuel filler neck all the way. If the odor persists, inspect all EVAP hose connections, both in the engine compartment and under the vehicle. Be sure to inspect each hose attached to the canister for damage and leakage along its entire length. Repair or replace as necessary. Inspect the canister for damage and look for fuel leaking from the bottom. If fuel is leaking or the canister is otherwise damaged, replace it.
7. Poor idle, stalling, and poor drivability can be caused by a defective fuel vapor vent valve or canister purge solenoid, a damaged canister, cracked hoses, or hoses connected to the wrong tubes. Fuel loss or fuel odor can be caused by fuel leaking from fuel lines or hoses, a cracked or damaged canister, or a defective vapor valve.
8. To check for excessive fuel vapor pressure in the fuel tank, remove the gas cap and listen for the sound of pressure release. If the fuel tank emits a whooshing sound when you open the filler cap, fuel tank vapor pressure is excessive. Inspect the canister vapor hoses and the canister inlet port for blockage or collapsed hoses. Also inspect the vapor vent valve. A complete test can only be done with a professional-level OBD-II scan tool, which will run a series of checks to detect excessive pressure. You’ll have to take the vehicle to a dealer service department or other qualified repair shop to have the EVAP system professionally diagnosed.
Component replacement EVAP canister purge solenoid
Non-6.01_ models
Note: The EVAP canister purge solenoid valve is located on the driver’s side of the intake manifold, behind the throttle body.
9. Remove the air intake duct (see Fuel and exhaust systems).
10. Disconnect the canister purge solenoid electrical connector (see illustration).
16.10 Canister purge solenoid location (4.3L V6 engine shown)
11. Disconnect the EVAP purge line quick-connect fitting from the canister purge solenoid. If you’re unfamiliar with quick-connect fittings, refer to Chapter Fuel and exhaust systems.
12. Remove the canister purge solenoid mounting bolt and remove the purge solenoid.
13. Installation is the reverse of removal.
6.0L V8 models
Note: The EVAP canister purge solenoid valve is located above the front end of the left fuel rail.
14. Remove the air intake duct (see Fuel and exhaust systems).
15. Disconnect the outlet purge line quick-connect fitting from the canister purge solenoid (see illustration).
16.15 Canister purge solenoid details — 6.0L V8 engine
1 Canister purge valve
2 Electrical connectors
3 EVAP line quick-connect fitting
4 EVAP vacuum line quick-connect fitting
16. Disconnect the electrical connector from the EVAP canister purge valve.
17. Disconnect the inlet purge line quick-connect fitting from the canister purge solenoid.
18. Cap both EVAP purge lines to prevent dirt, dust and moisture from entering the EVAP system while the line is open. If you’re unfamiliar with quick-connect fittings, refer to Fuel pump / fuel pressure — check.
19. Remove the canister purge solenoid from its mounting bracket.
20. Installation is the reverse of removal.
EVAP canister vent solenoid
Note: On some models, the EVAP canister vent solenoid is located in front of the EVAP canister. On other models, the vent solenoid is located at the back of the fuel tank.
21. Raise the vehicle and support it securely on jack stands.
22. Disconnect the electrical connector from the vent solenoid (see illustration).
16.22 Location of the EVAP canister vent solenoid (standard-bed pick-up)
23. Follow the inlet and outlet EVAP lines from the vent solenoid to their quick-connect fittings, then disconnect the fittings. Also, disconnect the lines from any clips securing them.
24. Remove the vent solenoid from its mounting bracket.
25. Installation is the reverse of removal.
EVAP canister
26. Raise the vehicle and place it securely on jack stands.
27. Clearly label all EVAP hoses connected to the EVAP canister, then disconnect the quick-connect fittings from the canister (see illustration). For more information on quick-connect fittings, see Fuel and exhaust systems.
16.27 EVAP canister mounting nut (A, accessed from above) and canister vent solenoid line fitting (B)
31. Remove the fuel tank (see Chapter Fuel and exhaust systems).
32. Remove the fuel tank pressure sensor from the fuel pump/fuel level sending unit mounting flange (see illustration).
Pry the fuel tank pressure sensor out of the fuel pump / fuel level sending unit mounting flange
33. Installation is the reverse of removal.