How to Diagnose a Purge Valve

The purge valve helps reduce vehicle emissions by introducing fuel vapors into the intake manifold.

As part of your vehicle's emission-control system, the purge valve controls fuel vapors collected inside a canister. The fuel tank produces these vapors, which the canister stores for burning whenever the engine is at operating temperature and running above idle speed. The purge valve, controlled by your car's computer, keeps emissions down and avoids rough idle. However, purge valves may become stuck. Diagnose the purge valve in minutes and help your engine maintain optimal performance. This guide applies to purge valves used on fuel-injected systems.

Things You'll Need

• Wrench
• Shop rag
• Labels
• Hand vacuum pump
• Jumper cables

Instructions

1. Locate the purge valve in your vehicle. Typically, this is a small cylinder on the left (driver's) side of the engine compartment. The valve has an electrical connection at the bottom and an input-output vacuum hose port. One side of the port connects to a vacuum hose that goes to the purge canister hiding on the front and bottom of the engine compartment. The other side of the port connects to the fuel tank and the intake manifold with a vacuum hose through a dual plastic connector.

2. Disconnect the black (negative) battery cable with a wrench and isolate the cable terminal with a shop rag to prevent the cable from accidentally touching the battery post.

3. Inspect the vacuum hoses connected to the purge valve and check for damage, tears or loose connections. A vacuum leak can cause the purge valve to fail. Reconnect any loose hoses and replace damaged ones, if necessary.

4. Label the vacuum hoses connected to the purge valve and the ports they connect to on the purge valve and carefully disconnect the hoses.

5. Unplug the electrical connector from the purge valve.

6. Apply 5 inches of mercury with a hand vacuum pump to the source port on the valve, which connects to the intake manifold. The valve should hold the vacuum. If the valve allows air to pass through the source port, replace it.

7. Hold the 5 inches of mercury to the source port with the hand vacuum pump. Connect a jumper cable from the negative post on the battery to the black wire on the purge valve electrical connector and another jumper cable from the positive battery post to the other wire on the purge valve electrical connector. This time, the valve should let air go through the source port. If not, replace it.

References

• "Modern Automotive Technology"; James E. Duffy; 2003
• "The Haynes Emissions Control Manual"; Mike Stubblefield and John H. Haynes; 2001