How should a technician test the EGR valve or system for proper functioning?

• A. Apply Vacuum at Idle and Listen for Increase in Engine RPM
• B. Apply Vacuum at Idle and Listen for Reduction in Engine RPM
• C. Remove EGR and Inspect
• D. Measure Vacuum with a Gauge

Answer: (B)

Opening the EGR valve with vacuum during idle is the direct way to check if it will actually recirculate exhaust gases. When a vacuum-operated EGR valve functions, applying vacuum at idle should cause the valve to open and bring exhaust gas back into the intake. That additional gas displaces some air and lowers combustion efficiency a bit, so the engine idle slows down. In other words, you should observe a reduction in engine RPM when vacuum is applied.

If there’s no change in RPM (or if RPM rises), the valve isn’t opening properly, indicating a fault such as the valve being stuck, a vacuum leak, or a problem with the vacuum source. Removing the EGR or simply measuring vacuum with a gauge doesn’t directly test the valve’s ability to open at idle, so they aren’t as effective for this functional check.