What is the most likely cause of this concern?

• A. Spark Knock
• B. Vacuum Leak
• C. Fuel Trim Decrease
• D. EGR Stuck Open

Answer: (B)

A vacuum leak is the most likely cause because it introduces extra, unmetered air into the intake, throwing off the air–fuel ratio and commonly producing a lean condition with idle instability or rough running. When the engine breathes in more air than the ECU expects, it responds by adjusting fuel delivery, but if the leak isn’t metered, you end up with symptoms like rough idle, hunts, or stalling at idle, and often codes indicating a lean condition.

The other options don’t fit as cleanly. Spark knock is detonation from overly advanced timing or a very lean mixture but is not the typical cause of idle instability caused by unmetered air. A fuel trim decrease would not align with a leak, since a vacuum leak usually causes the ECU to increase fuel trim to compensate for the extra air. An EGR stuck open can cause rough idle or stumble, but it tends to produce different patterns (usually under certain loads or speeds) and is less commonly the primary cause of a broad idle/air-fuel imbalance seen with a leak.