If a boost leak causes a lean condition, what would be the expected trend in fuel trims?

If a boost leak causes a lean condition, what would be the expected trend in fuel trims?

• A. Negative fuel trim
• B. No change in trims
• C. Positive fuel trim increasing
• D. Fuel trims oscillate randomly

Answer: (C)

Fuel trims show how the engine computer adjusts fueling to hit the target air–fuel ratio. A boost leak lets in extra air, creating a lean condition that the O2 sensor detects. In response, the ECU adds fuel, so the trims move positive and keep rising to compensate (first in short-term trims, and then in long-term trims if the condition persists). If the leak is fixed or the limits are reached, the trims stabilize. Negative trims would make the mixture leaner, no change wouldn’t correct the lean condition, and random oscillation isn’t how trims typically behave in a steady, unchanging problem.

Fuel trims show how the engine computer adjusts fueling to hit the target air–fuel ratio. A boost leak lets in extra air, creating a lean condition that the O2 sensor detects. In response, the ECU adds fuel, so the trims move positive and keep rising to compensate (first in short-term trims, and then in long-term trims if the condition persists). If the leak is fixed or the limits are reached, the trims stabilize. Negative trims would make the mixture leaner, no change wouldn’t correct the lean condition, and random oscillation isn’t how trims typically behave in a steady, unchanging problem.