Fuel System Lean During Acceleration (GM)
What does P1171 mean?
The P1171 code means the vehicle's onboard diagnostic system has detected: Fuel System Lean During Acceleration (GM). This is a moderate severity code.
Common Symptoms
- Check Engine Light on
- Hesitation or stumble during acceleration
- Poor fuel economy
- Engine misfires under load
- Rough idle that worsens when revving
Probable Causes (Ranked by Likelihood)
- Vacuum leak in the intake system Very High
- Weak fuel pump unable to maintain pressure under load High
- Clogged fuel filter or fuel injectors Moderate
- Dirty or faulty MAF sensor Moderate
- Leaking fuel pressure regulator Low
Step-by-Step Diagnostic Procedure
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Monitor fuel trims on a scan tool -- note if lean condition worsens during acceleration
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Perform a smoke test for vacuum leaks on the intake manifold and hoses
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Test fuel pressure at idle and under load (pressure should not drop significantly)
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Clean the MAF sensor and inspect for contamination
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Check fuel injector flow rates and balance
Frequently Asked Questions
Why does the lean condition only show during acceleration?
During acceleration, the engine demands more fuel and air. A weak fuel pump or partially clogged injectors may supply adequate fuel at idle but cannot keep up under increased demand, causing a lean condition only during acceleration.
Can I drive with P1171?
You can drive short distances, but the lean condition during acceleration can cause engine damage over time. Running lean generates excessive heat that can damage pistons, valves, and catalytic converters.
How do I test the fuel pump?
Connect a fuel pressure gauge to the fuel rail test port. Check pressure at idle (should match specifications) and have someone accelerate the engine while you watch the gauge. Pressure should remain stable. A significant drop indicates a weak pump.