P1171

Code

CHEVROLET P — Powertrain

Fuel System Lean During Acceleration

Brand: CHEVROLET

Type: P — Powertrain

Views: UK: 32 EN: 63 RU: 35

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Page language: EN

Causes

Low fuel pressure or insufficient fuel flow (weak fuel pump, clogged fuel filter, restricted return)
Faulty or contaminated mass air flow (MAF) sensor
Faulty manifold absolute pressure (MAP) sensor or barometric sensor
Intake vacuum leak or leaking intake manifold gasket
Leaking or damaged vacuum hoses, PCV system leak
Faulty or slow O2 sensor reporting lean

Symptoms

Check Engine Light (MIL) illuminated
Hesitation or lack of power during acceleration
Engine may stumble, surge, or misfire under load
Poor fuel economy
Possible engine knock/pinging under throttle
Hard starting or longer cranking if fuel delivery is severely reduced

What to check

Retrieve trouble codes and freeze frame data with scan tool; note RPM, load, throttle position and fuel trim at fault
Inspect air intake for leaks, damaged hoses, and loose clamps
Visually inspect wiring and connectors for MAF, MAP, O2 sensors, fuel pump and injectors
Check/replace fuel filter if maintenance interval exceeded
Measure fuel rail pressure and pump flow under load with a gauge
Monitor live data: MAF (g/s), MAP (kPa), STFT/LTFT, O2 voltages or wideband lambda, injector pulse widths

Signal parameters

MAF sensor: grams/sec (should rise with throttle; abnormally low values indicate under-reporting)
MAP sensor: kPa or inHg (should increase with throttle/load)
Short Term Fuel Trim (STFT): large positive numbers (+10% to +25% or more) during acceleration indicate lean correction
Long Term Fuel Trim (LTFT): sustained positive values confirm ongoing lean condition
O2 sensor voltage: low/lean voltage (~0.1–0.3V for narrowband) or wideband lambda >1.0
Fuel rail pressure: psi or bar (compare to spec; low under load indicates delivery problem)

Diagnostic algorithm

Record codes and freeze frame, then clear codes. Reproduce by road test under the same conditions while monitoring live data.
Monitor STFT/LTFT, MAF, MAP, O2 (narrowband/wideband), throttle position and injector PW during acceleration. Note which parameter deviates from expected behavior.
Perform fuel pressure test at the rail: verify static and dynamic (under load) pressure and pump flow. Replace pump or filter if below spec.
Inspect and test the MAF sensor: clean with approved MAF cleaner, check output vs expected airflow for given RPM/load, and replace if faulty.
Perform a smoke test and visually inspect for intake and vacuum leaks (intake manifold gasket, vacuum lines, PCV, throttle body, brake booster). Repair leaks found.
Check MAP/baro sensor operation and compare to expected values. Verify sensor wiring and connectors.
Inspect O2 sensor response: slow or stuck-lean readings can falsely indicate lean; replace if out of spec. Verify exhaust leaks upstream of the sensor are not causing false readings.
Check injector operation and resistance; perform injector balance/flow test if needed. Repair or replace leaking/clogged injectors.
Inspect wiring and grounds for fuel system sensors and PCM. Repair damaged wiring and connectors.
If no mechanical or sensor failures found, check for PCM software updates or salvage calibration issues before proceeding to component replacement.
After repairs, clear codes and perform verification road test under load to confirm P1171 does not return.

Likely causes

Low fuel pressure from a failing fuel pump or clogged fuel filter
Contaminated or failed MAF sensor under-reporting air flow
Intake vacuum leak at throttle body, intake manifold gasket, or vacuum lines
Faulty MAP or reference sensor causing incorrect fueling under load
High positive short-term/long-term fuel trim values indicating fuel delivery problem

Fault status

⚠️ Status

Fuel system lean detected during acceleration — ECU set P1171 indicating insufficient fuel or excess air under load.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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P1171 — Fuel System Lean During Acceleration