Code
P1171
GMC
P — Powertrain
Fuel System Lean During Acceleration
AI status
Completed
Completed
100%
Causes
- Intake vacuum leak (hose, intake manifold gasket, throttle body, PCV)
- Contaminated or failing MAF sensor or faulty MAP sensor
- Low fuel pressure (weak fuel pump, clogged fuel filter, faulty regulator)
- Partially clogged or leaking fuel injectors
- EVAP purge valve stuck open or leaking
- Faulty or slow oxygen sensor or sensor wiring
Symptoms
- Hesitation, stumble or reduced power during acceleration
- Poor throttle response or surging under load
- Engine may run rough or misfire under load
- Check Engine Light (MIL) illuminated, code P1171 stored
- Possible poor fuel economy or backfiring under heavy throttle
What to check
- Read freeze frame data and live PID values (STFT/LTFT, MAF, MAP, RPM, throttle position, fuel trim during acceleration)
- Visual inspection for cracked/vacuum hoses, intake leaks, loose clamps or damaged intake components
- Measure fuel rail pressure at idle and during commanded acceleration
- Check MAF sensor signal and clean with approved MAF cleaner if contaminated
- Perform smoke test of intake and EVAP system to find leaks
- Verify EVAP purge valve operation and vacuum lines to it
Signal parameters
- Short-term fuel trim (STFT) spike positive > +25% during acceleration is suspicious
- Long-term fuel trim (LTFT) > +20% indicates persistent lean condition
- MAF sensor: unexpected low airflow signal for given RPM/load or abnormal voltage (g/s or V depends on sensor)
- MAP sensor: readings not matching expected manifold pressure for throttle position and RPM
- Fuel rail pressure: typical port-injection range ~40–60 psi; sustained pressure below ~30–35 psi under load is problematic (specs vary by model)
- Upstream O2 voltage low (lean) during acceleration and not recovering as expected
Diagnostic algorithm
- Retrieve freeze frame and live data. Note conditions when P1171 set (RPM, throttle position, load, STFT/LTFT, fuel pressure).
- Visually inspect intake tract, vacuum hoses, PCV hoses, and intake manifold for obvious leaks or damage.
- Perform a smoke test of intake and EVAP system to locate leaks not visible by inspection.
- Check fuel rail pressure at idle and under commanded acceleration (use a gauge). Compare to OEM spec. If low, inspect fuel filter, pump, pressure regulator and fuel pressure sensor.
- Monitor MAF and MAP signals while performing a road or simulated load test. Clean MAF with proper cleaner if contaminated; replace if erratic or out-of-spec.
- Verify EVAP purge valve is not stuck open during acceleration (it should be closed or commanded appropriately).
- Test injectors for proper pulse and spray pattern; check injector resistance and wiring. Repair or replace leaking/clogged injectors.
- Check upstream O2 sensor response time and wiring. A slow/failed O2 sensor can cause incorrect trim corrections during transients.
- Inspect wiring, connectors, and grounds for sensors and the fuel delivery system. Repair any damage or corrosion.
- If all hardware checks OK, check for relevant technical service bulletins or PCM software updates. Clear codes and perform controlled road test to verify repair.
- Replace suspect components one at a time guided by test results; recheck and clear codes after each repair.
Likely causes
- Vacuum/air intake leak that becomes most evident under acceleration
- Low dynamic fuel rail pressure (fuel pump or filter problem)
- Contaminated/incorrect MAF signal (dirty or failing MAF)
- Faulty purge/EVAP system allowing extra air during acceleration
- Wiring or connector problem at MAP/MAF/O2 sensor
Fault status
Status
Fuel system lean detected during acceleration — PCM observed a lean condition (excess air or insufficient fuel) under transient load. Check fuel delivery, intake leaks, airflow sensors, and O2 sensor/wiring.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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