P1171

Code

OLDSMOBILE P — Powertrain

Fuel System Lean During Acceleration

Brand: OLDSMOBILE

Type: P — Powertrain

Views: UK: 29 EN: 98 RU: 38

AI status

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Page language: EN

Causes

Vacuum leak or intake manifold gasket leak that opens under throttle
Low fuel pressure (weak pump, clogged filter, faulty regulator)
Restricted or leaking fuel injectors or incorrect injector pulse
Faulty or contaminated mass airflow (MAF) sensor
Faulty throttle position sensor (TPS) or incorrect accelerator pedal position signal
Faulty or slow oxygen sensor or heater circuit (incorrect feedback)

Symptoms

Hesitation or stumble on acceleration
Reduced power or sluggish throttle response
Surging or stumbling under load
Engine may feel lean or misfire under acceleration
Check Engine Light (CEL) with stored P1171 code
Possible poor fuel economy if long-term trim affected

What to check

Scan tool: read freeze frame and live data (fuel trims, O2 voltage, MAF, TPS, fuel pressure if available)
Visual inspection for vacuum leaks, cracked hoses, loose intake bolts and PCV fittings
Fuel pressure test at rail with engine cranking and under load
Inspect and test MAF sensor operation and wiring; check for contamination
Verify TPS voltage and sweep vs. throttle movement; check for deadspots
Check upstream O2 sensor voltage response and heater circuit

Signal parameters

Short-term fuel trim (STFT): large positive values (+10% or higher) during acceleration
Long-term fuel trim (LTFT): elevated positive values if condition present over time
Upstream O2 sensor voltage: low or slow to rise on acceleration (e.g.,
MAF flow (g/s) or frequency: lower than expected for throttle position/engine RPM
TPS voltage: smooth increasing voltage with throttle; no dropouts or dead zones
Fuel rail pressure: below manufacturer specification under load or dropping during acceleration

Diagnostic algorithm

Retrieve codes and freeze frame: confirm P1171 and record engine conditions (RPM, throttle, coolant temp, fuel trims) when code set.
Clear codes and perform monitored road test with a scan tool while repeating the acceleration that sets the code. Watch STFT/LTFT, O2 voltage, MAF, TPS, and fuel pressure.
Inspect intake system for obvious vacuum leaks: visually check hoses, intake boots, manifold gaskets, and PCV. Use smoke test if available to find hard-to-see leaks.
Test fuel pressure and volume: measure static and dynamic (running) fuel pressure at the rail and compare to spec. Check for pressure drop under acceleration/load.
Check MAF sensor: inspect for contamination or damage. Clean with appropriate MAF cleaner and re-test readings vs. expected airflow for RPM and throttle.
Verify TPS operation: measure voltage at closed, mid, and wide-open throttle for smooth linear change and compare to reference values. Replace or recalibrate if erratic.
Observe upstream O2 sensor: verify it switches and responds to rich/lean conditions. Check heater circuit resistance/power if sensor is slow to respond.
Inspect fuel injectors: check for proper spray pattern, leaks, and correct pulse width. Consider cleaning or flow-testing injectors if uneven.
Check for exhaust leaks upstream of O2 sensor which can cause false lean readings during acceleration.
Inspect wiring and connectors for PCM, MAF, TPS, fuel pressure sensor, and O2 sensor for corrosion, shorts, or intermittent connections.
If all systems check OK, consider controlled substitution/testing of suspect sensors (MAF, TPS, O2) or perform ECM/PCM reflash if a manufacturer service bulletin indicates software issue.
After repair, clear codes and verify correction by re-running the failed acceleration test and confirming normal sensor readings and that P1171 does not return.

Likely causes

Vacuum leak that increases with throttle opening (intake manifold gasket, vacuum hoses, PCV)
Insufficient fuel pressure or volume under load (weak pump or clogged filter)
MAF sensor under-reporting airflow (dirty or failed) causing low commanded fuel
TPS out of range or miscalibrated causing incorrect fueling strategy
Faulty upstream O2 sensor giving low-voltage (lean) reading during acceleration

Fault status

⚠️ Status

P1171 — Fuel system lean detected during acceleration. Check for vacuum leaks, low fuel pressure/volume, faulty MAF/TPS/O2 sensors, injector or wiring issues.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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P1171 — Fuel System Lean During Acceleration