Initial Findings and Diagnostic Steps for P3191
Freeze frame data for P3191 revealed a crankshaft speed of 500 RPM (lower than normal), a mass airflow (MAF) reading of approximately 3 grams per second (likely acceptable given the low RPM), and an air-fuel sensor (AFS) voltage of 2.59V, indicating a rich fuel mixture. The fuel gauge displayed half full, and the customer confirmed no recent refueling.
After clearing the codes, starting attempts resulted in occasional engine “kicks,” accompanied by a drop in RPM, raising suspicion about potential timing issues.
Investigating Potential Causes of P3191
Several key areas were examined:
- Cam Timing: Oscilloscope analysis revealed proper cam timing.
- Ignition Timing: Inspection of the ignition trigger (IGT) and ignition feedback (IGF) signals showed acceptable patterns, albeit with some unusual spikes on the falling edge of the IGT signal every 120 degrees.
- Coil Ground: The coil ground connection was confirmed to be sound.
- Spark Plugs: A spark plug examination revealed a dark color and strong fuel smell, indicative of a rich condition. The vehicle had recently received new Denso coils approximately 50,000 miles prior.
- Compression: A compression test on cylinder #6 yielded 150 PSI, within an acceptable range.
- Fuel System: Fuel pressure measured at 45 PSI, considered normal. The fuel itself smelled normal, with no indication of diesel contamination.
- Throttle Body: While initial oscilloscope readings of the throttle position sensor (TPS) showed intermittent unresponsiveness, a subsequent active test using Techstream confirmed consistent throttle valve movement.
Live Data Analysis and Further Investigation of P3191
Live data recording during cranking showed both air-fuel sensors reading full rich (B1 and B2), accompanied by negative fuel trims. This indicated the engine control module (ECM) was attempting to lean out the mixture, further confirming the excessively rich condition. Injector pulse width on injector 2 was measured at 4.6 milliseconds, decreasing during cranking.
Despite confirming adequate compression, seemingly correct timing, spark, and an abundance of fuel, the engine refused to start.
Seeking Additional Insights into P3191
The diagnostic process ruled out many common causes of a no-start condition. Further investigation is required, potentially focusing on less common issues such as intermittent wiring problems, ECM malfunctions, or perhaps a more in-depth analysis of the unusual IGT signal spikes. Waveform examples of IGT #1 and the crankshaft position sensor (CKP) could provide valuable comparative data.
