Experiencing issues with your Nissan 350z can be frustrating, especially when it affects performance and diagnostic capabilities. One common hurdle for 350z owners involves problems with the 350z Obd2 port, which is essential for reading diagnostic trouble codes and monitoring engine performance. This article delves into a specific case of troubleshooting a 350z where an initial OBD2 port power issue led to the discovery of a persistent throttle cut problem.
Diagnosing and Fixing the OBD2 Port Power Loss: Fuse #34
The journey began with the inability to communicate with the car’s computer via the OBD2 port. A crucial first step in diagnosing any OBD2 issue is to check the power supply to the port itself. In the Nissan 350z, fuse number 34, located within the relay box in front of the battery, is responsible for providing power to the OBD2 port.
Upon inspection, fuse #34 appeared visually intact. However, appearances can be deceiving. Using a multimeter to test for continuity revealed a surprisingly high resistance within the fuse, indicating it was indeed blown despite looking normal. This highlights the importance of using a multimeter for accurate fuse testing rather than relying solely on visual inspection.
Replacing the faulty fuse #34 immediately restored power to pin 16 of the 350z OBD2 port. This allowed connection with an OBD2 scanner, enabling the technician to proceed with further diagnostics. With OBD2 access restored, the ECU’s learning parameters were reset, and NATS (Nissan Anti-Theft System) codes were cleared. While this resolved the immediate OBD2 communication problem, it unveiled another underlying issue: a persistent throttle cut.
Investigating the Persistent Throttle Cut
Even after fixing the OBD2 power issue and resetting the ECU, a noticeable throttle cut remained. The engine would rev up smoothly at low throttle inputs, but applying anything beyond 50% throttle resulted in the engine bogging down and feeling like it was experiencing a severe throttle cut. This symptom pointed towards a potential sensor malfunction or an issue within the engine management system still hindering performance despite the OBD2 port being functional.
To further investigate this throttle cut, data logging was employed to capture real-time sensor readings during the problematic acceleration. Key parameters logged included RPM (Revolutions Per Minute), AFM (Air Flow Meter) Volts, Accelerator Pedal Volts, and TPS (Throttle Position Sensor) readings.
Analyzing the Data Logs
The data logs provided valuable insights into the engine’s behavior during the throttle cut. Examining the TPS readings revealed that the sensor was indeed registering full throttle input (4V+). However, coinciding with the full throttle input, the RPM readings showed a significant drop, followed by a slow recovery to around 1200 RPM when the throttle was released. This RPM drop in response to full throttle strongly suggested that the engine was not receiving the correct air-fuel mixture under higher load conditions.
Time, RPM, Air Flow V, Accel P1, TPS 1
000.078, 962, 1.38, 0.78, 0.66
000.156, 950, 1.38, 0.78, 0.66
000.203, 950, 1.38, 0.78, 0.66
000.265, 950, 1.38, 0.78, 0.66
000.328, 962, 1.38, 0.78, 0.66
000.406, 962, 1.38, 0.78, 0.66
000.468, 962, 1.38, 0.78, 0.66
000.531, 962, 1.38, 0.78, 0.66
000.593, 962, 1.38, 0.78, 0.66
000.687, 950, 1.39, 0.78, 0.66
000.750, 950, 1.39, 0.79, 0.66
000.828, 950, 1.38, 0.78, 0.66
000.875, 950, 1.38, 0.78, 0.66
000.953, 950, 1.38, 0.79, 0.66
001.062, 950, 1.4, 1.84, 0.67
001.187, 862, 2.92, 4.6, 4.13
001.265, 775, 2.35, 4.6, 4.13
001.343, 700, 1.94, 4.6, 4.12
001.421, 775, 2.62, 4.6, 4.12
001.500, 738, 2.98, 4.6, 4.12
001.593, 650, 2.18, 4.6, 4.12
001.687, 750, 2.27, 4.6, 4.13
001.750, 688, 2.3, 4.6, 4.12
001.828, 638, 2.41, 4.6, 4.13
001.890, 600, 1.82, 4.6, 4.12
001.968, 538, 1.79, 4.6, 4.12
002.062, 675, 2.56, 4.6, 4.12
002.140, 650, 2.46, 4.6, 4.12
002.250, 700, 2.68, 4.6, 4.12
002.328, 638, 1.91, 4.6, 4.12
002.390, 662, 1.95, 4.6, 4.13
002.484, 662, 2.46, 4.6, 4.12
002.562, 588, 1.8, 4.6, 4.12
002.640, 638, 2.34, 4.6, 4.13
002.703, 700, 2.67, 4.6, 4.13
002.765, 662, 2.09, 4.6, 4.13
002.875, 725, 1.94, 4.6, 4.12
002.984, 700, 2.52, 3.52, 4.06
003.078, 875, 1.62, 0.78, 0.88
003.171, 1125, 1.4, 0.78, 0.69
003.265, 1212, 1.48, 0.78, 0.69
003.359, 1188, 1.5, 0.78, 0.69
003.437, 1200, 1.52, 0.78, 0.69
003.546, 1262, 1.51, 0.78, 0.68
003.609, 1250, 1.52, 0.78, 0.68
003.703, 1262, 1.48, 0.78, 0.68
003.765, 1262, 1.5, 0.78, 0.68
003.859, 1275, 1.5, 0.78, 0.68Suspecting the MAF Sensor: Potential Cause
Recalling a previous diagnostic code “P0113 – Intake Air Temperature Circuit High Input,” and considering the symptoms, the Mass Air Flow (MAF) sensor became a prime suspect. A faulty MAF sensor can lead to inaccurate airflow readings, causing the engine to run lean, especially at wide open throttle.
As highlighted in the original observation:
At wide open throttle, the fuel system goes into open-loop to provide maximum power. With a dirty MAF, the engine will run lean because of the under-reported airflow. Fuel trim will become more extremely positive but no code will set because of the (programmed) open-loop condition. Still, the engine will run so lean that it might not accelerate beyond a certain rpm, and this often leads to random misfire.
This perfectly describes the observed throttle cut and lack of Check Engine Light (CEL) or any stored diagnostic codes. The engine is likely running lean under high throttle demand due to the MAF sensor underreporting airflow, but not severely enough in open-loop to trigger a specific fault code.
Further Diagnostic Steps
To confirm the MAF sensor as the culprit, further diagnostic steps should include:
- Checking Fuel Trims: Analyzing short-term and long-term fuel trim data via the 350z OBD2 port can reveal if the ECU is compensating for an inaccurate airflow reading by adding excessive fuel. Positive fuel trim values would support the lean running theory.
- Monitoring Oxygen Sensor Readings: Observing the oxygen sensor readings during full-throttle acceleration can indicate a lean air-fuel ratio if the sensor reports lean conditions when it should ideally be rich under full load.
By focusing on the 350z OBD2 system and systematically diagnosing the power supply and sensor readings, it becomes possible to pinpoint complex issues like this throttle cut. In this case, a faulty MAF sensor appears to be the likely cause of the persistent performance problem, even after resolving the initial OBD2 port power issue. Addressing the MAF sensor will likely restore the 350z to its optimal performance.
