Sensor Causes Mitsubishi Low Power

Posted 7/8/2010
By Tom Vadnais

I recently had a call on a Mitsubishi Montero with codes P0171 and P0174 stored in memory and a low power complaint, indicating a lean condition on both banks.

Testing and Diagnostics:

Step 1: We checked the O2 sensor activity and the fuel trim numbers. The O2 sensors were switching, but the long-term fuel trims on both banks were positive at 12.5 percent, indicating that the powertrain control module (PCM) was adding the maximum amount of fuel as programmed. Confirming this, we could see that the injector "on time" was increasing above 3 milliseconds.

Step 2: The technician checked the volume air flow sensor input to the PCM. It was reading slightly high, at 40 hertz, which (if anything) should cause the vehicle to run rich.

Step 3: The barometric pressure sensor and temperature sensor inputs were checked; they were normal.

Step 4: The fuel pressure was tested; it read normal at 48 psi, with the vacuum hose off of the pressure regulator.

Step 5: The intake system was smoke tested for vacuum leaks; none were found.

Step 6: I suggested disconnecting a vacuum line and adding some additional fuel while monitoring the O2 sensors and long-term fuel trim. The vehicle reacted positively, the O2 sensor voltage went up, and the long-term fuel trims came down. The vehicle had a noticeable increase in power when adding the fuel.

Step 7: At this point, the tech attempted cleaning the fuel injectors. There was an increase in performance. But after clearing the fault codes and the fuel trims and test driving the vehicle, the same two codes reset, and the fuel trims once again maxed out at 12.5 percent.

Step 8: There was one sensor left to test, the manifold differential pressure (MDP) sensor. Normally, this sensor is used to monitor the exhaust gas recirculation (EGR) valve operation, but it is an input to the PCM that could cause a lean condition based on the negative pressure in the intake manifold (vacuum). The normal reading at "key on/engine off" would be in the range of 4 volts. In this case, the voltage was reading under 3 volts at "key on/engine off."

Step 9: I asked the tech to disconnect the sensor and jump the signal wire to the 5-volt reference wire and the voltage on the signal wire went up to 5 volts.

Step 10: I also asked him to check the vacuum hose and the vacuum port on the intake; everything appeared clear.

The fix: The technician replaced the sensor, and the fuel trims went back to normal.

Tom Vadnais is an Identifix Chrysler specialist. He is an ASE master with L1 and A/C recovery and recycling certifications.

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