High fuel trims on your Honda? This article explores common causes and diagnostic steps using an Autel scan tool, focusing on a 2008 Honda Odyssey and 2009 Pilot case study.
A 2008 Honda Odyssey with 168,000 miles, scanned with an Autel DS708, exhibited consistently high fuel trims (+10 short term, +10 long term) at both idle and 3000 RPM. This indicates a lean condition, where the engine is receiving insufficient fuel. Despite replacing upstream A/F ratio sensors and cleaning the MAF sensor, the issue returned after a few hundred miles. The MAF sensor readings (10 g/s at 2500 RPM) align with the service manual specifications (9-12 g/s). Vacuum leaks were suspected but ruled out after testing the PCV, brake booster, and purge valve lines. A slight momentary improvement was observed when pinching the purge valve, which operates on a duty cycle and was observed to be open at approximately 22%.
An EGR test performed with the Autel tool completed successfully. Both commanded and actual EGR positions read 0. The EGR valve was replaced under warranty at 17,000 miles. Valves were adjusted at 140,000 miles. The rear O2 sensors read a steady 640mV at idle. While Hondas with upstream A/F sensors utilize downstream sensors for fuel trim adjustments, the extent of their influence is unclear.
A similar case involves a 2009 Honda Pilot with 167,000 miles, presenting lean codes (P0171, P0174) and high fuel trims (over 20% long term, 7-8% short term). This vehicle recently underwent a timing belt replacement and valve adjustment.
Several potential causes for these lean conditions are being considered. Could the rear O2 sensors be contributing to the issue despite their seemingly normal readings? Given the mileage, replacing them might be a viable option.
Further diagnostic steps might include:
- Fuel Pressure Test: Verify adequate fuel pressure and pump performance. Low fuel pressure can cause lean conditions.
- Intake Manifold Inspection: Thoroughly inspect the intake manifold for cracks or leaks, especially around gaskets and seals. Even small leaks can disrupt fuel trim.
- Smoke Test: A smoke test can pinpoint even the smallest vacuum leaks that might be missed during visual inspection.
- Exhaust System Inspection: Check for exhaust leaks before the rear O2 sensors. Leaks can introduce fresh air and skew sensor readings.
Resolving persistent lean conditions requires systematic diagnosis. Using an Autel scan tool, coupled with careful inspection and testing, will help pinpoint the root cause and ensure proper engine performance and fuel efficiency. Further investigation is needed to determine the exact cause of the lean conditions in both the Odyssey and Pilot.
