Trouble Codes Triggered
Q: I currently own a 1998 F-250 4x4 with a 5.4-liter engine and automatic transmission. Until recently, the truck had performed flawlessly. After diagnosing with a scan tool and replacing the two rear oxygen sensors, the truck ran like new. Fuel mileage and performance both increased. Then, while towing a vehicle up a long grade, the "Check Engine Light" came on. I scanned it again and came up with codes PO171, 174 (Bank 1, 2 lean). Now it's using fuel like crazy and I'm having a hard time finding the cause. The truck's equipped with a K&N filter and Flowmaster exhaust, both three years old.
A: Your F-250 uses four oxygen sensors in the exhaust system, two in front of the catalytic converters, and two behind them. This enables the system to test the converters' operation by measuring the amount of oxygen in the exhaust gasses before and after they pass through this key emissions-control device. The two trouble codes stored in the Powertrain Control Module's memory indicate a significant lean condition (too much air or not enough fuel) on both sides of the engine. The correct way to figure it out requires a practiced technician with a scan tool, who can analyze the system and take the appropriate diagnostic steps. But here are a few possibilities: (1) A clogged fuel filter, or any other cause of a loss of fuel pressure could set the two codes, but would often be accompanied by a loss of power. (2) A failure of the Mass Air Flow sensor (measures the amount of air entering the engine) can also simulate a lean condition by sending deceptive data to the PCM. (3) A significant vacuum leak, which would also promote excessive fuel consumption. Do a careful inspection of the intake manifold, paying special attention to the vacuum hose leading to the PCV valve. It may have deteriorated or fallen apart and is now allowing excessive air to enter the combustion process. If everything looks good, take it to a professional.