P0171 Lean Mix Error in 1997 Chevrolet K3500 Vortec 454: Diagnosis and Solutions
I own a 1997 Chevrolet K3500 dually with a Vortec 454 engine that has been intermittently displaying P0171 (System Too Lean - Left Bank) for several months. The vehicle previously had a small coolant leak that rusted the left bank exhaust manifold (cylinder 1), which was repaired. I currently use a K&N air filter and have cleaned the Mass Air Flow (MAF) sensor with brake cleaner, though I now suspect this may have caused damage. I also replaced the oxygen sensor closest to the exhaust manifold. The check engine light comes on intermittently—often right after refueling—and then goes off for periods of time. When towing, especially on steep grades or during heavy loads, a significant loss of power is noticeable. However, under normal driving conditions without towing, performance remains stable and fuel economy stays consistent. I have replaced the gas cap and regularly change the fuel filter, but I haven't yet tested fuel pressure or volume. I am hesitant to install a new MAF sensor due to cost, as it would likely not explain an intermittent issue if that were the root cause. The engine runs well most of the time, so I don’t believe plug wires are faulty. Given the symptoms—especially the sudden power loss and recovery during towing—I suspect deeper issues such as fuel starvation or vacuum leaks. Any insights into potential causes for this P0171 code with intermittent behavior? It's becoming increasingly difficult to diagnose without proper testing.
Brake cleaner can damage MAF sensors—use electrical contact cleaner instead. Fuel pressure and volume readings are essential for diagnosis. In my experience with Vortec engines, intermittent low power is often caused by a failing fuel pump.