2000 Ford Taurus 3.0L with 135K Miles - P0171 Code Appears Only in Cold Weather with Rough Idle
My 2000 Ford Taurus 3.0L has started exhibiting a rough idle and stalling issue when the engine is cold, particularly on mornings when outside temperatures are below 60°F. Once the engine warms up, the idle becomes smooth again. On three consecutive cold-start days with temps under 60°F, the vehicle set a P0171 diagnostic trouble code (indicating a lean condition in Bank 1). I've reviewed several posts about P0171 on this model, but none specifically mention rough idling only during cold starts or the symptom improvement after engine warm-up. The air filter is currently new and has been replaced. Given that the issue occurs exclusively under cold conditions, I'm looking for targeted troubleshooting steps—especially those related to temperature-sensitive components. While vacuum leaks are a common cause of P0171 on Fords, I'd like to know if there's a more specific starting point based on this unique symptom pattern. I've already checked the PCV hose (replaced it 2–3 years ago due to swelling), cleaned the MAF sensor (which is known to cause lean codes when dirty), and swapped out the fuel filter. None of these resolved the issue. Today, I tested accessible areas with throttle body cleaner and noticed a drop in RPM when spraying near the lower intake (front side). Since the code indicates Bank 1 is lean but I observed a potential problem on Bank 2 (which was harder to access), I'm now considering whether a leak at the lower intake gasket could be responsible. Does this point seem valid? Any other temperature-sensitive components worth checking?
A vacuum leak is a common cause of rough idle and P0171 in cold weather on Ford Taurus models. Many of the same root causes that trigger this code also lead to poor cold-start performance. If you haven't already, check for any loose or cracked hoses—especially around the intake manifold and throttle body. These issues often worsen when temperatures drop.