03 v6 Running Lean after Load


New Member
Feb 4, 2022
Hey all!
My mustang (2003 3.8 automatic) drives fine for about an hour after cold start, but then it begins to run lean. It starts off as a minor kick, and progresses to missing on one cylinder, and eventually it will die. Stopping the engine and letting it sit for about 5 minutes will allow it to drive about 30 more minutes.
The only code is P1131, which is lean on bank one.
I have done the following to attempt to fix this: new fuel filter, made sure emissions is all hooked up, higher octane gas with cleaner, new plugs and plug wires (not for this problem, but they have less than 2,000 miles on them), new coil pack, and visual inspection of fuel lines/injectors. I know not all these seem like fixes to a lean problem, for a while I figured it was the ignition system.
What do yall think I should do next? I'm going to get an air filter, because I haven't put a new one on for about a year (20,000 miles), but I can't imagine that causing this issue on its own.