1996 Mercury Grand Marquis mileage: 42,000. Every several hundred miles or so (between code clearing), the check engine light will come on. The codes are 0171 and 0174 indicating a lean condition. Gas mileage is also down about 10%. I have replaced the fuel filter, fuel pressure regulator (fuel pressure is 31 psi with vacuum and 42 psi without) and EGR valve. I have also tuned the car replacing plugs, plug wires and air filter. Any ideas as to what is triggering the codes?