I have a 99 ford ranger 2.5, it's throwing the code p0171, (system too lean on bank 1.) I've tried everything I can think of including, Tried Replacing PVC vavle, IAC, o2 sensor, all gaskets for the air intake manifold, mass airflow sensor, throttle body gaskets, fuel injectors, fuel filter, air filter, temp sensor, thermostat, both up and down stream o2sensor, checked all vacuume lines, and egr vavle, egr tube, transmission, starter, all electronical relays, and none have gotten rid of the problem, any ideas on what to do?