I am trying to diagnose a misfire/bogging/backfire problem with my 03 Legacy GT 2.5 with 125,000 miles. It runs better when cold but not perfect but when it gets warmed up the condition gets real bad. It has trouble climbing hills and accelerating, it backfires, pops, shakes and runs horrible.
Plugs are only 10,000 miles old and were gapped to spec, wires look to be in good condition, fuel filter is 3000 miles old. valves were adjusted 10,000 miles ago along with new head gaskets, timing belt and all that good stuff. If I run 93 octane the condition gets better but is not gone completely. My scan tool shows the following on a long drive:
Front oxygen sensor is reading 2-5 volts, fluctuating but averaging about 4v
Had a flashing check engine light once, shut car off and restarted and it never came back. Showed misfire on cyl3. After that No codes ever returned. If i feather the throttle VERY lightly it will drive ok but anymore than 5-10% it runs bad. I tried unplugging the front o2 sensor and running it and it had no change. There is no exhaust leaks, no vacuum leaks. Stumped and do not want to start throwing unnecessary parts at it unless I run out of options.
According to what I have read, it is running rich or the computer thinks it is and it could be, coil pack, front o2 sensor, stuck open injector, bad wires, bad fuel pressure regulator. The fact that it runs a little better cold makes me think o2 sensor or coil. Any thoughts???