Hi I have a 2008 Mitsubishi Lancer 2.0 naturally aspirated with 160,234 miles and it is running lean with long term between +9.2 to +12.5. I have checked and repaired known vacuum leak which was the intake boot and everything else at the intake system seems fine. I also have repaired known exhaust leaks by the muffler and after the second cat and oxygen sensors. The car still runs lean even after fuel trim resets. I hooked up a vacuum gauge and only during a cold start if car hasn’t been started in a few hours or a day the vacuum readings go around 10 to 15 in/hg while the engine runs rough and engine seems to cut off a bit but then after a few seconds the engine runs fine and vacuum stays around 16 to 17 in/hg until engine reaches operating temp. The vacuum gauge I’m using shows that reading is late ignition timing. At operating temp the vacuum stays to 17 to 19 in/hg. I also hooked up my scan tool and found out during the cold start ignition timing goes down to 6.0 degrees until engine smoothed out and comes around 20 degrees. If someone can help explain what’s going on here it will be much appreciated.
When running lean main causes are vacuum leak, faulty or failing fuel pump or injectors, clogged fuel filter, worn or damaged manifold gasket or throttle body gasket. My advice to you is double check all vacuum lines, check the manifold gasket and throttle body gasket, check the intake boot after the MAf. No cracks, holes and properly connected. Pressure test fuel system, replace fuel filter and run cleaner. @doc do what he said and run cleaner through the system. If your going to use techron run it at double strength. Some car manufacturers recommend that. If none of that works it's get really complicated. Especially if there's no codes.
Dirty fuel injectors, dirty fuel filter. Run some Techron through it and give it the Italian Tune Up. It might help. Without codes, it's kind of hard to diagnose.
Any codes?
No codes present