If you’re experiencing a lean fuel mixture (too much air), this could be caused by a large number of problems. Here are just a few to help you start investigating: air or vacuum leaks, a weak fuel pump, a pinched fuel line, a faulty fuel pressure regulator, a dirty mass airflow sensor, filthy fuel injectors, ignition misfire, compression leaks, a crack in the exhaust manifold or a leak in the gasket, or a bad oxygen sensor.