Running lean because fuel trims are +15 or higher, or actually running Lean because engine is pinging/knocking?
If it is just fuel trims or codes then engine is running OK but MAF sensor air flow data is sending computer a lower air flow than the intake is actually getting.
Computer calculates the 14:1 ratio of fuel needed based on MAF air flow, if air flow data is wrong then O2 sensors see that as too much oxygen in exhaust so computer adds more fuel, +5, +10, +15, ect...fuel trim and then computer sets Lean code because original calculation is Lean, not the actual engine operation.
The engine is not running Lean because computer is adding the extra fuel needed, but computer is warning the driver of a problem of incorrect data.
Any "extra" air entering the intake from MAF sensor to intake valve ports will cause this, commonly called a vacuum leak, although it can be on MAF sensor air plenum.
MAF sensor could just be dirty, it should be cleaned every now and then.
Low fuel pressure can also cause this, '99 engine computer would expect 70psi fuel pressure, it can't monitor pressure, it bases its fuel calculations on 70psi, so opens fuel injectors for a specific time(pulse width) knowing X amount of fuel will be released if there is 70psi of pressure at the injector.
If pressure is lower then less fuel goes in and O2 sensor reports that, so +5, +10, +15, ect.....fuel trims and Lean code set.
O2 sensors are the final say as far as the computer is concerned, so it trims the fuel accordingly, on a V engine there would be two O2 sensor used by the computer for fuel trims, one on each bank, there can be other O2 sensors by the Cat converters but these are not used for short term fuel trim.
If you only have a Lean(or Rich) fuel trim on one bank then it could be a faulty O2 sensor, but could also be a misfiring cylinder on that bank(that shows as lean).
If both banks/O2s are showing similar fuel trims then it won't be an O2 sensor issue
Last edited by RonD; 03-22-2015 at 11:30 AM.