Possible Causes

Fuel control system uses fuel trim tables stored in PCM to compensate for variances in fuel system components from wear and aging. The fuel trim strategy learns deviations during closed-loop engine operation. Fuel trim has 2 means of adapting; a long term fuel trim and a short term fuel trim. Inputs from ECT, IAT and MAF sensors are required to activate fuel trim adjustments. The HO2S detects presence of oxygen in exhaust and provides PCM with feedback identifying air/fuel ratio. A fuel trim factor is added to fuel injector pulse width calculation according to long and short term fuel trims. Long term fuel trim values at their rich or lean limits indicate a fuel system malfunction. Possible causes are:

• Low fuel pressure or running out of fuel.
• Excessive fuel pressure.
• Leaking fuel injector(s).
• Leaking fuel pressure regulator.
• Vapor recovery system.
• Contaminated fuel injector(s).
• Air leaks after MAF sensor.
• Vacuum leaks.
• Restricted air intake.
• PCV system.
• Improperly seated engine oil dipstick.
• Leaking EGR gasket.
• Stuck EGR valve.
• Leaking EGR diaphragm or EGR vacuum regulator.
• Engine oil overfill.
• Engine cam timing.
• Cylinder compression.
• Exhaust leaks before or near HO2S(s).