What Causes Positive/Negative LTFT Values?

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Causes of Positive LTFT Values:

Positive fuel trim is caused by ‘leaner’ combustion than the ECM or PCM predicted. Here are some common causes of positive long-term fuel trim:

  1. Air or vacuum leaks within the manifold, throttle body, or during a vacuum hose.
  2. A defective fuel pump not generating enough pressure or volume.
  3. Restricted fuel flow because of a pinched hose or plugged filter.
  4. Defective fuel pressure regulator unable to take care of enough fuel pressure.
  5. Air leaks within the PCV system.
  6. Malfunctioning MAF (Mass Airflow) sensor.
  7. Clogged or unresponsive fuel injectors.
  8. Ignition misfires are caused by the fouled plug, weak induction coil, or bad sparking plug wires.
  9. A damaged valve that permits unburned oxygen into the exhaust.
  10. Air moving into the manifold through a crack or gasket leak.
  11. Defective oxygen sensor or its wiring.

 

Causes of Negative LTFT Values:

Negative fuel trim is caused by ‘richer’ combustion than the ECM of PCM predicted. Here are some common causes of negative long-term fuel trim:

    1. Leaky or chattering fuel injector
    2. Defective fuel pressure regulator unable to take care of fuel pressure or restricted fuel return line.
    3. Restricted air intake system or clogged filter.
    4. Restricted exhaust thanks to the clogged converter, crushed pipage, or plugged muffler.
    5. Defective oxygen sensor or its wiring.

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