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:
- Air or vacuum leaks within the manifold, throttle body, or during a vacuum hose.
- A defective fuel pump not generating enough pressure or volume.
- Restricted fuel flow because of a pinched hose or plugged filter.
- Defective fuel pressure regulator unable to take care of enough fuel pressure.
- Air leaks within the PCV system.
- Malfunctioning MAF (Mass Airflow) sensor.
- Clogged or unresponsive fuel injectors.
- Ignition misfires are caused by the fouled plug, weak induction coil, or bad sparking plug wires.
- A damaged valve that permits unburned oxygen into the exhaust.
- Air moving into the manifold through a crack or gasket leak.
- 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:
-
- Leaky or chattering fuel injector
- Defective fuel pressure regulator unable to take care of fuel pressure or restricted fuel return line.
- Restricted air intake system or clogged filter.
- Restricted exhaust thanks to the clogged converter, crushed pipage, or plugged muffler.
- Defective oxygen sensor or its wiring.
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