Engine Mixture Adaptations

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Engine Adaptation Values

Additive mixture adaptations: This adaptation is referred to as “short term fuel trim”. This value is measured in milliseconds and can occur in positive or negative values. These adaptations are made to the engine while at idle.

Excessively positive Additive values indicate a lean condition. Some common causes are:

  • Unmetered air leaks, torn intake boot, leaking intake manifold gaskets
  • Crankcase vent system fault, system valve stuck open
  • Low fuel pressure, pressure regulator malfunction
  • MAF malfunction, sending incorrect signal information

Excessively negative Additive values indicate a rich condition. Some common causes are:

  • A restriction at engine intake, such as clogged air filter blocking air intake into engine
  • Crankcase vent valve stuck closed or blocked
  • High fuel pressure, caused by pressure regulator or restricted fuel return line
  • MAF malfunction, sending incorrect signal information to ECM

Multiplicative mixture adaptation: This adaptation is referred to as “long term fuel trim”. This value is measured in percentages and can occur in the positive or negative values. This adaptation occurs during part-load driving conditions.

Excessively positive multiplicative adaptation values indicate lean condition. Some common causes are:

  • MAF malfunction, incorrect signal information sent to the ECM
  • Low fuel pressure, faulty fuel pump
  • Unmetered air leak, such as torn intake boot, etc
  • Pressure regulator malfunction

Excessively negative multiplicative adaptation values indicate rich condition. Some common causes are:

  • Sensor failure that could result in erroneous information to falsely enrich mixture, such as engine coolant temperature sensor or intake air temperature sensor
  • High fuel pressure, restriction in return line or fault with fuel pressure regulator
  • MAF malfunction, incorrect signal information being sent to the ECM
  • Restriction at air intake blocking airflow into engine, such as a clogged air filter

Hint: To determine if the MAF is sending incorrect information to the ECM, check the additive and multiplicative mixture adaptations. When the mixture adaptations are contradicting each other, this indicates that the MAF is faulty and sending incorrect signal information.

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