Katerina Swanson
Fuel trim is the adjustment the engine computer makes to the fuel mixture to maintain a balanced air/fuel ratio. It is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT). The engine computer uses fuel trims to maintain a stoichiometric balance between fuel and air on petrol engines.
| Characteristics | Values |
|---|---|
| Fuel trim is the adjustment the engine computer makes to the fuel mixture to maintain a balanced air/fuel ratio | 14.7 parts of air to 1 part of fuel |
| Fuel trims are adjusted according to the data provided by an oxygen sensor, located in the exhaust manifold | 14.7 pounds per square inch |
| Short-term fuel trims refer to the immediate adjustments made by the PCM to the fuel delivery system based on immediate input from the upstream O2 sensors | 14.7 parts of air to one part of fuel |
| Long fuel trim responds when short fuel trim drifts too far from zero | 14.7 pounds per square inch |
| Long-term fuel trim (LTFT) and short-term fuel trim (STFT) are reported in two different forms | 14.7 parts of air to 1 part of fuel |
What You'll Learn
Air/fuel ratio is balanced by adjusting the fuel mixture
The difference between the calculated and the actual quantity of fuel being delivered to the cylinders is what is called “fuel trim”. Fuel trim is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT). The concept of fuel trims, and how an ECU uses fuel trims to maintain a stoichiometric balance between fuel and air on petrol engines is probably among the least understood aspects of modern engine and fuel management systems today. However, the process of trimming the air/fuel mixture to maximise power and at the same time, save fuel and minimise emissions is relatively straightforward, provided the subject is approached in a logical manner.
Atmosphere at sea level is 14.7 pounds per square inch. It just so happens that the cleanest burn at sea level for the air fuel mix by weight is 14.7 parts of air to one part of fuel. When a petrol engine runs with a stoichiometric air/fuel mixture (14.7 parts of air to 1 part of fuel), all of the fuel is combusted using all of the available air. Engines with two banks of cylinders (e.g. H4, H6, V6, V8) have one oxygen sensor per cylinder bank, so each bank’s fuel trims are adjusted according to the data from the oxygen sensor located in its exhaust manifold.
Fuel trims are adjusted according to the data provided by an oxygen sensor, located in the exhaust manifold. A restrictive air filter may not set a code, but the restriction and compromised fuel trim will hurt the power and efficiency. Replacing the filter and resetting the fuel trims may return the system to normal. The keyword where fuel trim is concerned is stoichiometry, which is a branch of chemistry dealing with relationships (in this case) between atomized fuel and atmosphere in the combustion chambers. That’s the optimum balance of fuel and air by weight for the absolute cleanest burn.
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Stoichiometry is the optimum balance of fuel and air
Fuel trims are adjusted according to the data provided by an oxygen sensor, located in the exhaust manifold. Engines with two banks of cylinders (e.g. H4, H6, V6, V8) have one oxygen sensor per cylinder bank, so each bank’s fuel trims are adjusted according to the data from the oxygen sensor located in its exhaust manifold.
The difference between the calculated and the actual quantity of fuel being delivered to the cylinders is what is called “fuel trim”. Fuel trim is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT).
Short-term fuel trims refer to the immediate adjustments made by the PCM to the fuel delivery system based on immediate input from the upstream O2 sensors. Long fuel trim responds when short fuel trim drifts too far from zero.
A restrictive air filter may not set a code, but the restriction and compromised fuel trim will hurt the power and efficiency. Replacing the filter and resetting the fuel trims may return the system to normal.
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Oxygen sensors provide data for adjustments
Fuel trims are adjustments made by the engine computer to the fuel mixture to maintain a balanced air/fuel ratio. The difference between the calculated and the actual quantity of fuel being delivered to the cylinders is what is called “fuel trim”.
The process of trimming the air/fuel mixture to maximise power and at the same time, save fuel and minimise emissions is relatively straightforward, provided the subject is approached in a logical manner. Atmosphere at sea level is 14.7 pounds per square inch. It just so happens that the cleanest burn at sea level for the air fuel mix by weight is 14.7 parts of air to one part of fuel.
Fuel trims are adjusted according to the data provided by an oxygen sensor, located in the exhaust manifold. Engines with two banks of cylinders (e.g. H4, H6, V6, V8) have one oxygen sensor per cylinder bank, so each bank’s fuel trims are adjusted according to the data from the oxygen sensor located in its exhaust manifold.
Short-term fuel trims refer to the immediate adjustments made by the PCM to the fuel delivery system based on immediate input from the upstream O2 sensors. Long fuel trim responds when short fuel trim drifts too far from zero.
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Long-term and short-term fuel trims are reported separately
Fuel trim is the adjustment the engine computer makes to the fuel mixture to maintain a balanced air/fuel ratio. It is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT).
Short-term fuel trims refer to the immediate adjustments made by the PCM to the fuel delivery system based on immediate input from the upstream O2 sensors. Long fuel trim responds when short fuel trim drifts too far from zero.
Fuel trims are adjusted according to the data provided by an oxygen sensor, located in the exhaust manifold. Engines with two banks of cylinders (e.g. H4, H6, V6, V8) have one oxygen sensor per cylinder bank, so each bank’s fuel trims are adjusted according to the data from the oxygen sensor located in its exhaust manifold.
The concept of fuel trims, and how an ECU uses fuel trims to maintain a stoichiometric balance between fuel and air on petrol engines is probably among the least understood aspects of modern engine and fuel management systems today. However, the process of trimming the air/fuel mixture to maximise power and at the same time, save fuel and minimise emissions is relatively straightforward, provided the subject is approached in a logical manner.
Atmosphere at sea level is 14.7 pounds per square inch. It just so happens that the cleanest burn at sea level for the air fuel mix by weight is 14.7 parts of air to one part of fuel. The keyword where fuel trim is concerned is stoichiometry, which is a branch of chemistry dealing with relationships (in this case) between atomized fuel and atmosphere in the combustion chambers.
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Air filter replacement may reset the system
Fuel trim is the adjustment the engine computer makes to the fuel mixture to maintain a balanced air/fuel ratio. It is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT).
A restrictive air filter may not set a code, but the restriction and compromised fuel trim will hurt the power and efficiency. Replacing the filter and resetting the fuel trims may return the system to normal.
Short-term fuel trims refer to the immediate adjustments made by the PCM to the fuel delivery system based on immediate input from the upstream O2 sensors. Long fuel trim responds when short fuel trim drifts too far from zero.
Atmosphere at sea level is 14.7 pounds per square inch. It just so happens that the cleanest burn at sea level for the air fuel mix by weight is 14.7 parts of air to one part of fuel.
Engines with two banks of cylinders (e.g. H4, H6, V6, V8) have one oxygen sensor per cylinder bank, so each bank’s fuel trims are adjusted according to the data from the oxygen sensor located in its exhaust manifold.
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Frequently asked questions
Fuel trim is the adjustment the engine computer makes to the fuel mixture to maintain a balanced air/fuel ratio.
Fuel trim is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT).
The purpose of fuel trim is to maximise power and at the same time, save fuel and minimise emissions.
