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. In this article, we will briefly discuss what fuel trims are, why they are needed, and how to use fuel trims as diagnostic aids, starting with this question-.
However, since all petrol engines require more or less fuel as the load on them changes, the balance between air and fuel must necessarily change to accommodate in changes in the demand for fuel.
Short-term fuel trims occur as a direct result of changes in the oxygen content of the exhaust stream. The exhaust stream is monitored by an oxygen sensor upstream of the catalytic converter, and the signal voltage it generates is directly proportional to the oxygen content of the exhaust stream.
It must be noted that on most applications, the signal voltage ranges from about 0. As a practical matter, a signal voltage of 0. Since the driver controls airflow load with the accelerator pedal, the ECU can only control fuel. This adjustment is called fuel trim and can be read using a scan tool.
The LTFT stored value in memory can be reset using a bi-directional scan tool or on some vehicles by removing the battery power for 15 minutes. They provide input on air entering the engine and the state of the exhaust gasses. These parameters will allow the ECU to adjust fuel delivery accordingly to ensure that the ratio of fuel and air is perfect and the ignition timing is perfect for maximum power.
To retrieve STFT information, the engine must be operating in closed-loop mode, which means running it for a few minutes until the oxygen sensor reaches normal operating temperature. It is usually best to monitor the fuel trim under a few different load conditions. 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. The DTCs directly related to fuel trim problems are:. Here are some common causes of positive long-term fuel trim:. Here are some common causes of negative long-term fuel trim:. The long-term fuel trims LTFT are driven by the short-term fuel trims. A negative fuel trim percentage indicates a taking away of fuel while a positive percentage indicates an adding of fuel.
A normal long-term fuel trim reading will appear to stay the same, giving a long-term average of fuel added. It, too, should be close to zero, positive, or negative single digits under normal circumstances.
It will fluctuate much slower, possibly appearing static. So, when you shut your car off, it can reference these LTFTs the next time it starts up. When the car is shut down, all data is lost for STFT and learned again on the next start-up.
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