How do fuel trims work

Clearing fault codes has the effect of erasing stored fuel trim data as well, and the ECU generally requires at least a few drive cycles to relearn how to adapt fuel delivery strategies.

Note though that since sudden changes in the engine speed can cause short-term fuel trim values to fluctuate wildly, all fuel trim values should be taken at a minimum of at least three steady engine speeds, these being at idle, at about RPM, and at about RPM.

Note also that fuel trim values should only be taken when the engine had been running at the above speeds for at least 30 seconds. Note that if the long-term fuel trim values fluctuate in a pattern that resembles the fluctuations of the short-term fuel trim values suspect a defective catalytic converter.

Nonetheless, as a rule of thumb, long-term fuel trim values that hover around the 5 to 8 percent mark — either negative or positive- are not necessarily indicative of a problem.

If fuel trim values are interpreted correctly and their implications are understood, they offer a technician an almost foolproof way to diagnose problems and issues that may otherwise take hours to diagnose. In fact, fuel trim values offer insights into the overall condition of the engine and fuel system in ways that few other diagnostic methods can match, so use them to your advantage. Toggle navigation Navigation.

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- What are fuel trims?

When the engine is running lean too much air and not enough fuel , the O2 sensor generates a low voltage signal that tells the engine computer more fuel is needed. When the engine is running rich too much fuel and not enough air , the O2 sensor produces a higher voltage signal that tells the engine computer the engine is getting too much fuel and to cut back the fuel delivery. Accurate fuel trim values require an accurate feedback signal from the Oxygen sensor, otherwise the engine computer has no way of knowing whether the fuel mixture is running rich or lean.

When a cold engine is first started, it may take 10 to 30 seconds or more for the heaters inside the oxygen sensors to warms the sensors up to operating temperature. Until that point is reached and the fuel feedback control system goes into "closed loop", the fuel mixture is fixed at a predetermined value so no fuel trim adjustments are made. But once the Oxygen sensors are hot and the coolant temperature is high enough for the computer to go into closed loop, the computer starts to generate fuel trim values and make adjustments in the fuel mixture.

When the engine is shut off, the fuel trim values are retained in the computer's memory so the next time the vehicle is driven it can pick up where it left off. Erasing the computer's memory with a scan tool or by disconnecting the battery or the PCM power supply to clear codes also wipes the fuel trim values, which means the computer has to start learning the fuel adjustments all over again the next time the engine runs.

The fuel trim value is read by plugging a scan tool into the OBD II diagnostic connector located under the instrument panel on the drivers side near the steering column. When the key is turned on, the scan tool will initialize and start to communicate with the vehicle's onboard computer.

Depending on the tool and the vehicle, it may be necessary to enter the vehicle year, make, model and engine VIN code before the scan tool can read the data. The engine must be started and running to read the fuel trim information. Depending on the scan tool and how its menu options are set up, you choose the option that allows you to read system live data. On this list will be two fuel trim values for inline four and six cylinder engines, and four fuel trim values for V6 and V8 engines one pair for each cylinder bank.

This value changes rapidly and can bounce around quite a bit depending on engine load, speed, temperature and other operating conditions.

Values normally range from negative 10 percent to positive 10 percent, though the readings may jump as much as 25 percent or more in either direction. 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:. Skip to content. What is Fuel Trim?