Fuel trim is technology used to keep the fuelling control operating correctly over vehicle lifetime, in order to comply with emissions regulations. Trim values are accessible via scan tools and this knowledge can help your diagnostic procedures.
What does it do?
Short-term fuel trim (STFT) and long-term fuel trim (LTFT) are expressed as a percentage, Positive fuel trim percentages indicate that the ECU is attempting to richen the fuel mixture, to compensate for a lean condition. Negative fuel trim percentages indicate the opposite. Fuel trim values are generated and stored with respect to engine operating condition – generally speed or load.
As vehicles operate over their lifetime, and accumulate mileage. The engine system components accumulate operating hours, and thus experience wear and tear. With respect to the fuelling control system – air flow meters can become dirty and contaminated, and this has an effect on their initial calibration and response characteristic. Oxygen sensors can also become contaminated and this impacts on their accuracy and reliability of the signal. The fuel injection wet system also suffers from long term fatigue effects – injectors can clog, thus affecting their spray pattern, and ability to provide a homogenous charge for combustion. Fuel pressure regulators lose their calibration and accuracy due to the continuous operation mode. Add to these factors, general engine wear, due to loading and thermal effects and it’s clear that over a long period of time, the accuracy and capability of the engine control system, with respect to correct fuelling, becomes compromised. In the past, this gradual loss of accuracy for fuel metering would have been accepted – just wear and tear. However, due to ever tightening emissions regulations, this is no longer accepted! The engine control system must maintain emissions within prescribed limits over vehicle life, and if it can’t (perhaps due to component failure) then the system must inform the driver that his vehicle has exceeded critical limits. So that’s why we need fuel trim – to monitor and compensate the closed loop emission control system, over a long term period (the life of the car).
Adaptive Learning
Fuel trim values are the result of an adaptive learning strategy within the ECU, this monitors the effectiveness of the control system during engine operation. For example, a gasoline engine ECU system will determine deviations from stoichiometry in the exhaust gas content over time. The information is stored in non-volatile memory and used to adjust or offset the fuelling value from the original stored fuel map value, to the required value, taking into account wear and tear factors. This is known as a long term trim value – it is learned by the ECU and it is not lost when the battery is disconnected, it generally can only be reset by a technician with a scan tool.
In addition, there are short term trim values, these are offset values based on short term effects, in response to temporary changes. Typically, for a gasoline engine, the short term trim values are a result of the closed loop lambda control system, comprising of the pre-catalyst oxygen sensor signal, that helps maintain fuelling at the stoichiometric point. Short and long term fuel trim correction values work together to maintain the correct, required lambda value. The short term trim will provide immediate correction, and if this correction is needed to maintain the required value on an on-going basis, then the long term fuel trim will learn this and provide a permanent offset to the basic fuelling map. Note though, this process is monitored and maintained within calibrated limits that correlate with tailpipe emissions. If the long term trim value becomes greater than a certain, pre-set limit. Then this is used as a trigger in the diagnostic (OBD) system to inform the driver of an emissions related problem, via the MIL (Malfunction Indicator Lamp) and storage of an appropriate DTC (Diagnostic Trouble Code).
Fig 1 - Fuel trim is effected by modifying the basic injector demand, for the engine operating condition
Fuel trim diagnostics
A useful aspect of fuel trim values is the potential of being able to use them as part diagnostic procedures. Many fault coder readers, including low cost, generic devices, can provide fuel trim values. If you know and understand what these values are, and how they are generated, then you can use them to help you understand the nature and root cause of many fuel systems related faults that you may come across.
Assuming you can access fuel trim information, how does it help you? During normal engine operation, the ECU records long (LTFT) and short (STFT) term fuel trim values as ECU labels (accessible via a scan tool). STFT normally cycles at the same frequency as the oxygen sensor switches, in effect it reflects the operation of the closed loop lambda control. LTFT is normally quite stable, and any adjustment of the value is made over a long period of time, so is less noticeable. Remember that LTFT is held in non-volatile memory (also known as KAM – keep alive memory), but STFT is dynamic, thus constantly changes during run-time, but starting at zero, both values re-set when DTC’s are cleared.
Gasoline engines are inherently unstable! The cycle-to cycle, and cylinder to cylinder variations cause loss of efficiency and roughness. So the ECU is continually adjusting spark timing and fuelling to compensate for this fact. As injectors wear, the delivered quantity and spray pattern are compromised, and this can cause cylinder specific fuelling faults that can be identified very efficiently using fuel trim values! The long and short term fuel trim value will reflect any variations in injector delivery or combustion efficiency for a specific cylinder, and can help diagnose faults like leaking blocked or dirty injectors.
Fig 2 - Typical fuel trim values at idle, as shown on a standard scan tool, connected to the vehicle OBD connector. Note that STFT is negative and follows closed loop lambda operation. LTFT is much more stable over time, and is a positive value. Both are within acceptable limits
Trim values should generally not exceed 10% either way. Positive means the ECU is trying to compensate for a weak mixture, it does this by extending the basic injector pulse width. This could indicate faults caused by air leaks in the flow system, manifold or vacuum pipework. In addition, MAF (mass airflow meter) or oxygen sensor issues. Lean mixture problems are the most common failure mode. Negative trims mean that the engine is running rich, and the base pulse width is reduced to compensate. This could indicate problems with EGR, MAF, fuel pressure regulator, leaking injectors or oxygen sensor problems. Individual cylinder contributions can also be evaluated. Disconnecting the injectors one-by-one and monitoring fuel trim values allows the technician, to compare each cylinder with the others. Thus individual misfire or injector problems can be identified clearly.
Fig 3 - Fuel trim - high idle speed. STFT is now positive, LTFT has switched negative, this shows how both work together to achieve the required target of Lambda 1.
Summary
It is worth to remember though that different ECU’s will respond differently to a given fault or diagnostic stimulus, in particular with respect to how LTFT and STFT work together to address a deviation or system problem. This is part of the ECU calibration strategy, so there are no hard a fast rules, but if you understand what LT and ST fuel trims are, and what they do. Then you can observe what you see and make an informed judgement. There is no doubt that being familiar with the concept will help your diagnostics of fuelling related problems. If you have time, it’s always worth making some observational measurements of a known ‘good’ vehicle, and record/note the data. Ready for the next faulty vehicle of that type or make to appear!
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