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. 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 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. 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. Thus, put simply, fuel trims are the continual adjustments an ECU makes to fuel delivery strategies to keep the air/fuel mixture as close to the stoichiometric point (also known as Lambda = 1) as it possibly can throughout the engine’s operating range.
In practice, the ECU manages two distinctly different types of fuel trims, these being-
Short Term Fuel Trim (STFT)
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. During normal operation of the vehicle, and provided the oxygen sensor is in closed loop operation, the ECU will react to changes in the composition of the exhaust stream almost instantly, hence the term, “short term fuel trim”, and it will do so several times per second.
It must be noted that on most applications, the signal voltage ranges from about 0.2 volts, to about 0.9 volts, which the ECU interprets as changes in the oxygen content of the exhaust stream. Depending on the signal voltage value, the ECU will alter the pulse width of the injectors to either add fuel to the air/fuel mixture, or alter the injector pulse width to subtract fuel from the air/fuel mixture, and it this process of adapting the injector pulse width that is known as fuel trims.
As a practical matter, a signal voltage of 0.45 represents an air/fuel mixture that is at, or close to the stoichiometric value, and signal voltages above or below this value represent either a rich, or a lean mixture. Note that oxygen sensors can merely indicate whether the air/fuel mixture is either lean or rich: these sensors cannot measure the actual composition of the exhaust stream in the way that air/fuel ratio sensors are able to do.
Long Term Fuel Trims
Long-term fuel trims are measured over longer time intervals by oxygen -, or air fuel ratio sensors downstream of the catalytic converter, and in practice, the signal voltages these sensors generate are not directly implicated in fuel delivery strategies, although there some exceptions to this rule.
The primary purpose of downstream oxygen sensors is to monitor the efficiency of the catalytic converter, which is accomplished by means of the ECU comparing its signal voltage to that of the upstream oxygen sensor. More to the point though, the pattern of changes in the signal voltage of the downstream oxygen sensor is compared to the pattern of changes in the signal voltage of the upstream oxygen sensor, and based on the differences or similarities between the signal voltage patterns, the ECU calculates an efficiency value for the catalytic converter.
In a fully functional engine management system, the signal voltage of the upstream sensor fluctuates rapidly as a result of changes that occur in the composition of the exhaust stream. However, the signal voltage from the downstream sensor should remain fairly constant around the mid-point of the voltage range that applies to that sensor, provided the catalytic converters’ efficiency is at about 75% or higher.
Thus, in the absence of faults, failures, or malfunctions that can affect fuel trims, long-term fuel trim values represent an average of the trims/adaptations the ECU had performed to correct the air/fuel mixture as measured over a predetermined length of time.
How to interpret fuel trim data
For the purposes of this article, we will assume that all oxygen -, or air/fuel ratio sensors work in the same way, in the sense that a low signal voltage indicates a lean mixture, and that conversely, a high signal voltage indicates a rich mixture. We will also assume that the engine is in good mechanical condition, there are no engine vacuum leaks, there are no misfires present, and there are no exhaust leaks upstream of the oxygen sensors.
If you connect a scan tool to such a vehicle, both the short and long-term fuel trim data will be displayed as percentages. Ideally, both values should be at, or close to 0%, when the engine is running at a steady speed, such as when it is idling. However, in some cases, the displayed value for long term fuel trims can be as high as 6 to 8 percent (depending on the application) and can be either a negative or a positive number. So what does this mean? Let us look at positive numbers first-
Positive fuel trim values
If the displayed fuel trim value is a positive number, it means that the ECU is increasing the injector pulse width to add fuel to the air/fuel mixture to enrich the mixture because the input data it is receiving tells it that the mixture is too lean.
Negative fuel trim values
If the displayed fuel trim value is a negative number, it means that the ECU is decreasing the injector pulse width to subtract fuel from the air/fuel mixture to lean out the air/fuel mixture because the input data it is receiving tells it that the mixture is too rich.
TIP: Bear in mind that fuel trim values are only reliable if it is known that the oxygen -, or air/fuel ratio sensors are fully functional and in closed loop operation, and that there are no air/fuel metering or ignition related codes stored. However, even if the sensors are known to be good, it can happen that displayed fuel trim values may be off by as much as 20% or more; this can be the result of a malfunction, or, it can be the result of someone having recently cleared all fault codes. 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.
So what should fuel trim values be?
While fuel trim values of 0% would be ideal, there is no such thing as a perfect engine, which means that in practice, achieving 0% fuel trim values consistently gets progressively more difficult as an engine ages. However, while fair wear and tear on engines is unavoidable and modern ECU’s are programmed to compensate for increased oil consumption rates, sensors losing sensitivity, and other factors that affect fuel trims, ECU’s have a limited capacity to compensate for some factors, so keep the following in mind when interpreting fuel trim values-
Short-term fuel trim values
Assuming that the engine is in excellent mechanical condition, and that all sensors implicated in metering both intake air and fuel, short-term fuel trim values should generally be between positive 10%, and negative 10% when the engine is running at a steady speed. 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 2500 RPM, and at about 3500 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.
Long-term fuel trim values
Ideally, long-term fuel trims should be at, or close to 0% when the engine is running at a steady speed. However, while changes to the engine speed will (and must) produce changes in the long-term fuel trim value, this value should return to a point close to 0% when the engine speed steadies. 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. However, when long-term fuel trims exceed about 10% or so to either side of 0%, there is a problem that needs to be investigated, but note that codes indicating rich or lean running conditions will generally only be set when the deviation reaches about 25% or so.
Short-term fuel trim values on the other hand, often deviate by as much as 10% to either side of 0% when the engine is running at a steady speed, which does not necessarily indicate a problem. However, once the deviation reaches about 25% with the engine running at a steady speed, there is a problem that will almost always be indicated by a rich or lean running trouble code.
How to use fuel trim values as diagnostic aids
As stated elsewhere, fuel trim values that deviate by a few percent from the ideal 0% are not necessarily indicative of serious problems. However, fuel trim values (both long and short) that range from small deviations to about 25% on either side of 0% are certainly worthy of attention, even if no codes have set as the result of the deviation.
Below are some tips and tricks to diagnose the most common faults /defects / failures /malfunctions that cause lean air/fuel mixtures that result in high positive fuel trim values-
- Small vacuum leaks; if both the short and long terms fuel trim values are higher than about 10% with the engine at idle, increase the engine speed to about 2000 RPM for about 30 seconds or so. If the fuel trim values return to a more normal level after this time, there is a small vacuum leak whose effects are reduced or eliminated at high engine speeds.
- Defective MAF sensors can over report the volume of air passing over them, thereby causing lean running conditions
- Inadequate fuel pressure; test fuel pressure with a dedicated fuel pressure gauge
- Clogged or dirty fuel injectors; test injector operation by performing a spray pattern and volume test
- Marginally defective oxygen -, or air/fuel ratio sensor(s); sensors lose some sensitivity after long use, or sensors may be stuck at reading lean conditions as a result of not reacting to switching signals from the ECU, or may be slow to respond to switching signals
Below are some tips and tricks to diagnose the most common faults /defects / failures /malfunctions that cause rich air/fuel mixtures that result in high negative fuel trim values-
- Excessive fuel pressure; test fuel pressure with a dedicated fuel pressure gauge
- Leaking fuel injectors; test injector operation by performing a spray pattern and volume test
- Defective MAF sensors can under report the volume of air passing over them, thereby causing rich running conditions
- Exhaust leaks; repair all leaks found since exhaust gas contaminates the ambient air that oxygen sensors use for reference purposes
- Loss of some compression on one or more cylinders; loss of compression causes poor or incomplete combustion, which results in unburned hydrocarbons “fooling” oxygen sensors into reporting a rich mixture
- Marginal misfire; not all misfires are bad enough to set a misfire code, so check or replace suspect spark plugs, and check the operation of all ignition coils with an oscilloscope
- Marginally defective oxygen-, or air/fuel ratio sensor(s); sensors may be stuck at reading rich conditions as a result of not reacting to switching signals from the ECU, or may be slow to respond to switching signals
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