Fuel trims (LTFT and STFT):
Fuel trims are formed from the data of the lambda sensor. In a petrol engine, the fuel trims are used to maintain the ideal air/fuel ratio for complete combustion. This is 14.7 kg of air to 1 kg of fuel and is called the stoichiometric air-fuel ratio.
Fuel trims form a correction factor to adjust the base amount of injected fuel when necessary. Wear and contamination of engine components, sensors and actuators are taken into account. With the help of fuel trims, the exhaust gas emissions are kept within legal limits over the entire life cycle of the car. With a positive fuel trim, the ECU tries to enrich the lean mixture. With a negative fuel trim, the opposite happens; the rich mixture is made leaner. In doing so, the injector control pulse will be lengthened or shortened.
In the following image, the fuel trims with a rich mixture (-25%) and with a lean mixture (+25%) can be seen.
- The negative fuel trim means that the injectors must inject less fuel.
- The positive fuel trim means that the injectors must inject more fuel.
With a fuel trim of 0%, no compensation needs to be carried out, because at that moment the stoichiometric air-fuel ratio is present.
There are two types of fuel trims:
- Short Time Fuel Trim (abbreviated as STFT) is what the engine management is doing at this moment to adjust the air/fuel mixture. The STFT changes constantly while the engine is running due to short-term adjustments and temporary changes. This is also called the “short-term adjustment”. The STFT is reset when the engine is switched off.
- Long Time Fuel Trim (abbreviated as LTFT) consists of adaptive learning values that are formed from the STFT over a longer period of time. This is also called the “long-term adjustment”. The LTFT is stored in the “Keep Alive Memory” (KAM), which is not reset when switching the engine off and on. The LTFT is stored in the readiness test. Erasing it is only possible with diagnostic equipment or by disconnecting a battery terminal. The latter is not always possible.
Both the STFT and LTFT values must be as close to 0% as possible. Depending on the condition and operating conditions of the engine, the LTFT values can vary from 5 to 8%. The LTFT and STFT values shown by the scan tool in the image below fall within the tolerances and are therefore fine.
In the image above, the STFT and LTFT of “Bank 1” and “Bank 2” are shown. This engine therefore has two cylinder banks, so it will be a V-type engine. The engine is often marked to show which cylinder bank is number 1 and which is number 2. Otherwise, consult the engine specifications if there is any doubt.
With fuel trim values of more than 10%, there is often a problem. A fault code does not yet have to be stored. With fuel trims lower than -20% or higher than 20%, the engine management will store a fault code relating to a rich or lean mixture.
The LTFT values remain constant for a long period, because these values have been measured over a long time span and are stored in the readiness test (see the OBD page). The STFT values often jump across the screen during varying engine loads, as a result of the throttle opening further or closing.
Studying the fuel trims can be useful when making a diagnosis. In cases where there are no faults stored, or when the fault is not related to the complaint, the fuel trims can provide an answer. With an LTFT of just under 10%, no fault is stored, but it does indicate that the mixture is on the lean side.
The origin of the STFT and the transition to the LTFT:
The following image shows at the top the voltage pattern of the lambda sensor (zirconium / switching sensor), in the middle the short-term adjustment, and at the bottom the long-term adjustment.
The lambda sensor signal does become negative (0.1 volts) but not sufficiently positive (0.25 volts). The engine management recognizes this as a lean mixture.
To enrich the mixture, extra fuel is injected. We see this correction reflected in the STFT percentage: the blue line rises. At that moment, nothing yet happens with the LTFT.
While the STFT is rising, we see the lambda sensor measuring an increasingly rich mixture. The STFT continues to rise until the voltage has reached the desired value of 0.9 volts. This point is indicated by the green vertical line.
Now that the STFT has taken a certain value, it is kept constant for a certain period of time. If it turns out that the lambda sensor signal is correct as a result, the LTFT takes over the value of the STFT. The purple vertical line indicates the moment of this transition.
The STFT drops to 0% and the LTFT has taken over the positive value. The percentage exceeds the threshold value of 10%. The MIL will light up. Thanks to the correction factor, the engine will continue to run well.
After repairing the problem, the learned values can be erased. This is not strictly necessary: the fuel trims will be corrected by themselves over time.
Example: a vacuum leak has caused an LTFT of 7.8%. After the repair, a test drive is carried out. Because there is no longer any false air, the correction now results in a rich mixture. The STFT picks this up immediately and becomes negative. The following four images were taken at different moments during the test drive.
The LTFT in the previous image is 5.5%. To compensate for this, the STFT is -5.3%. This can also be seen in the second, third and fourth images: the positive LTFT value is compensated by a negative STFT value.
The following graphs show the percentage in relation to time.
- Before the repair, the STFT was 0% and the LTFT positive;
- During the test drive after the repair, the STFT becomes negative to cancel out the LTFT value;
- The LTFT decreases step by step: between each correction the value remains constant for a moment;
- The LTFT eventually becomes 0%.
For a technician, it is important to look at this: after the repair, are the values of the STFT and LTFT mirrored:
- +15 and -15, or
- -5 and +5.
This indicates that the resultant is 0%, so that the repair has been successful.
Possible causes of a rich mixture (negative fuel trim):
- Defective fuel injector; if the injector leaks, more fuel will end up in the combustion chamber than the engine management has calculated and commanded.
- Problem in the air supply to the engine as a result of a heavily contaminated air filter or a blockage in the intake.
- Problem with the lambda sensor; a defect or a clogged hole with which the lambda sensor measures the oxygen content in the ambient air.
- Problem in the fuel supply due to a defective fuel pressure regulator or a problem with the fuel return.
- Incorrect coolant temperature.
- Problem with EGR.
- Loss of compression.
- Valve clearance too small.
Possible causes of a lean mixture (positive fuel trim):
- Leakage in the exhaust, as a result of which not all exhaust gases are measured by the lambda sensor.
- Vacuum leakage, for example in the engine intake hoses (between the mass air flow sensor and the intake valve), a cracked crankcase ventilation hose, a cracked hose of the vacuum brake booster, etc.
- Defective fuel injector; it injects too little or nothing at all.
- Defective or contaminated lambda sensor.
- Defective or contaminated mass air flow sensor.
- Restrictions in the fuel supply due to, for example, a clogged fuel filter
- Defect in the fuel pump, as a result of which not enough fuel pressure is delivered.
Possible cause of both a positive and negative fuel trim in an engine with two cylinder banks:
An engine with two cylinder banks (V engine) has two exhaust manifolds and therefore also two (control) lambda sensors that can determine the mixture ratio per cylinder bank. When the engine is equipped with one mass air flow sensor and, in the event of an engine fault (e.g. misfire), the fuel trims are read out, it may be that bank 1 shows a negative trim and bank 2 a positive trim, for example:
- bank 1: LTFT -10
- bank 2: LTFT +12
In this case, a correction takes place on bank 1 to make the mixture leaner (due to oxygen deficiency) and on bank 2 richer (oxygen surplus). This may be due to incorrect valve timing. In this case, check the timing of the crankshaft in relation to the camshafts. Do note that during the electrical timing check (using the scope to check the relationship between crankshaft and camshafts) there may be variable camshaft timing. One can also choose to carry out a mechanical check with locking tools. For engines with two mass air flow sensors (one for each cylinder bank), this is not applicable.
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