An air-fuel mixture ratio can be long-term fuel trim or short-term fuel trim. I’ve explained what the former means. In this article, I’ll discuss what STFT entails.
The performance of your engine depends largely on the response of the vehicle engine control unit (ECU). Relatively, the normal operating percentage range of short-term fuel trim is -22% to +25%. Any range above this range means that your system is faulty.
By knowing how STFT value affects your engine performance and fuel system. What is the normal range of STFT B1? A good way to start is to get familiar with the concept of fuel trim.
Fuel Trim: How Does It Work?
For a system to function, all the components involved must maintain balance. The same goes with the engine system, the right amount of fuel and air must flow into the engine for efficiency.
Fuel trim is the report of the air and fuel ratio determined by the engine computer. The engine computer uses the MAF sensor and the fuel injections to mix the right fuel mixture. Then, the oxygen sensors signal the engine computer.
In turn, the engine computer measures the fuel mixture to determine whether it is lean or rich. The PCM finds out the quantity of air in the engine through the sensors. In response, it either adds or subtracts fuel for optimization of combustion.
Once the oxygen sensor detects a lean (too little fuel and too much air) in your car’s exhaust gases, it will present the data in a positive percentage value. To adjust, the amount of time your fuel injectors stay open increases. It continues until the oxygen sensors return to the normal voltage.
If the fuel system trims the fuel amount by allowing more fuel flow into the combustion chamber, it will record it with a positive percentage number. Another case is if the fuel system trims the fuel amount, by reducing the fuel flow to maintain a proper mixture of air and fuel in the engine. It will reflect as a negative number on the scanning tool.
The calculated data will be compared with the actual fuel quantity injected into the cylinders, using an accurate ratio of air to fuel. The calculated amount must neither be too little nor too much.
While fuel trim comes in two types -short-term fuel trim (STFT) and long-term file trim (LTFT)- I‘ll focus on the STFT and its normal range.
Short-Term Fuel Trim (STFT): What is it?
Short-term fuel trim is the quick and short response of the PCM to the change in exhaust gases. In other words, the STFT relies on the response of PCM in receiving data from the oxygen sensor.
Many factors affect STFT value. This includes a difference in the quantity of oxygen in exhaust cases, engine temperature, engine load and a few more.
The PCM STFT ensures that the fuel system maintains an air-fuel mixture ratio. To achieve this, the PCM adjusts the fuel injection, making corrections to the STFT as the fuel system enters a closed loop. An increase occurs to the STFT value if the oxygen voltage of the data input from the oxygen sensor to the PCM drops. The reverse occurs if the voltage increases.
As soon as the oxygen levels of the exhaust gases change, the oxygen sensor tracks the flow and brings about a response. In effect, the PCM quickly responds to this alteration in the exhaust gases. The generated data usually are not saved or stored. The whole process restarts when the vehicle is turned off and on again.
In an automatic control system, the STFT, represented by STFT B1, helps adjust the air/fuel ratio to maintain a rich and lean balance. For a V-shaped engine, there are usually two cylinder banks.
STFT B1 is the short-term fuel trim in engine cylinder bank 1. If an imbalance in the air-fuel ratio occurs in any of the cylinder banks, it can affect the entire fuel delivery system. As a result, an accurate emission level is not maintained.
What is the Normal STFT B1 range?
Earlier, I said that the relative normal STFT range of the PCM is -22% to +25%. In an absolute real-world application, the range of STFT to monitor is -10% to +25%, equivalent to ±5% and ±10%. The -ve value indicates that the system is losing fuel while the +ve value means that the system is adding fuel.
I need to mention here that a mechanical/ignition misfire may account for a fuel trim range value of +15% to +25%. The problem could also result from failure of fueling which accounts for a range of -10% to +15%. Conditions like engine speed, load, and throttle lift-off may also alter fuel trim value.
Yet, once the value goes beyond the normal range, it may be a sign of issues associated with the sensors or engine. This is often signaled to the vehicle controller or driver through the trouble code. STFT and LTFT values together make the total fuel trim.
Generally, each should float between ±5% and ±10%. STFT in cylinder bank 1 should not exceed ±10% under normal conditions. A value closer to zero is perfect. If the numbers are higher than +10, it means your vehicle’s engine is running lean.
An ignition or mechanical misfire naturally causes excess air (oxygen) in the exhaust gas stream. Once the oxygen sensor detects excess oxygen in the exhaust system, its voltage will drop. Hence, the PCM will interpret the low voltage as a low air-fuel mixture.
As a result, the PCM adds more fuel by making the injector ‘ON’ time drive wider. At this stage, the STFT will show a positive sign. If this condition remains constant, the STFT remains positive.
Otherwise, an intermittent ignition misfire will cause fluctuations of STFT corrections, featuring both positive and negative. The consequence is that the system will record lower STFT percentage correction (-ve) and lower STFT percentage value (-ve).
Is STFT different from LTFT?
Yes, there are areas of difference between STFT and LTFT. You can gain insight into the health of our car’s engine. Long-Term Fuel Trim, LTFT, is the long-term response of the engine computer to keep the fuel mixture in balance over some time. Conversely, the STFT drives the adjustments made by LTFT. The PCM makes use of the accumulated data stored in the memory to monitor the air-fuel mixture.
If STFT fails to maintain a fuel: air value over time, LTFT reacts to this persistent issue. It corrects the fuel map by compensating for the issue. Hence it saves the data in the PCM memory even if the engine goes off.
STFT is the immediate or short-term response of the PCM to any change in the value of exhaust gases. STFT B1 (short-term fuel trim in engine cylinder bank 1) normal range is ±5% to ±10%, under good condition. A deviation from the 0% to -99% range means that the system is rich while 0% to +99% indicates a lean system.
Understanding fuel trim values provided a faster way of diagnosing problems. Moreover, for your vehicle’s engine to function, balance is critical. Otherwise, it could cause restraints, backfires and more problems.