Written by intimidator on mightymustangs.net

Fuel Trim refers to feedback adjustments to the base fuel schedule.
Short Term Fuel Trims refers to dynamic or instantaneous adjustments (i.e. during braking/acceleration).
Long Term Fuel Trims refers to much more gradual adjustments to the fuel calibration schedule than short-term trim adjustments. Long term adjustments compensate for vehicle differences and gradual changes that occur over time.

Some of the most common fault codes (DTC's) pertain to fuel trims (rich mixture, lean mixture, etc.) Here is an explanation of fuel trim and what it does for us.

The ECU controls Air/Fuel mixture in order to maintain power, efficiency, and emissions. A/F is expressed as either a ratio (14.7:1 for example) or as a Lambda value. With iso-octane ("ideal" gasoline), Lambda of 1.0 is equal to 14.7:1 A/F. This is known as "Stoichiometry", a condition where there is a perfect balance between oxygen molecules and the various hydrogen and carbon based molecules in petroleum. With the oxygenated gasoline that most of us use, actual A/F ratio of 15:1 is closer to stoichiometric.

If Lambda is greater than 1.0, then there is a surplus of air and the engine is running lean. If Lambda is less than 1.0, then there is a surplus of fuel and the engine is running rich. It should be noted that the ratios are mass-based, not volume-based.

So, why don't we always run at 1.0 all the time? Well, we do MOST of the time. At cruise and idle, mixture is held tightly to 1.0 to keep the catalytic converter at optimal efficiency, so the emissions are minimized. However, when we need acceleration, the mixture gets richer. Why? Maximum power is made between 0.85 to 0.95 Lambdas (12.5 to 14.0 A/F with iso-octane). So, under acceleration, mixtures get richer. Sometimes you want to get even richer under acceleration to keep detonation (pre-ignition of the mixture from excess cylinder temperatures) away. The 1.8T has a relatively high compression ratio for a turbocharged engine, which especially under lots of boost is very susceptible to detonation).

So, now that we know that the ECU wants to be able to control the A/F ratio. It has a prescribed set of values (maps) for a given RPM, Load, etc. So, the ECU tells the injectors to pulse for exactly XX.X milliseconds and that SHOULD get us the proper A/F ratio that we want. Well, if you tell an employee to go do something, you want to make sure they actually did it, right? The ECU has some snitches (the front O2 sensor and the MAF, for the most part) that will report back whether or not the desired mixture has been attained. The rear O2 sensor is used mostly to monitor the condition of the catalytic converter, although in some applications it also contributes to trim information.

Based on feedback from the snitches, the ECU learns to apply a correction factor to its commands to the fuel injectors. If you know that your employees take longer than the standard allotted time to do a specified job, you will need to adjust for that in your planning (injectors are in a union, so it is tough to fire them). The learned values go between the maps in the ECU's Flash ROM (the "chip") and the signal to the fuel injectors. These learned compensations are known as "trim". So, when you see "trim", it means "compensation".

"Add" means additive trim, which is addressing an imbalance at idle. When the ECU is using additive trim, it is telling the injectors to stay open a fixed amount longer or shorter. The malfunction (e.g. vacuum leak) becomes less significant as RPM increase. For additive adaptation values, the injection timing is changed by a fixed amount. This value is not dependent on the basic injection timing.

"Mult" means multiplicative trim, which is addressing an imbalance at all engine speeds. The malfunction (e.g. clogged injector) becomes more severe at increased RPM. For multiplicative adaptation values, there is a percentage change in injection timing. This change is dependent on the basic injection timing.

You can check your current state of trim by using LiveLink or equivalent to look at your engine fuel trims. The first two fields will have percentages. The first field tells the fuel trim at idle (Additive). The second field tells the fuel trim at elevated engine speeds (Multiplicative). Negative values indicate that the engine is running too rich and oxygen sensor control is therefore making it leaner by reducing the amount of time that the injectors are open. Positive values indicate that the engine is running too lean and oxygen sensor control is therefore making it richer by increasing the amount of time that the injectors are open.

It is totally normal for both the first and second fields to be something other than zero. In fact, zeros IN BOTH FIELDS indicates that either you just cleared codes (which will reset fuel trim values) or something isn't working properly. If values get too far away from zero, it will cause a DTC (fault code) and can set off the MIL (commonly referred to as the Check Engine Light, or CEL). Specifications for normal operation are usually somewhere near +/- 10%.

In general, an out-of-spec value in the first field (Additive) indicates a vacuum leak since it is mostly presented at idle, when vacuum is highest. An out-of-spec value in the second field (Multiplicative) indicates a fault at higher RPM, and may point to a faulty MAF.

If you have the means to log over the OBD-II port you can watch what the short term and long-term fuel trims (STFT and LTFT) are doing. These are the corrections that the PCM is applying to add or remove fuel based on its O2 feedback. There are different cells (spots or ranges of correction) based on what MAP voltage level you're at and if it's above or below ~2k rpm's (as well as separate corrections for idle/decel). Short term is on the spot temporary changes of up to 25% correction to add or remove fuel. If the PCM continually has to add 10% more fuel under certain operating conditions/ranges (above 16psi boost and above 2k rpm's, for example), the long term would get adjusted so the short term can go back towards zero correction. The PCM always likes to keep short term as close to zero correction as possible. Long term can also provide up to 25% correction, and both together can make up to a total of 50% correction. The PCM makes these corrections by altering the injector pulse width which is the time the fuel injectors are "on" and flowing fuel.

Both the short and long term fuel being cell based means they use a matrix based on RPM and load (determined from the MAP sensor) to determine which cell the engine is operating in. Each cell can store its own fuel trim correction. Under normal driving with open throttle the engine management uses 12 cells, referencing MAP voltage against RPM's. It basically uses low RPM (below around 2k rpm's) and high (above 2k), referenced against 6 different MAP voltage set points. There's also a deceleration and idle fuel trim cells for above and below 2k rpm's.

So as an example, one cell might have a range of 3.6V to 3.9V on the MAP sensor with the engine operating above 2k rpm's. So if you had a LTFT correction of 10% it would be adding 10% fuel when the boost is between ~10-12 psi (3.6-3.9V). As soon as the MAP voltage goes over 3.9V you'd enter the next cell which may have a different LTFT correction.

When you're looking at fuel trims remember that there are only two different reference points for rpm's in regards to the fuel trims. There's one set of fuel trims for above ~2k rpm's and one set of fuel trims for below. For above and below that 2k rpm point there are six different fuel trim corrections stored (called cells) that is MAP voltage based.

So if you're at 4k rpm's and 3.5V on the MAP that would fall in a certain cell. At 5k rpm's and 3.5V it's still in the same cell (because the MAP voltage didn't change and it's still above 2k rpm's). Once the MAP voltage changes a certain amount or the engine drops below 2k rpm's you may drop into a different cell that may or may not have different fuel trim corrections. A lot of the older PCM's used many more tables and cells to store different fuel trims. My educated guess is that because the NGC (Next Generation Controller) used in the SRT-4 is model-based and a lot more powerful than older PCM's they can rely on it being able to use O2 and other feedback as well as calculations to make faster, more accurate "live" decisions on changes to the fuel trims without having to rely on extensive cells for different rpm and MAP voltage ranges like the old PCM's.

These short and long term fuels trims can be viewed on most scan tools or data loggers that can monitor generic OBD-II information and will be represented as a percentage. A positive number (such as 15%) means the PCM is adding fuel and a negative number (-12% for example) means its removing fuel. Some older pre-OBD-II cars like GM use a different system based off a block learn value between 0 and 255. In those types of systems 128 would be the middle or average value with zero correction and a higher number would mean the PCM is adding fuel and a lower number meaning that it's removing fuel.

We cannot change anything in the way of changing how the PCM calculates the long and short term trims. The only things we can alter that changes the fuel trims are the injectors and the fuel pressure. These are 2 of the 3 key factors used to calculate the flow rate the PCM uses. The 3rd is the injection pulse width. If you lower your fuel pressure and do not change your injectors or the amount of time the injectors are open, you get less fuel than what the PCM calculated. This causes the o2 to show a leaner condition than it should. The PCM then adjusts the trims to add more injection pulse width to compensate for this.

The opposite is true also. If you do not change your fuel pressure and change over to big injectors, the o2 sensor shows the car is running too rich. The PCM then alters the trims to remove injection time.

You have to balance the bigger injectors out by lowering the fuel pressure, but you have to do this keeping in mind the drivability of the car. If you lower the fuel pressure too much, you don't get good atomization of the fuel. This is why selecting the right injector for your power goal is important. There are many different sites on the internet that help you do all the calculations for this. Sometime in the near future I'll make a post with all these links so everyone can learn from them.

Most OBD II vehicles have a maximum fuel trim value of 20% to 25%. But a key question here is do the long-term and short-term fuel trim percentages truly reflect how much fuel is being added or subtracted? And if the relationship is not equivalent, is it at least linear?