11 minute read
Fuel trim analysis
By Des Davies AAE MIMI, Top Gear Motor Services
This article explains in detail how to test and diagnose faults within the engine and fuel systems of the vehicle using the fuel trim PIDs on your scanner
We are very fortunate to have this data within the PCM to help us diagnose faults on engine related systems – for engine, fuel and ignition misfires and faults, in particular.
I find a lot of faults on vehicles are misinterpreted by some because of a lack of knowledge and understanding of fuel systems and components.
Fitting new parts without proper testing, by just checking the fault codes and replacing parts that are related to that code, often results in the fault remaining, ending in poor relationships between the garage and customers.
Normally one of the first codes to pop up in your scanner is the O2 sensor for a fault within the engine system PCM, this is because the set parameters for this component has deviated from the manufacturer’s specifications.
This does not indicate that you need a new O2 sensor but it tells you that it cannot operate within the set window of operation because of a fault within the system and this will flag up a fault code. Don’t be tempted to change the sensor, test if first, by looking at and interpreting the fuel trims and then testing the O2 sensor to confirm a fault.
Fuel trim analysis is one of the best tools that we have at our disposal to help us diagnose and fix poor engine running faults – if we can interpret the readings and then analyse them correctly.
Fuel trims
What are fuel trims? These are the adaptation, or self-learning, where the vehicle system can be adjusted or compensate its output if a change occurs in its basic operating environment. The key to adaptation is ‘closed loop control,’ which is a simple system of providing feedback to the ECM. Fuel trims are the adaptation values that the engine control unit is applying to the fuel injection set point. The engine control unit uses the fuel injection set point to meter the fuel, so that the air/fuel mixture is at 14.7:1, lambda1 fuel ratios are around 11.8:1 AFR to 17.6:1 AFR.
The engine control unit uses inputs such as engine speed, engine load (throttle position, mass air flow meter), coolant temperature and battery voltage to calculate the set points. These are stored in the control unit’s memory as a fuel map and each cell in the map may have a different value. The system uses the oxygen sensor to provide feedback –whether or not the set point achieved its goal of a correct mixture to suit the operating conditions.
Short-term fuel trim (STFT)
The O2, or oxygen sensor, voltage trends drive the STFT short term fuel trim.
This is adjusting to what is happening currently, it is a temporary adaptation adjustment to the fuel injection set point, based on the oxygen sensor’s output. When you switch the engine off, the STFT's memory is cleared. Basically, STFT is live feedback.
Long-term fuel trim (LTFT)
The short-term fuel trim's job is to drive the long-term fuel trim, LTFT, adjusting to bigger changes and over a longer time period. It reacts to major shifts of the STFT, is stored in the control unit’s memory and is incorporated into the fuel injection set point calculation.
Check using a scanner for closed loop and the STFT for movement, if it is not moving it is in open loop or has reached its maximum and in limp-home mode. The LTFT remains in the memory of the computer as a starting point for fuel calculations.
If the fuel trim is 0%, this is equal to 100% of the fuel used (correct amount of fuel used according to the vehicle manufacturer).
If the fuel trim is +5%, this is equal to 105% fuel used (5% more fuel used).
If the Fuel trim is -10%, this is equal to 90% fuel used (10% less fuel used).
STFT short term fuel trims should be around -10% to +10%.
LTFT long term fuel trims should be around -5% to +5%.
The total fuel trim (STFT + LTFT) should be less than +/- 10%, if more than this , you need to investigate the fault (+10% are lean fuel trims, – 10% are rich fuel trims).
Air/Fuel Mixture Strategy
In Figure 1, starting at the top left and working in a clockwise direction, we see the O2 has become rich with a 10:1 AFR(0.70 λ). The ECM sees this figure and takes fuel away, by shortening the on time of the fuel injectors (pulse width) with negative fuel trim figures to try to lean the fuel mixture to keep the AFR at 14.7:1 or Lambda 1 λ
With the vehicle being driven at this lean mixture, the O2 goes too lean at 17:1 AFR(1.20 λ), the ECM will now see this and will add fuel by opening the fuel injectors longer (pulse width) with positive + values called fuel trims to get the AFR back to 14.7:1 or Lambda 1 λ. This positive fuel trim, adding fuel, will continue until the O2 detects a rich mixture of 10:1 AFR again, and the ECM now starts to lean the mixture by subtracting the fuel again to try to keep and maintain an AFR of 14.7:1 or Lambda 1 λ. It will continue to repeat this strategy or process to keep the AFR at 14.7:1 or Lambda 1 λ
As explained, LTFT & STFT should both be within 10% when you add them together, this indicates that the fuel trims are good e.g. if the LT fuel trims are 30% and ST fuel trims are λ 20%, take the -20 away from 30, the total fuel trim is 10%, within the 10% operating range specifications. Fuel trims above 15% will normally set a fault code.
If the LT & ST fuel trims are at around 30% + λ , it has reached its limit (33% max) and then goes into a fixed fuel strategy by the engine computer, sometimes called a (limp home mode). For Example, after you have fixed a fault with the fuel system, the fuel trims may be LT + 25% and the ST may be λ-20% trims, then you have fixed the fault because the +25% and λ 20% figures are then added up together as a total fuel trim and in this case it adds up to 5% + or a positive fuel trim, which are within the 10% operating range.
Remember, the ECM is constantly altering the fuel trims to keep the engine management system at the correct stoichiometric fuel ratio of 14.7:1 or Lambda1, depending on engine speed, load and temperature conditions amongst some other parameters.
Fault-finding clues
Fuel trim readings tend to go higher with an injector/ fuel fault
Fuel trim readings tend to go lower with an ignition fault
A misfire with a small fuel trim difference, (single digit) is an ignition fault
A misfire with a larger fuel trim difference, (double digit) is usually an air/fuel ratio fault (fuel or air)
When fuel trims are better at higher rpm, rough idle, stalling, check for vacuum leaks using a smoke meter. When fuel trims are worse at higher rpm, check fuel pressure, fuel delivery, or for a MAF/MAP fault.
If the fuel trims indicate a weak fuel mixture, I normally keep snapping throttle to drive the fuel trims rich, if the signal goes to 0.8V then this indicates or points towards a rich mixture, there’s no need for a new O2 sensor, check for fuel issues.
An exhaust leak will normally give you positive fuel trims.
I always look at LTFT first to see if the PCM is doing its job correctly, or is it struggling to keep the air/fuel mixture ratio balanced correctly? This is one of the first checks I will perform when I have a poor running engine with no fault codes to help me. The fuel trims will help us to determine which side of the air/fuel ratio we need to focus on – are the exhaust gases showing a rich or lean condition?
This is where the 4-gas analyser comes into its own, checking the exhaust gases after burning the mixture in the combustion chamber. This makes it a lot easier to help you diagnose and interpret the waste gasses at the tail pipe for finding faults. I love my 4-gas analyser, it indicates what is happening inside the combustion chamber and I can then interpret the exhaust gas readings to help me diagnose engine and fuel related faults.
Not all vehicle manufacturers use the same terminology to display their data PIDs within the information extracted with your scanner live readings, these are some you will come across that are different than what you are used to:
Scanner data PIDs on some vehicles use these figures –typically Subaru and some others
Air/fuel correction is the STFT data on the scanner
Air/fuel learning is the LTFT data on the scanner
VAG Group Fuel Trims
Group 032 Fuel data PIDs
Group 032 in your engine measuring blocks is the LTFT (Lambda control mixture adaption)
Group 033 in your engine measuring blocks is the STFT (Lambda control)
Adaption (idle) in group 032, the first field, is the value of the LTFT at idle speed for bank1 sensor1 in %
Adaption (partial) in group 032, the second field, is the value of the LTFT when the engine is under load for bank1 sensor1 in %
Adaption (idle) in group 032, , the third field, is the value of the LTFT for bank2 sensor1 in %
Adaption (partial) in group 032, the forth field, is the value of the LTFT when the engine is under load for bank2 sensor1 in %.
Group 033 Fuel Trims
Group 033 the first field is your STFT for bank1 sensor1, lambda control in%
Group 033 the second field is the sensor voltage for bank1 sensor1
Group 033 the third field is bank 2 sensor 1 STFT %
Group 033 the forth field is bank 2 sensor 1 voltage
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).
In general, an out-of-spec value in the first field (Additive) indicates a vacuum leak since it is mostly present 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.
I have also come across these on older VAG and some other vehicles:
Integrator = short term fuel trim
Block learn = long term fuel trim
128 = 0% fuel trim
Below 128 = taking fuel away (rich command)
Above 128 = adding fuel (lean command).
Nissan fuel trims
A/F Alpha -B1 , 100% = lambda 1 or 14.7:1 fuel ratio this is Nissan's total fuel trims above 100% = total correction
Numbers above 100% = adding fuel
Numbers below 100% = subtracting fuel
96% fuel trims = negative fuel trims taking away fuel at -4%
98% fuel trims = negative fuel trims taking away fuel at -2%,
104% fuel trims = positive fuel trims adding fuel at 4%.
Go Global OBD or EOBD (European)
If you are confused with the above fuel trims you can instead go to OBD Global on the scan tool to get the generic data for the vehicle fuel trims. This is a lot easier, and this mode of fuel trims is not controlled by the manufacturer’s own data with its built-in strategy in the software in order that you can still drive the vehicle safely as it operates in a limp-home mode strategy.
The OBD Global is a generic mode that has real live readings not readings supplied by the vehicle manufacturer – this is very good for diagnostic purposes with true, live readings and is normally easier to interpret and understand.
In summary
We need to understand fuel trims and Air/Fuel ratio readings to help us diagnose vehicle faults correctly. If all manufacturers all had the same systems and components it would be much easier to test and fix vehicles more quickly and cheaply.
Testing systems and components is time consuming and difficult at times but we can get a lot of satisfaction by fixing the vehicle correctly, resulting with a very happy customer.
