For whatever reason the project Octavia RS was running a bit rich everywhere except at high loads and high RPMS. My solution to reduce the fuelling and increase the AFRs was to do some MAF tuning using a piggyback ECU. By using short-term fuel trim data, I could see how much I needed to reduce the MAF signal.
Why Not Use AFRs?
The car is equipped with a wideband lambda sensor from the factory. This means it has target AFRs it wants to meet under all conditions. MAF tuning using AFRs is not going to work in this case because the target is going to be constantly moving as the car’s ECU adjusts the fuelling to meet its AFR targets. Tuning the MAF signal on a wideband equipped car means allowing the car’s ECU to achieve its AFR targets with as small as possible adjustment of the short-term fuel trims.
Luckily, we can see the short-term fuel trims in real time using an OBD Bluetooth dongle and an Android app called Car Scanner. If you have someone else driving you can adjust the MAF signal in real-time or alternatively you can use Car Scanner to datalog the short-term fuel trims, feed the data into LogWorks analysis software and find the adjustments you need to make.
The Problem on the Octavia
The problem with the Octavia was that it was maxing out the adjustment range of the ECU. Specifically, the short-term fuel trims could be pegged at -25% to try to get to its AFR targets. 25% I believe is the limit of adjustment which means even with -25% fuelling the ECU was still not able to get to the AFR target, the fuelling would still be rich.
Short-Term Fuel Trim Adjustment Range
By tuning (reducing) the MAF signal going to the car’s ECU, we can reduce the fuel entering the engine, meaning the ECU would not need to take out so much fuelling (using short-term fuel trims) to get to the AFR target.
If the short-term fuel trims were only -10% to achieve the AFR target we know the fuelling is correct.
If on the other hand the short-term fuel trims were -25% we can probably assume that the target had not been met because the ECU was at the limit of its adjustment range. It would still take out more fuel to get to the AFR target, but it can’t. By reducing the fuelling via MAF tuning the sensor signal, we bring the fuelling closer to the centre of the short-term fuel trim adjustment range. This gives the car’s ECU more scope to adjust fuelling. By reducing the MAF signal the ECU no longer needs to use -25% short-term fuel trim to get to the target, with a reduced MAF signal it may only need to use a -10% adjustment in the short-term fuel trim.
The Objective & Result
I wanted to get the average short-term fuel trim down to less than +/-10% and preferably less than +/-5%. By reducing the MAF sensor signal by varying amounts through the range of the MAF sensor signal I achieved the goal. Now the fuel trims are less than 10%.
To confirm the MAF tuning was a success I checked the minimum fuel trims between the original setup i.e. no tuning to the MAF signal and the final tune of the MAF signal (using a piggyback ECU). The unaltered setup was showing a consistent minimum value of the short-term fuel trims of -25% ie the adjustment range was maxed out. This was confirmed by the fact that fuelling was still rich according to the AFRs (also supplied through the Car Scanner OBD app). With the tuned MAF signal the minimum value for the short-term fuel trims were at most -15% and most around -10%. This confirms that the ECU was able to meet its AFR targets, which in turn means the fuelling is correct. This was confirmed by looking at the AFRs which showed normal AFRs i.e. around 14.7 at cruise and light loads and high 11s, low 12 AFRs under high loads.
Why Did The MAF Signal Need Tuning?
To be honest I do not know why there was an issue with fuelling. It could be that the MAF sensor is tired, perhaps the map was done with the different MAF sensor, perhaps the signal from the MAF sensor was not good/consistent when the mapping was done (a real possibility) and the list of possibilities goes on. Whatever the case may be, adjusting the MAF signal has done what I needed it to do ie allow the ECU to achieve its AFR targets.
If it turned out that the AFR targets in the engine map were the problem, then MAF tuning would not have been the solution. I was prepared for this possibility before starting the adjust the MAF sensor signal. However, having got the short-term fuel trims down ie closer to zero, the AFRs that are being reported are what I would expect them to be, so I believe the AFR targets are in the map are correct.
More MAF Tuning Info
You can read more articles about MAF tuning here.