Theory: MAF Sensor Placement!

One of the major contributors to abnormal and erratic fuel trims, I believe, is Mass Air Flow (MAF) Sensor placement!

So let's have a look at this interesting topic!

There are four places you could possibly put the MAF Sensor when you add a turbo to your monster. This may be altered depending on the position of the blow-off valve as you could possibly place the blow-off valve before the intercooler, but the relative positions of the MAF should be the same.

1. Before Turbo:

This is actually a problem. When you accelerate and build boost, then go to switch gears, the throttle snaps shut and pressurised air gets expelled through the blow-off valve, BUT... The turbo is still spooling and is still actually drawing air through the MAF and hence it's possible that you will get a rich condition just before or when you get back on the throttle. You'll also get skewed MAF readings especially when you are decelerating with partial throttle (e.g. going down a hill). Although you are only touching the throttle, the turbo is spooling more and drawing in far more air than normal operations but all the factory computer sees is that more air is being drawn in so it will add more fuel. However, the O2 sensor will be reading rich, so this then leads to excessive fuel trimming and it is not uncommon to see -15 to even -20 STFT. This leads to abnormal behaviour in those RPM ranges.

2. After Turbo, Before Intercooler :

Again this placement is not ideal. It is still in the same airflow as under the same conditions with the MAF before the turbo, the turbo will still be pushing air through the MAF resulting in metering (measuring) airflow that may or may not eventually be used in the motor.

3. After Intercooler, Before BOV:

Again, it is not certain that the metered air that blows through the MAF will eventually be used in the motor as it could be by-passed by the blow-off valve.

4. After BOV, Before Motor:

This is the optimum placement. With the MAF in this position air that will indeed end up being used in the motor will be metered accordingly. When you are at partial throttle, the turbo and draw as much air as it wants but the actual amount of air that goes to the motor will be predicable. Even in the blow-off condition, when boost is built and by-passed, there wouldn't be any airflow to the MAF which is logical and is expected by the factory computer. The only challenge here is that the MAF will be in a region of pressurised air rather than just 'drawn in' air. Largely dependent on air temperature, you'll need to look at your tune to see if you need to adjust your stock MAF levels.

The main factors that drive the positioning of the MAF Sensor includes:

• Physical space: perhaps there is not enough room?
• Cable length: perhaps there's not enough cable length to move the MAF sensor?

Cable length shouldn't be a factor as you can extend the wires, so if you can move your MAF, do so!

So how can we avoid funky fuel trims due to MAF sensor placement?

At this present time, the most cost-effective method that retains the most amount of hair folicles on your head is to move the MAF to the appropriate position (After BOV, Before Motor). This may mean re-positioning and fabricating piping as well as rewiring to accomodate the MAF sensor.

The not so cost-effective method that may change you from being Zack Fair to Rude (refer Final Fantasy VII) is through intervention tuning by re-mapping the MAF sensor and compile a whole new MAF map.

Instead of taking in a MAF value from the sensor and adjusting it by a percentage and pushing that to the factory computer, you basically remap the entire map and for every sensor value you allocate a completely different sensor value. So for each breakpoint in the MAF map you specifically define a sensor value and regardless of what the MAF sensor sends to the piggy back, you output the value in the corresponding cell dependent on RPM and load (i.e. MAP). This is no easy feat. It maybe do-able but you'll need to get a log of all MAF values throughout the entire RPM range in a range of scenarios (idle, acceleration, deceleration, cruising etc) then extrapolate those values and add 'fudge' factors to account for the turbo. To add to the complexity, one thing that you don't have control of is intake air temperature which is sensed by the MAF sensor and is used by the factory computer to further 'fudge' the numbers to account for fluctuations. This may then necessitate two MAF maps (you can switch between two tunes on the AEM FIC by the way), one for hot days and one for cold days. As you can see, the complexity of doing this rises exponentially and the MAF map that you develope will never by ideal and you'll be tuning for weeks... months... years......

There's probably a couple of other ways to some how switch the MAF sensor signal to a single output during partial or off throttle but again, it's too complex and time consuming to consider. It would probably be easier and less stressful to just live with it and just understand what not to do to prevent fuel trims going crazy.

Tips on living with MAF induced fuel trim problems:

• Have a scan tool that reads and ouptuts LTFTs and STFTs! See when this happens and adjust your driving style.
• When you are going downhill, avoid having your foot on the accelerator too much. Again, watch your trims! If it starts going crazy negative STFTs, take your foot off for a bit and resume when trims stabilise.
• Have a scan tool that reads and outputs LTFTs and STFTs, what? Did I say that already? This will become (if not already) a permanent feature on your dash.

So if you have MAF induced fuel trim problems, have a look at your MAF sensor placement! Hopefully this post gives you a few ideas in solving any fuel trim related problems!

Good luck!