Tuning: Fuel Trims!

Following on from Closed Loop Tuning, here's a bit of a spill about fuel trims! Modern factory computers are pretty clever and can automatically adjust the motor's tune in response to an array of environmental variables. There are a number of articles out on the interwebs about fuel trims but I'll have a whack at explaining it!

Let's have a closer look at fuel trims, what affects it and what can you do to help smooth it out on your tune!

Fuel Trims are adjustments that a Engine Control Unit (ECU) applies to compensate in changes in environmental variables to ensure the motor remains on its factory base map (usually stoich) in the 'Closed Loop' mode.

Closed loop is when the ECU is using an O2 sensor to get an air/fuel ratio reading from the exhaust to determine whether the motor is running in accordance with the base map. 'Open Loop' is when the ECU is relying solely on the base map for fueling and fuel trims are not applied. The ECU is in closed loop when the O2 sensor is sufficiently warmed up (needs to be at a working temperature to work) and utilised throughout the entire RPM range under partial throttle including idle and cruising. During start-up, when the O2 sensor is still 'cold', the ECU cannot trust the O2 sensor signals yet so it relies on the base map for fueling levels; hence the ECU would be in Open-Loop. The ECU also goes into open-loop in Wide Open Throttle (WOT) as the changes in O2 readings are too rapid to use as a suitable data source as well as in 'off-throttle' deceleration (in many cars).

Factory and standalone computers have fuel trim capabilities but piggy backs usually don't as it requires other inputs and more sophisticated mappings which defeat the purpose of having a simple (and cheap) piggy back computer.

What affects Fuel Trims?

The environmental variables that affect fuel trims include intake air-temperature, water temperature, oxygen (O2) readings, air flow readings and Manifold Absolute Pressure (MAP). Different computers may also use other environmental variables (e.g. Exhaust Gas Temps (EGT), ignition, cam/crank sensors), but to get a general feel for fuel trims, we'll look at these guys:

Intake air-temperature: as temperatures rise, the ECU can compensate by adding more fuel to cool the intake charge to keep the motor at stoich or the factory map.

Water temperature: if temperatures are too low (cold), the ECU can actually cause the motor to run slightly lean to increase cylinder temperatures and hence increase water (coolant) temperatures. If temperatures are 'normal' or 'hot', the ECU usually shouldn't need to adjust fueling (please correct me if I'm wrong!).

O2 sensor readings: This is the primary mechanism for fuel trims. As explained earlier, deviations in O2 readings from the base map gets trimmed. If exhaust gases are detected as being too fuel rich, the ECU can adjust fueling to bring it back to the factory map value (usually stoich); likewise if the exhaust is too lean.

Air flow (MAF) / MAP: these guys are the primary loads (reference points) of ECUs. Technically, fuel doesn't get trimmed because of these, rather, the changes in these affect fuel delivery, which in turn affects the O2 sensor readings and deviations from the base map gets trimmed accordingly. So if you increase MAF / MAP, you move the break point in the computer's map, and then from the increase in fuel detected by the O2 sensor, the computer trims that fuel back to base map value.

The cool thing with fuel trims is that as the environment changes, the ECU automatically compensates and always has your motor running as close as possible to its base map. If it gets too cold, the ECU adjusts, if it gets too hot, the ECU adjusts.. Clever!

The problem surfaces when you want to adjust the ECU's base map for turbo applications. It's not a problem for a standalone computer as you have direct access to the map. It's the primary problem of piggy backs as you are trying to adjust the map indirectly by feeding different signals to the factory ECU. Its the number one cause of having your tune 'disappear' on you!

Short-Term and Long-Term Fuel Trims

Before we get into fuel trim management, let's have a look at the two types of fuel trims; short-term and long-term. If you hook up your factory computer to a scan tool you should be able to find these trim values.

Short-Term Fuel Trims (STFT) are immediately applied compensation values that the ECU actions in response to changes in environmental variables. These adjustments are applied quickly in real-time and done many times per second. If the ECU has to continuously apply compensation values to the same breakpoint, it eventually 'learns' that there is a lean/rich spot and semi-permanently records that trim as Long-Term Fuel Trims (LTFT). So long-term fuel trims are semi-permanent adjustment to the base map, while short-term fuel trims are adhoc adjustments to the base map. It's semi-permanent as you can reset these trims by resetting the factory ECU (full power-down).

It is common to see the ECU trim out fuel in response to environmental variables, here is an example:

No change.
STFT: 0
LTFT: 0

Increase in fuel detected by O2 sensor, mixture too rich.
STFT: -6
LTFT: 0

Mixture normal again.
STFT: 0
LTFT: 0

Increase in fuel detected again by O2 sensor, mixture too rich.
STFT: -6
LTFT: 0

Increase in fuel detected again by O2 sensor, mixture too rich.
STFT: -6
LTFT: 0

Due to constant trimming down of fuel, ECU adds long term adjustment.
STFT: -3
LTFT: -3

Mixture normal again.
STFT: 0
LTFT: -3

Increase in fuel detected again by O2 sensor, mixture too rich.
STFT: -3
LTFT: -3

Increase in fuel detected again by O2 sensor, mixture too rich.
STFT: -3
LTFT: -3

Due to constant trimming down of fuel, ECU adds long term adjustment:
STFT: 0
LTFT: -6

Mixture normal again.
STFT: 0
LTFT: -6

Mixture normal again.
STFT: 0
LTFT: -6

From the example above, you can see that the STFTs gets applied immediately and can fluctuate quite rapidly. LTFTs however don't change as rapidly and the increments are usually quite subtle.

Tuning Fuel Trims

So how can we 'manage' fuel trims in your piggy back tune?

Fuel trims usually become a problem in boost situations when you want to have a richer air fuel ratio than the factory base map. Open-Loop isn't a problem as the ECU is ignoring the O2 sensor and is running purely on the base map. The output of that map is usually consistent and doesn't fluctuate as there are no fuel trims applied. Closed-loop, is where the action is!

Once you've sorted out your Open-Loop tune (usually WOT) you can start venturing through the wondrous world of closed-loop tuning. The requirement for close-loop tuning is usually during partial throttle where boost is already building.

The primary way of compensating for the increase in fuel delivered by the piggy back fuel map is to adjust the O2 sensor reading that gets sent to the factory ECU. For the AEM FIC, the AEM team have already compiled instructional videos for both narrow band and wide band O2 sensors (note that the 1NZFE uses narrow band).

I used their guide to construct a basic O2 map but from then on, it was a long and iterative process in order to get a stable tune. Tuning the O2 map to stabilise fuel trims will take a nice chunk of your time as the factory ECU can behave very... Naughtily. But persevere!

Here are some tips for O2 tuning:

1. Make sure you motor is warm/hot and that fuel trims have settled (i.e. not too much fluctuations in STFT). If you don't, you'll be trying to tune out trim that the ECU is compensating for start-up temps.
2. Adjust the O2 map in small increments. The voltage range of some O2 sensors are quite narrow and a little adjustment could create large changes.
3. An external wideband Air/Fuel Ratio (AFR) meter is KING. Make sure you reach your target AFRs! No point in stabilising trims if you are running too lean/rich. Stabilise the trims to HOLD your AFR!
4. Tune to stabilise LTFT! Don't worry so much about STFTs as the factory ECU will always be compensating for something. If you can stabilise the LTFTs to +/- 5 or so, then you've pretty much stabilised STFT (remember how they are related?).
5. Positive fuel trim means the ECU thinks the motor is running lean and is adding fuel.
6. Negative fuel trim means the ECU thinks the motor is running rich and is removing fuel.
7. STFTs are considered 'additive' as it adds a factor to the base map.
8. LTFTs are considered 'multiplicative' as it multiplies a factor the the base map.
9. Make your cell transitions as smooth as you can, large jumps between cells has the potential to confuse the factory ECU.
10. Tune as much as you can for one day, then leave it for a couple of days. LTFTs can take several drives before it becomes locked in. Once it gets locked in, then you'll have an indication on where you need to go with STFTs.
11. If you get absolutely stuck and cannot adjust the trims any further, you can try to amend the MAF map, but make absolutely sure that your fuel map is in order by checking your wide band air/fuel ratio.

Further to Tip 11; you can adjust the trim by adjusting MAF (if your primary load is MAP) but be warned that it will affect the amount of fuel being injected. So check that you are getting your target air/fuel ratio and check back in a couple of drives' time to ensure that the MAF adjustment isn't trimmed out. You have to be careful as you might get into a vicious loop of adjusting MAF, getting trimmed, adjusting fuel, getting trimmed, adjusting MAF again to a point where the values are way too large/small.

When I was tuning the O2 map, I would warm the motor, make the adjustments to the point that trims were stable or when I got sick and tired of it. I then stopped tuning and just monitored trims for the next day or so. With the 'updated' LTFTs I could see where in the RPM and MAP range I needed to tune, I would then make the adjustments and just monitor trims again for another day.

The more you tune O2, the more you get to know the behaviour of the factory ECU. Eventually, you'll be able to predict what state it is in and what it could do next. By this time, your trims should be stable 'enough' and that the trim behaviour is predictable. At this point, you can call it a day.

Remember, piggy back tuning is unfortunately not an accurate art. If you can tune it to almost +/- 5, 6, 7 or even 10 LTFT, then that's good enough! When the weather changes, all those trims will get mucked up anyway!

And just to reiterate... Your wideband AFR is KING!! Stabilise your LTFTs to HOLD your target AFR! Don't just tune to stabilise your trims!

O2 tuning requires a crap loads of patience, numerous days of tuning runs and the aim for stability rather than outright performance. 

Good luck!