Disassembly: Closed Loop/Fuel Trims

[Greg E]

Nothing really new to report about this. Most of all this we already know.

Basically, fuel trims are the percentage of change in fuel over time. For the engine to operate properly the air/fuel ratio needs to stay within a small window of 14.7:1. It has to remain in this zone under all the various conditions an engine encounters every day. So, the ECU is trying to maintain this proper air/fuel ratio by fine-tuning the amount of fuel going into the engine. As it adds or takes away fuel, the narrow bands monitor how much oxygen is in the exhaust and reporting it to the ECU. The ECU monitors this input from the continuously when in closed-loop. If the o2 sensors inform the engine computer that the exhaust mixture is lean, the computer adds fuel by lengthening IPW to compensate. Conversely, if the o2 sensors report a rich condition, shortens IPW.

This change in fuel being added or taken away is called Fuel Trim. Changes in O2 sensor voltages cause a direct change in fuel. The short term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. The long term fuel trims (LTFT) are driven by the short term fuel trims. LTFT refers to changes in STFT but averaged over a longer period of time. A negative fuel trim percentage indicates a taking away of fuel while a positive percentage indicates an adding of fuel.

A normal STFT reading will generally fluctuate between negative and positive single digits 2-3 times per second. Usually they'll stay around positive or negative 5%, but they may occasionally go up towards 8 or 9% depending on the efficiency of the engine, age of the components, and other factors. A normal long term fuel trim reading will appear to stay the same, giving a long term average of fuel added. It should be close to zero, positive or negative single digits under normal circumstances. It will fluctuate much slower, possibly appearing static.

The LTFT is divided only by the MAF sensor readings. This table was included in Chrome V1 but I don't think I ever explained this very well.



Anything under 94hz is usually idle and I almost never see the High LTFT change as usually we're in open loop by that range. You can adjust this if you'd like but I never had any reason to. Maybe if you want a very high idle with cams and ported heads, I could see raising the low -> mid & mid -> low.

The other table you see is the NB voltage the ECU considers to be 14.7. This needs to be tested, but I believe raising this value will also change the target AFR the ECU uses to cycle the NBs thus improving gas mileage.

There's a trade off though... A leaner AFR will also raise your EGTs so it could have some serious long term effects on the life expectancy of your motor so be careful.

ALSO! I have also patched Chrome V2 so that the rear fuel trims won't be disabled anymore when Periphery0 Bit2 is disabled (you need this when the secondary O2 sensors are removed). This should help keep the motor running smoother when you have a crappy tune.

[Chris@Rvengeperformance]

very nice!

[R/T93]

Greg you are a mad scientist! Keep up the good work, expect a donation soon

[Greg E]

Something else....

I see this a lot where guys try to tune closed loop based on their wide band readings. You can't do this! In fact, completely ignore the wideband while the ECU is in closed loop. The ECU is always going to follow the narrow bands. What they report is how the ECU responds. If you want that picture perfect stoich ratio, you need to wire in the 1V output from your wideband.

You are always going to have a variance in your WB -vs- NB readings because there are many variables. Age/location of the sensor for example...

[Toni]

Greg, not sure if this helps at this point but when I get the car back on the road I can log my narrowband and widebands with pretty good resolution. This is for a 1st gen ecu. Not sure if this will help you any?

[keo92stealth]

this is great info. when you guys say closed loop, you mean what? that the car is cold? and open loop is operating temp?

[Greg E]

this is great info. when you guys say closed loop, you mean what? that the car is cold? and open loop is operating temp?

Closed loop = the ECU is using the stock O2s to correct the fuel (idle, cruise and light throttle acceleration)
Open loop = the ECU is ignoring the stock O2s and is using the fuel map (mild accel and WOT)

[Greg E]

Greg, not sure if this helps at this point but when I get the car back on the road I can log my narrowband and widebands with pretty good resolution. This is for a 1st gen ecu. Not sure if this will help you any?

Nah. Just tune the injectors & fuel map to keep the stock NB cycling and your fuel trims in check and don't ever worry about it again.

[Toni]

The other table you see is the NB voltage the ECU considers to be 14.7. This needs to be tested, but I believe raising this value will also change the target AFR the ECU uses to cycle the NBs thus improving gas mileage.

There's a trade off though... A leaner AFR will also raise your EGTs so it could have some serious long term effects on the life expectancy of your motor so be careful.

Greg,

This is something I am interested in as I usually don't run with a cat so I don't care to have the car tuned for it. According to Jeff Lucious's info just above 16 is best for economy. Have you or anyone else been able to do some testing in this area? I realize you are busy with life and what not but just curious

ECU closed-loop fuel injection control (feedback control)
In closed-loop mode, the ECU is using oxygen sensor voltage (see figure below) in a feedback loop to restrict the air-fuel ratio to a narrow range where the catalytic converter is most efficient. That range is A/F equal to 14.7 plus or minus 1% air (0.99-1.01% lambda, where a theoretical lambda of 1.0 equals an A/F of 14.7), or ~14.55 to ~14.85 A/F. The oxygen sensor indicates a (theoretical) stoichiometric air-fuel mixture (A/F equal to ~14.7) with a voltage typically in the range of 0.45 to 0.5 volts. When the sensor output is greater than this range, there is much less oxygen in the exhaust gas than in the atmosphere and the mixture is rich (A/F is less than ~14.7). When there is excess oxygen concentration in the exhaust gas, that is, as the oxygen content approaches that of the atmosphere, the sensor sends a signal less than 0.45 to 0.50 volts to indicate a lean mixture (A/F is greater than ~14.7). If fuel injection is stopped in the engine, oxygen content in the exhaust stream will equal that in the atmosphere and the O2 sensor voltage output will be zero. Typically the ECU uses a reference voltage of about 0.4 V, which is just on the lean side of ~14.7. The ECU reacts to these rich and lean signals by reducing and increasing, respectively, the injector activation duration. By using this feedback control to maintain the oxygen content in the exhaust stream within a very narrow range, the three-way catalytic converter operates at its peak efficiency to reduce carbon monoxide (CO), hydrocarbon (HC), and nitrous oxides (NOx) emissions.

Closed loop mode is used generally during warm idle, low-load cruising, and low-load acceleration to reduce engine emissions. Best fuel economy actually occurs with a slightly lean mixture, with A/F a little more than 16. Despite the need to reduce emissions, there are certain operating conditions where closed loop mode is not used in order to prevent overheating the catalytic converter or driveability problems. These situations include:
While cranking the engine.
During engine warmup when the coolant temperature is below 45ºC (113ºF).
During strong acceleration or deceleration.
During high-load operation.
When the oxygen sensor is not functioning properly.