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Learning the value of fuel trims

Thursday, August 1, 2019 - 06:00
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Most of us as technicians know that fuel trims are part of the ECM (Engine Control Module) data stream that we can access with our scan tools. However, a lot of techs either struggle with or fall short of using fuel trims to their full diagnostic value. This article will explain fuel trims and expand on how we can employ them as part of our regular diagnostic routine to increase our diagnostic efficiency and accuracy.

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What are fuel trims?

What are fuel trims and why were the originally added to the ECM’s data stream? Fuel trims were a mandated OBDII Parameter ID (or PID) that reflected the ECM’s correction to the injectors base pulse width (IPW) to achieve a desired air fuel ratio (AFR) based on the oxygen sensor and other engine sensors input to the ECM in closed loop. In layman’s terms, they were a feedback loop system that was developed to use an “if /then” strategy for fuel control to the engine. But why? Why did we have to have this strategy? A big factor to their development was the adoption of the three-way catalytic converter or TWCC. 

Any air that enters the engine without passing through the MAF sensor first will create a "lean" condition.

TWCC’s required the feed gases to be close to Stoichiometric (14.7:1) air-fuel ratio to function at their greatest efficiency. Using the closed loop feedback system helped to achieve this. Most techs realize that fuel trims and the O2 sensors work in tandem with one another. So, if the O2 sensor reports too lean of a condition in closed loop, the ECM will make a correction to “fatten up” or add more fuel to the engine. Conversely, if the O2 reports to the ECM that the exhaust is too rich, the ECM will make a correction to “lean out” or subtract fuel from the air fuel ration entering the engine.

An example of a simple feedback loop would be a modern HVAC thermostat on the wall of your home that is capable of both cooling and heating. Let’s say it’s set at 70 degrees F. It’s chilly in your home in the morning and the room’s ambient temperature is 60 degrees F. The thermostat senses that “there isn’t enough heat” and tells the furnace to “add 10 degrees of heat” until the 70 degree F mark is met. If later in the day, the house heats up to 80 degrees F, the thermostat senses this and signals the A/C system to “remove 10 degrees of heat” until a desired temperature of 70 degrees F is attained. The thermostat would be the input that is responsible for the “if” part of the “if-then” equation. The HVAC unit would be the “then” part. The intelligent circuit board in the HVAC unit would be analogous the ECM.

From OBDI confusion to OBDII clarity

Fuel trims have been around since the early years of self-diagnostics in OBDI vehicles. In the old GM OBDI world, there were the fuel control PIDs called Block Learn and Integrator. GM used a sliding scale between 0 and 256 with 128 being the center point. If the number was greater than 128, it reflected the amount of enrichment that the ECM commanded to compensate for a lean condition. If the Integrator read 139, the ECM had sensed the engine was 11 percent lean and made the appropriate correction. In addition, if the Integrator PID in the scan tool’s data stream read 120, it is reflecting the ECM sensing the exhaust is too rich and it made the appropriate correction to subtract 8 percent fuel to lean out or bring the Integrator back closer to 128 or the zero mark. The Integrator revealed the ECM's quick response where the Block Learn was the learned value that accumulated over time and learned the trends or cells for the Integrator corrections. Back in the day, I always struggled to remember which PID was which.  Manufacturers could use any term they wished back then to describe the ECM’s correction.

A restricted exhaust on one side of a dual exhaust system can cause an imbalance of fuel trim between banks. Look at the bank with the more negative correction for the cause.

Fast forward to OBDII and now OEMs are required to call the real time correction Short Term Fuel Trim or STFT. The learned value now had to be called Long Term Fuel Trim or LTFT.  The correction is now also required to be displayed on the scan tool in the form of a percentage. Positive percentages reflect the percentage of correction when the ECM is adding fuel to correct for a lean condition. Negative percentages now reflect the percent of fuel that is being subtracted to correct for a rich condition.

Fuel trim response to common problems

Using the scan tool's ability to graph PIDs like Short Term Fuel Trim can be a great asset when diagnosing drivability issues such as air metering issues, vacuum leaks, fuel delivery and volumetric efficiency issues. They give the tech the ability to see the ECM corrections to the problems listed above and aids techs in a getting diagnostic direction.

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