Learning the value of fuel trims

Aug. 1, 2019
Understanding how the ECM controls fuel delivery can add a powerful diagnostic aide to your troubleshooting toolkit.

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.

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.

Vacuum leaks —Let's say the vehicle has vacuum leak. On a MAF-equipped vehicle, this means it has air that is entering the engine that is not being measured by the Mass Air Flow sensor. Most of us know that the air-fuel mixture is based on the amount of air entering the engine on a vehicle that has a MAF. If unmetered (often known as pirate or false air) enters the engine from a breach like a leaking intake manifold gasket, the ECM does not deliver enough fuel to achieve the proper air-fuel ratio (AFR). This results in a lean condition which is observed by the oxygen sensor. The ECM, if in closed loop, will make the corresponding if /then correction to STFT (Short Term Fuel Trim) and will add additional fuel to try to achieve stoichiometry.

The largest percentage of the breach or vacuum leak occurs at idle.  This is because the throttle blade acts as a restriction to airflow. As the throttle blade is opened, the size of the breach or leak is diminished due to the fact the throttle blade's restriction has been removed and manifold vacuum is replaced by atmospheric pressure under wide open throttle acceleration. From a diagnostic standpoint, our fuel trims will reflect this. If we graph the STFT correction, we would see high positive fuel trim corrections at idle that waned a way closer to 0 when the throttle blade was opened.

MAF input error — Let's say we suspected the MAF sensor was not reporting the correct amount of air entering the engine. How would we expect our fuel trims to behave? 

An example of this could be a MAF hot wire that is covered up with debris, oil or just a failing MAF sensor. Since most Mass Air Flow failures tend to overestimate air flow at idle and underestimate air flow under acceleration or load, we would see slightly negative fuel trims at idle and the more we opened the throttle blade, the more positive the fuel trims would react. Techs often notice that the positive fuel trims tend to “follow the throttle” when this problem is present.

A failing or contaminated MAF sensor will often cause negative fuel trims at idle and progressively more positive fuel trims as engine speed and/or load increase.

Bank-to-bank imbalance — Fuel trims can also be a great aid in diagnosing bank-to-bank air imbalance issues, especially on MAF-equipped engines. The bank-to-bank imbalance in airflow is usually caused by a problem with the engine’s Volumetric Efficiency (VE) or, in simpler terms, its ability to inhale and exhale. This can be caused by restrictions hampering the engine ‘s ability to breathe or a cam timing issue on a V-configured OHC motor that use separate camshafts for each bank. 

How do you suspect the fuel trims to behave on an engine when such a bank-to-bank airflow imbalance is present? The answer would be it depends. It would be dependent on a couple of factors: the amount of the imbalance in airflow bank-to-bank, the engine RPM and load and the degree of restriction present for starters. A general statement would be that when bank-to-bank airflow imbalance is present, the fuel trims are usually opposed — in other words, there is a significant difference side-to-side, sometimes techs refer to this as the fuel trims are skewed bank to bank.

Think about why this happens. As an example, let’s look at a MAF equipped V-6 where each head has its own camshaft and its own pre- and post- O2 sensors on each bank. The MAF sensor’s job is to measure the air entering the engine and report it to the ECM so it can add the correct amount of fuel to the engine to try to maintain stoichiometry of 14.7 to 1 air-fuel ratio.  Let’s say at a steady state throttle opening, the MAF sensor reads and reports that 48 grams of air per second (gps) is entering the engine. The ECM in closed loop is going to provide enough fuel for each cylinder to have the correct amount for 8 grams of airflow per cylinder (48/gps of measured airflow/6 cylinders = 8/gps of airflow per cylinder). 

Misfires can be caused by ignition or fuel delivery issues. Fuel trims may help point you in the right direction.

What if the same v-6 engine has an exhaust that incorporates catalytic converters for each bank and one side is restricted? How are the engine fuel trims going to appear and why?

For example, say the same engine under the same conditions that would normally report 48/gps of MAF has an exhaust restricted on bank 1 and only reads and reports 6/gps per cylinder due to the Bank 1 exhaust restriction. Bank 2’s VE or breathing is unaffected and still pulls 8/gps per cylinder. So, what does the MAF report to the ECM? 

Answer: Bank 1 has 3 cylinders @ 6/gps = 18/gps and Bank 2 has 3 cylinders @ 8/gps = 24/gps. The MAF now reports 42/gps (B1 18 + B2 24) to the ECM. The ECM sees this and assumes that each cylinder should get the same amount of fuel for the 7/gps of airflow per cylinder (42/gps divided by 6 cylinders = 7/gps per cylinder). Most vehicles only have one MAF sensor that is centrally located in the intake tract so it cannot differentiate between each bank’s VE. The unaffected or normally breathing side (Bank 2) gets delivered fuel for 7/gps of airflow delivered to cylinders that actually flow 8/gps. These cylinders are now under fueled and lean. 

The oxygen sensor on that bank reports this to the ECM which, in closed loop, counters by adding more fuel through a change to that side of the engine's injector pulse width (IPW). This is reflected in the fuel trims for Bank 2 reading positive. Now let’s examine bank 1, the side with the exhaust restriction.

Bank 1 Volumetric Efficiency has been altered by the exhaust restriction, in this example the Bank 1 Cat is damaged. The cylinders that should be flowing 8/gps are now only flowing 6/gps due to the restriction. This changes the MAF reading and because the MAF can’t differentiate between each bank,s airflow, Bank 1 now receives fuel for 7/gps for cylinders that are only flowing 6/gps. The result is that Bank 1 is over fueled and is now running richer than it should. Consequentially, the Bank 1 O2 reads rich. The ECM sees this and, if in closed loop, subtracts fuel by reducing the Bank 1 cylinders IPW. This is reflecting in the scan data for Bank 1, now showing negative fuel trims.

The MAF reports total air entering the engine — something that doesn't change whether it's ignited or not.

So, when we have a vehicle that has catalytic converters and oxygen sensors on each bank and a restriction in the exhaust occurs, the fuel trims will be opposed from one bank to the other. The question diagnostically is when we have opposed fuel trims which side do we diagnose?  The above example hopefully illustrated that we examine the side or bank with the most negative fuel trims.

We could also have a similar situation where the trims are opposed bank-to-bank caused by an airflow imbalance that is not caused by a restriction, but rather by a Volumetric Efficiency issue caused by valve timing — the valve timing is off.  Let's say we had an overhead cam engine that was a V-6 and each cylinder head had its own camshaft. Moreover, the scan data revealed while in closed loop the fuel trims for bank 1 were +3 percent and the fuel trims for bank 2 were +22 percent. Which side of the engine has the most negative fuel trims? If your answer was Bank 1, you are correct. Bank 1 is the side that has the air flow imbalance.

Misfire — Fuel trims can be help us determine whether a misfire is fuel-related or ignition-related. For example, if I have a 4-cylinder engine that is misfiring and I have already ruled out a mechanical issue using a relative compression test or a cranking vacuum transducer test, my next suspect will be either fuel or ignition. 

If I had a fuel injector that the wiring harness had rodent damage and caused an injector to be electrically offline, how would that affect my oxygen sensor readings? The answer would be 02 would read lean due to the fact the ECM could not deliver enough fuel to the engine. Obviously if the oxygen sensor told the ECM it was too lean my fuel trims would be positive. The question is how positive would they be?

Let's examine this. So, if the rodent damage was done to a single injector, and the ECM attempted to inject the correct amount of fuel into the cylinders but couldn't due to the wiring harness damage, how much of the fuel got delivered to the engine? If you said 75 percent, you are correct. If the above vehicle is misfiring and the ECM shows that the oxygen sensor is still lean it will try to compensate in closed loop by adding positive fuel trims close to +25 percent.

A failed injector, however, doesn't deliver its share of fuel to the cylinder its assigned to, and results in less fuel overall for the amount of air reported to the ECM.

So diagnostically, if I have a misfiring vehicle that has high positive fuel trims in a single cylinder misfire, I might want to examine fuel injection for that cylinder. Consequently, if I have a vehicle that has a misfiring cylinder and my fuel trims appear to be normal, I am going to look into ignition as the probable cause. It has been discussed in previous articles how ignition misfires have very little effect on fuel trims in closed loop.

Let ‘s review, fuel trims have been around for a long time, basically since the ECM took over fuel control via a feedback loop. OEMs got to call it whatever they wanted and use however they chose. OBDII changed this. Now it is a mandated PID in the data stream. They have to now be referred to as short term and long term and given in percentages. Positive percentages indicate the ECM’s correction for a lean exhaust by adding fuel. Conversely negative fuel trims indicate the ECM correction for rich exhaust content by subtracting fuel. Graphing scan data on MAF can be very helpful for diagnosing lean conditions like vacuum leaks and air metering issues. Opposed fuel trims bank to bank indicate an air flow imbalance bank to bank. When trims are opposed look to the side with the most negative trim as the side with the issue. Moreover, fuel trims can be used to differentiate whether a misfiring cylinder and ignition or fuel related. Hopefully this article gave you some insights regarding using fuel trims as a diagnostic aid.

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