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Fuel trim: How it works and how to make it work for you

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Fuel trim: How it works and how to make it work for you

Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. His began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.

When diagnosing a check-engine light or drivability issue, you can learn a lot by looking at fuel trim data on a scan tool. Fuel trim can be affected by just about anything between the air filter and the muffler, including sensors, injectors, ignition, EGR, the engine’s mechanical condition and even the crankcase ventilation system. Fuel trim numbers alone won’t provide a complete diagnosis, but if you know what to look for, those numbers can lead you in the right direction.

Global OBD-II fuel trim reporting was standardized for all manufacturers in 2005 and later models equipped with a CAN bus control system. That’s what we’ll focus on here, but the same basic principles apply to all model years.

What is fuel trim?

The amount of fuel required to operate an engine correctly depends upon the amount of air flowing into the combustion chambers. Since the driver controls airflow (load) with the accelerator pedal, the powertrain control module (PCM) can only control fuel. It uses sensors to measure or calculate airflow, consults an air/fuel ratio map in its permanent memory, then chooses the correct injector pulse width to match that airflow. That pre-programmed injector pulse will provide exactly one gram of fuel for each 14.64 grams of air (a stoichiometric air/fuel ratio), but the PCM almost always adjusts injector pulse width to provide more or less fuel than specified in the map. That adjustment is “fuel trim.”

There are two types of fuel trim, long-term and short-term, and the numbers are displayed on the scan tool as a percentage of fuel added to or subtracted from the pre-programmed fuel quantity.

Volkswagen calls fuel trim “oxygen sensor adaptation,” and Ford classifies fuel trim as a “continuous monitor” that runs when the fuel control system is in closed loop operation. This shows us that fuel trim is an ongoing calculation based on data reported by the oxygen sensor. Extra oxygen in the exhaust indicates a lean air/fuel mixture, so the PCM increases injector pulse width to add more fuel (positive fuel trim). Too little oxygen in the exhaust indicates a rich mixture, causing the PCM to reduce injector pulse width (negative fuel trim).

The formula for fuel trim calculation is:

Fuel mass = Air mass x (short-term fuel trim x long-term fuel trim) divided by (equivalence ratio x 14.64)

Understanding these terms and their relationship will help you understand how to use fuel trim data shown on a scan tool.

Equivalence ratio

This is the desired air/fuel ratio, a command issued by the PCM. Greater than 1.0 is the command for a rich air/fuel ratio, and less than 1.0 is a command for a lean mixture.

Under steady-state conditions, this command constantly rises and falls to alternate between rich and lean mixture for proper catalyst operation. Many scan tools will display and graph this number in Global OBD-II.

Short-term fuel trim (STFT)

During normal closed-loop operation, the PCM uses short-term fuel trim (STFT) calculations to constantly command the air/fuel ratio slightly rich and then slightly lean. This is necessary for the catalytic converter to do its job correctly, and the average of the rich/lean swings will be in or near the middle of the oxygen sensor’s signal range. You can see this by displaying graphs of equivalence ratio, the oxygen sensor and STFT on a scan tool.

At idle the graphs will mirror or “chase” each other: they won’t have the same shape, but as one rises, the others will follow or fall.

Long-term fuel trim (LTFT)

As the engine ages and the cylinders don’t seal evenly anymore, the short-term fuel trim may trend high or low most of the time. The PCM has the ability to learn this trend and store it in memory, and it will use that number in the fuel trim calculation to compensate for the changes causing the trend. That’s long-term fuel trim (LTFT). Since fuel trim is an ongoing calculation, keeping one factor in the calculation constant makes it possible to return STFT to its normal range, enabling a faster and more accurate reaction to bigger changes in operating conditions, such as acceleration.

In general, it takes anywhere from five to 20 seconds for LTFT to change, and since it’s stored in memory when the engine is switched off, that same LTFT value is used next time the system goes into closed-loop operation.

These three variables in the fuel calculation — STFT, LTFT and equivalence ratio — are all generated by the PCM. The only other variable, air mass, is measured with a sensor.

To understand how the variables influence fuel trim, consider this: When the system is in open loop, there is no fuel trim because the PCM holds the three controlled variables at 1.0. The only variable that affects fuel requirement is airflow, and the calculation looks like this:

Fuel mass = Air mass x (1 x 1) divided by (1 x 14.64)

Some people think of LTFT as a coarse adjustment and STFT as fine adjustment. A more technical definition would be additive and multiplicative. Additive fuel trim calculations (STFT) won’t change much with engine speed or load. For example, a vacuum leak would produce an additive calculation because the effects of that leak increase very little with increasing engine speed. Multiplicative fuel trim calculations (LTFT) are greater as engine speed or load increases because, for example, the effect of a partially clogged injector increases as speed and load increase.

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What’s normal, what isn’t?

When looking at fuel trim on a scan tool, it should be checked for at least 30 seconds at three different engine speeds: idle, 1,500 rpm and 2,500 rpm. If you make a scan tool recording while driving the vehicle, you can see how fuel trims change under changing loads.

When everything is stable and working correctly, fuel trim numbers should be no greater than 10%, and total fuel trim should be no more than 10% when the numbers are added together. For example, if LTFT is 4% and STFT is 4%, the total is 8%: that’s acceptable.

If LTFT is 12% and STFT is negative 6%, total fuel trim is 6%. This shows that the PCM has enough control to keep the catalyst working correctly, but the high LTFT number shows it’s compensating for something. On an older engine, LTFT is typically a bit higher as the PCM compensates for normal wear.

If fuel trim is significantly greater than 10% positive or negative, the PCM is compensating for more than just normal wear-and-tear. Whether it’s an older engine with a simple oxygen sensor or a newer model with a wide-band air/fuel ratio sensor, LTFT will continue to shift as needed to keep the STFT swings in the correct range.

LTFT can shift surprisingly far, but when it reaches plus or minus 25%, the MIL will be illuminated and a code will be set. Fault codes specific to fuel trim are:

  • P0170: fuel trim bank 1
  • P0171: system too lean (bank 1)
  • P0172: system too rich (bank 1)
  • P0173: fuel trim bank 2
  • P0174: system too lean (bank 2)
  • P0175: system too rich (bank 2)

By the time LTFT gets to 25%, there will be other codes, too.

But if LTFT is below that limit with or without other codes, you can still gain a lot of information from the scan tool before connecting additional test equipment to confirm your diagnosis.

Why are the fuel trim numbers high?

If LTFT or total fuel trim is greater than plus 10%, the PCM thinks the air/fuel ratio is too lean and it’s adding fuel to bring STFT control to the correct range. This presents three possibilities:

  • Unmeasured air is reaching the combustion chambers.
  • Less than the commanded amount of fuel is reaching the combustion chambers.
  • One or more sensors is reporting incorrectly.

When thinking about what would cause each of these conditions, the first thing to consider is how the PCM determines airflow. If the engine uses a mass airflow sensor (MAF), high fuel trims at idle are a classic symptom of a vacuum leak, especially if LTFT decreases at higher engine speeds.

Since the amount of air flowing through the vacuum leak doesn’t increase, the leak has less affect on air/fuel ratio at higher speeds and loads, so LTFT will come down as engine speed goes up. When you look for vacuum leaks, don’t forget the various “calibrated vacuum leaks” like crankcase ventilation, the evaporative emissions (EVAP) purge valve and, if equipped, air-shrouded injectors.

A dirty or faulty MAF sensor can also cause positive fuel trim numbers because it “under-reports” airflow, driving the base air/fuel calculation lean.

On engines that use a manifold absolute pressure (MAP) sensor to determine airflow, a vacuum leak does not affect fuel trim because the extra air (pressure) in the manifold is still measured by the MAP sensor.

Low fuel delivery will cause an increase in LTFT as the PCM tries to compensate for extra oxygen in the exhaust stream. Remember, the PCM doesn’t measure fuel flow; it only knows injector pulse width and assumes fuel delivery is correct as commanded. Is equivalence ratio changing, too? If you add propane and see LTFT and equivalence ratio numbers come down, there probably is a fuel delivery problem. If there’s no change, an oxygen sensor may be faulty or shorted to ground. Don’t forget to check this at different speeds and loads, because fuel flow problems often don’t show up at idle.

If the engine has two cylinder banks (even some four-cylinder engines are split into two banks), compare the readings to see if the problem affects both banks.

The PCM consults all oxygen sensors in the system when calculating fuel trim. Normally the rear (post catalyst) sensor voltage will be fairly stable near the middle of its range, but LTFT will likely increase if that sensor reading is low (remember, low is lean).

A catalyst code will influence both short- and long-term fuel trims. An exhaust leak after the catalyst will probably only affect the rear oxygen sensor.

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Why are the numbers low?

If LTFT or total fuel trim is more than 10% negative, the PCM thinks the air/fuel ratio is too rich, so it is leaning out the mixture calculation to return STFT control to the correct range. This presents three possibilities:

  • Not enough air is reaching the combustion chambers.
  • More than the commanded amount of fuel is reaching the combustion chambers.
  • One or more sensors is reporting incorrectly.

One thing that restricts airflow into the cylinders is a damaged catalytic converter that causes high exhaust back pressure. At idle this can drive fuel trims in opposite directions, producing a positive STFT and negative LTFT. You can see signs of high exhaust back pressure on a scan tool; calculated load will be low at wide-open throttle (WOT) and fuel trims will trend negative as engine speed increases.

The most obvious source of excess fuel is a leaking injector, especially at idle. In this case, STFT will be low but increase with engine speed as the extra fuel represents less of the total fuel requirement. Excessive crankcase vapors or a flooded EVAP canister can also mimic excess fuel, especially at idle. If the oil hasn’t been changed in a long time, especially in an older engine with a bit of blow-by, fuel in the oil can drive total fuel trim negative. Often simply changing the oil will demonstrate this condition by returning the fuel trim numbers to normal.

In a speed/density system, the PCM will substitute a pre-programmed value for a faulty barometric sensor. If the vehicle is more than a few hundred feet above sea level, this will show up as negative LTFT numbers.

Confirming the repair

There are two ways to use fuel trim to confirm a repair. One is to make sure total fuel trim is within 10% and then go for a test drive. Long-term fuel trim should almost immediately begin returning to normal.

It may take a few miles and/or cold-starts, but it’s a good way to watch the PCM learn the “new normal.”

A faster way is to clear the codes and, along with them, the PCM’s adaptive memory. When you start the engine with all the fuel trims at zero, watch the short-term fuel trim.

When the system goes into closed loop, STFT should stay within 10%, and as the engine warms up, total fuel trim will stay within 10% at all speeds and loads. If STFT quickly starts swinging into double digits, there’s still something wrong.

To learn more about how fuel trim is affected by various problems and conditions, connect your scan tool to a known-good vehicle and create some problems: introduce vacuum leaks, unplug an injector, disconnect a sensor, add propane: See how the PCM compensates with substitute values and fuel trim adjustments. Note the different reactions in MAF systems versus MAP systems.

On multi-bank engines, note how problems on one bank can affect fuel trim on the other, or not.

Like everything else in diagnostics, there’s nothing like first-hand experience with known-bad and known-good vehicles.

Once you have a feel for what fuel trim is supposed to look like under specific conditions, it will become one of the fastest and most useful diagnostic tools in your tool box.

Author’s note: Thanks to Snap-on Diagnostics for their assistance in preparing this article.

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