The modern engine measures twothings extremely well, the amount of air going into the combustion chamber withthe mass airflow (MAF) sensor, and the byproducts that are generated by thecombustion event with the oxygen sensor. These measurements allow the engine’scomputer to put the right amount of fuel and spark into the cylinder to givethe most efficient and cleanest combustion event.
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Carburetors are stuck with alimited range of fuel trim due to fixed jets. In order to get a cold engine tostart, a carburetor would block off the air to a Venturi with a choke. The fuelmix would be richer, and the engine could start. The same phenomenon happens onfuel-injected engines with dirty air filters.
But fuel-injected engines are ableto compensate by changing fuel trim. Since there is less oxygen, the enginewill need less fuel. Less fuel means a leaner fuel trim and less power. Thisadjustment comes at the detriment of performance by taking away fuel.
It also works in the oppositedirection. If unmetered air makes its way past the MAF sensor, the oxygensensor will detect a lean combustion event. The ECM will then instruct theinjectors to pulse for a longer period of time. This increases fuel consumptionand decreases fuel economy.
The ECM can only compensate solong before it becomes unbalanced and can no longer add or restrict fuel toachieve a proper oxygen sensor reading. This is when codes are set for a toorich or too lean fuel trim. A restrictive air filter may not set a code, butthe restriction and compromised fuel trim will hurt the power and efficiency. Replacingthe filter and resetting the fuel trims may return the system to normal.
What are Fuel Trims?
Fuel trim is the adjustment theengine computer makes to the fuel mixture to maintain a balanced air/fuelratio. Either the engine is taking away fuel or adding fuel so the exhaustbyproducts, measured by the oxygen sensor, add up to an efficient operation. Apositive number means the fuel system is adding fuel by lengthening theinjector pulse, so more fuel goes into the combustion chamber. A negative numbermeans the engine is taking fuel away by shortening the injector pulse.
A rich fuel mixture can producemore power (up to a point), but it also increases fuel consumption andemissions. Conversely, a lean fuel mixture reduces fuel consumption but can alsoincrease emissions if the mixture is so lean that it fails to ignite and causeslean misfire.
Accurate fuel trim values requirean accurate feedback signal from the oxygen sensor, otherwise the enginecomputer has no way of knowing whether the fuel mixture is running rich orlean. When the engine is shut off, the fuel trim values are retained in thecomputer’s memory so the next time the vehicle is driven it can pick up whereit left off. Erasing the computer’s memory with a scan tool, or by disconnectingthe battery or the PCM power supply to clear codes, also wipes the fuel trimvalues, which means the computer has to start learning the fuel adjustments allover again the next time the engine runs.
How to Read Fuel Trim
The engine must be started and runningto read the fuel trim information. Choose the option that allows you to readsystem live data, which varies depending on the scan tool. This will display along list of sensor outputs and other readings called PIDs (Parameter IDs).There will be two fuel trim values:
Short-Term Fuel Trim (STFT) values change rapidly and can bounce around quite a bit depending on engine load, speed, temperature and other operating conditions. Readings may jump as much as 25% or more in either direction depending on loads, throttle position and engine speed.
Long-Term Fuel Trim (LTFT) is a longer-term average of what the engine computer has been doing to balance the fuel mixture over a predetermined interval of time. Values normally range from -10% to +10%. This value is a more accurate indicator of how the fuel mixture is being corrected to compensate for changes in the air/fuel ratio that are occurring inside the engine.
With both fuel trims, it iscritical to see that the system is making corrections to bring the engine intothe correct fuel trim for efficient operation. The ECM is not immediatelyreacting to fuel trim; instead it is reacting and correcting and thenmeasuring.
If you see a fuel trim pegged highor low at the maximum limits, there could be problems like an intake manifoldleak or a fuel injector that is stuck open. Fuel trims are not the final wordfor diagnostics, only a starting point.
Understanding Air Filters and FuelTrims
If a filter is clogged orrestrictive, it will show a negative fuel trim value, which means the enginecomputer is subtracting fuel — decreasing the pulse width or on-time of thefuel injectors — to reduce the amount of fuel injected into the engine. This isdone to lean out the fuel mixture to compensate for what it perceives as a richrunning or choked condition.
Positive fuel trim air filter diagnostics are a little more complicated. If an air filter or air induction system is delivering more than the normal amount of air, it will cause the oxygen sensor to detect a lean condition. It will add more fuel and cause the fuel trim to be positive.
Andrew Markel
