Rick's Free Auto Repair Advice

What is fuel trim?

Fuel trim — what is it and how to read it

Why do we need fuel trim?

Fuel trim is an adjustment to the air/fuel mixture to maintain a balanced air/fuel mixture as engine speed and load change. If you run an engine in a controlled environment with a constant air temperature, constant barometric pressure, constant engine RPM and a constant load, you wouldn’t need to modify the air/fuel mixture. Once set, you could leave it at that.

But a car’s RPM and load are constantly changing as you speed up, go up/down hills and slow down; so the air/fuel mixture must constantly change with those engine changes.

The vehicle’s ECM is factory programmed to make those constant changes to air/fuel mixture as you change RPM and load. But those factory programs are theoretical. They don’t take into account how the engine and emissions systems are actually performing at the time.

However, since most modern vehicles are equipped with a mass airflow sensor, manifold absolute pressure sensor and oxygen sensors, the ECM can determine exactly how its air/fuel calculations have performed to maintain power and yet still meet emissions goals.

That’s where fuel trims come into play. Short term and long term fuel trims are the modifications the ECM makes to its initial air/fuel calculations based on the results it’s seeing from the oxygen sensors. In other words, it’s the self correcting portion of the vehicle’s fuel system in order to maintain a 14.7:1 ratio.

Short term versus long term fuel trim

The engine computer begins compensating by adding or subtracting a certain amount of fuel, from -25% to up to +25%. If the addition or subtraction helps reach the emissions goal, and the engine requires this modification for a long period of time, The engine computer will determine that it must make a semi-permanent adjustment to the factory programming. At that point it moves the correction in the long term fuel trim category and either adds or subtracts fuel on every start up and every trip.

What are troublesome fuel trim numbers?

Short-Term Fuel Trim (STFT) values change rapidly, responding to changes in engine RPM and engine load. In fact, they can bounce around by a large amount in response to changed in accelerator position. Readings can quickly to as high as  +25% and you push down on the accelerator or drop to as low as -25% when you take your foot off the pedal.

Long-Term Fuel Trim (LTFT), on the other hand, is a long-term average of the changes the ECM has been making to the STFT values. If the ECM is constantly adding STFT or a predetermined amount of time, the ECM will adjust the LTFT values as a new base.

So it’s not uncommon to see LTFT values in the -10% to +10% range. In most vehicles, both STFT and LTFT max out at +25%/-25%.

However, a LTFT above or below -10%/+10% is an indication of an air/fuel related issue.

What high or low LTFT trims mean

A high LTFT, +10% to +25% means the ECM is constantly adding more fuel due to a vacuum/gasket leak (unmetered air is entering the engine, forcing the ECM to add more fuel to get a balanced air/fuel mixture). Or, it could mean the system’s fuel pressure is low and the ECM is forcing a longer fuel injector opening time to compensate.

A high negative LTFT, -10% to -25% could be caused by a stuck open fuel injector that’s allowing too much fuel into the engine. In that case, the ECM would try to cut back on injector opening time.

In other words, in a port injected engine, a fuel trim beyond ±10% means you’ve got an engine, fuel, or ignition problem. However, since gasoline direct injection if far more accurate in fuel delivery, a fuel trim adjustment of as little as ±7% can mean you’ve got a significant issue.

Negative fuel trim causes

Keep in mind that an ECM commands a negative fuel trim because it’s seeing a rich exhaust report from the O2 sensor. So it’s trying to lean out the engine by cutting back on fuel by shortening the amount of time the fuel injectors are open.

Causes of negative fuel trim

• Stuck open fuel injector
• Fuel pressure is too high, allowing too much fuel to enter engine when injectors are open
• Clogged engine air filter (too little air entering engine based on TPS data)

Positive fuel trim causes

The ECM commands a positive fuel trim because it’s seeing a lean exhaust report from the O2 sensor. So it’s trying to add more fuel lengthening the amount of time the fuel injectors are open.

• Unmetered air is entering the engine or exhaust.
• Vacuum hose disconnected
• Intake manifold gasket leak
• Exhaust leak upstream from the O2 sensor

Long term and short term fuel trim effect on catalytic converter operation

Both a high and low LTFT can cause aP0420 or P0430.

A maxed out +LTFT will load the catalytic converter with fuel, reducing it’s ability to function, causing a Po42o because the downstream sensor will read a constant rich condition.

A maxed out -LTFT will cause the downstream O2 sensor to read a constant lean condition and the lack of fuel can prevent the catalytic converter from reaching full operating temperature.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat



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