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Your car’s engine needs three primary ingredients to run well: adequate spark, good mechanical compression, and a properly balanced mixture of air and fuel. The air-fuel mixture is compressed by pistons inside of the engine, then ignited by spark plugs, creating a series of small explosions that create the energy needed to propel the vehicle.

Without a properly balanced air-fuel ratio, the engine will suffer from incomplete combustion (misfiring) and run poorly. That might leave you wondering: What exactly is the ideal air-fuel ratio for a gasoline engine, anyhow?

What is the Perfect Air-Fuel Ratio?

air fuel ratio chart
An air-fuel ratio chart demonstrating how different mixtures affect engine performance and vehicle emissions.

In a perfect world, all gasoline engines would run the ideal air-fuel mixture of 14.7 parts air to 1 part fuel. This target mixture, which is referred to as the stoichiometric air-fuel ratio, is a compromise between optimum fuel economy and optimum power output.

A stoichiometric air-fuel ratio causes all of the oxygen and fuel to be consumed inside of the engine during combustion, resulting in nothing but harmless water and carbon dioxide exiting the vehicle’s tailpipe.

Although theoretically, the stoichiometric ratio is 14.7:1, the real-world mixture depends on the molecular structure of the gasoline. What’s more, the stoichiometric ratio is dynamic, meaning the air-fuel ratio often changes back and forth between a rich mixture and a lean mixture in response to engine operating conditions. A gasoline engine can operate without stalling using a mixture that’s anywhere within the range of 8:1 to 18.5:1.

It’s also worth pointing out that different types of fuels have different stoichiometric ratios. For example, while the stoichiometric air-fuel ratio for traditional gasoline is 14.7:1, the ideal mixture for E10 gasoline (gas that contains 10% ethanol) is around 14.04:1.

combustion engine pistons
In a perfect world, all gasoline engines would run the ideal air-fuel mixture of 14.7 parts air to 1 part fuel.

What is a Rich and Lean Mixture?

Simply put, a rich air-fuel mixture contains less air than the stoichiometric ratio, whereas a lean mixture contains more air than the stoichiometric ratio. Thus, an example of a rich air-fuel mixture is 9:1, and an example of a lean mixture is 17:1.

As was mentioned, the engine may run somewhat rich or somewhat lean depending on the operating conditions. For instance, when the engine is first started, it runs rich until the onboard computer management system enters “closed-loop” mode, where the oxygen sensors are warmed up enough to provide proper feedback.

On modern vehicles, the engine computer, which is often referred to as the powertrain control module (PCM), commands the air-fuel mixture from rich to lean and vice versa. The PCM makes its decisions based on the feedback from various sensors. Oxygen sensors, which measure the air in the exhaust stream, are the primary input to the PCM regarding mixture control.

mechanic using a scan tool
The best way to tell whether your engine is running rich or lean is to monitor the fuel trim data via a scan tool that displays live data.

How Do I Know if I am Running Rich or Lean?

On modern cars, the PCM continuously monitors and adjusts the air-fuel mixture to keep the engine running right. The adjustments the PCM makes are known as fuel trim.

The best way to tell whether your engine is running rich or lean is to monitor the fuel trim data via a scan tool (not a code reader) that displays live data. To use the scan tool, you simply plug it into the diagnostic port under the vehicle’s dashboard, then follow the product instructions to retrieve the live data stream from the car.

Fuel trim is typically displayed as a percentage on the scan tool. There are two fuel trim data sets: short-term fuel trim (STFT) and long-term fuel trim (LTFT). As you might guess, STFT refers to the mixture adjustments the PCM is currently making, whereas LTFT is an average of the adjustments the PCM has made over a certain period.

Also, if your vehicle has two banks of cylinders, there will be two separate sets of fuel trim data, one for each bank. Engines with a ‘V’ or flat configuration have two banks of cylinders, while engines with an inline or straight configuration have just one. Bank one is the bank that houses the number one cylinder in the engine’s firing order.

When the engine is running rich, the PCM will reduce fuel delivery, resulting in a negative fuel trim value. On the other hand, when the engine is running lean, the PCM will increase fuel delivery, resulting in a positive fuel trim value. So on your scan tool, you’ll see the short-term fuel trim readings switching back and forth between slightly rich and slightly lean, which is normal.

Generally, the engine is considered to be running too lean when fuel trim is above 10% (with the engine running in closed-loop mode). If fuel trim is below -10%, the engine is typically considered to be running rich. Though on some vehicles, the fuel trim specifications may vary, so it’s a good idea to consult a repair manual or repair database if the readings you’re seeing don’t make sense.

The PCM also notes the total fuel trim—the sum of the STFT and LTFT values on the same engine bank. Usually, if the module determines the total fuel trim has dropped below 20% or exceeded 20% for two drive cycles, it turns on the check engine light (CEL).

When the PCM turns on the CEL, it also stores a corresponding diagnostic trouble code (DTC) in its memory. Common codes associated with a fuel mixture problem include P0171 and P0174 (for mixture too lean), along with P0172 and P0175 (for mixture too rich).

smoke coming out from the tailpipe
An engine running either lean or rich can lead to reduced fuel economy, increased tailpipe emissions, and engine performance problems.

Is it Better to Run Rich or Lean?

Rich running conditions and lean running conditions are both bad news. An engine running either lean or rich can lead to reduced fuel economy, increased tailpipe emissions, and engine performance problems (e.g., misfiring).

Furthermore, an unbalanced air-fuel mixture can cause costly problems, such as a failed catalytic converter and internal engine damage. So, if your car is running significantly rich or lean, you’ll want to address the issue right away to prevent bigger problems down the road.

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