# Air To Fuel Ratio – What Is The Best A/F Ratio?

Have you noticed that your vehicle started to drink a lot of fuel lately and you are asking yourself if you have a bad air to fuel ratio? Well, if you are in this situation then you are at the right place because there will be a lot to cover on this topic.

If you didn’t know the air to fuel mixture is one of the most essential things that need to be in check so the car works as it should all of the time. If the air to fuel mixture is bad, you will have a ton of problems driving your car because the car will simply struggle to run properly.

You might have situations like rough idle, poor engine work, and poor performance and the gas bill will be higher and higher. All this leads to a problem with the air to fuel mixture and this problem needs solving. This is why you need to learn more about this A/F mixture and understand what is the right air to fuel ratio. But you shouldn’t worry because we are going to help you out.

First, we are going to learn what is the air to fuel ratio and then we will cover the different types of ratios. Then we will learn the possible causes for bad ratios and also the symptoms that your car will produce. Lastly, we will learn how you can diagnose and fix this problem quickly and effectively. So, follow along.

## What Is Air To Fuel Ratio?

Now first let’s elaborate more on what is the air to fuel ratio in general. Knowing this will help you out when trying to understand the problem that you have with your car and the ways you can solve it. This is why we first need to learn the basics and then move on to more complex topics. So, let’s get into the topic.

The air to fuel ratio is basically the mixture of fuel and air that is injected into the combustion chamber. As you probably know, the car gets the air from the intake, and the fuel is injected with the help of the injectors into the cylinders.

When these two compounds mix, they create an A/F mixture. Then the spark plug ignites this mixture and your car moves forward. Simple as that. But what is the ratio exactly?

Well, the ratio in simple words is the balanced mixture of air and fuel. This mixture has to be balanced in order so the car will run properly.

And what is worth noting is that this type of mixture is not constant. It should be but actually, it is not and there are a ton of conditions that need to be met in order so the mixture is perfect. But what is the perfect air to fuel ratio?

Well, more on this we are going to elaborate next where we will learn all the different types of mixtures that you can get. And then you will be able to tell what type of mixture or ratio is the perfect one. So, let’s get into it.

## Types Of Air To Fuel Ratio

Now let’s cover the different types of air to fuel ratios. In fact, there are three types of this condition based on the work of the engine. These ratios are the normal ratio, rich ratio, and lean ratio. All these three are different states and we will cover them one by one.

The first type that we will cover is the normal air to fuel ratio. The ideal air to fuel mixture is 14.7:1. But what do all these numbers mean?

Well, this means that the normal ratio should be 14.7 parts of air and 1 part of fuel. That’s it, simple as that. Whenever the ratio is not adjusted properly you have the two other types. A lean situation and a rich situation.

The lean air to fuel mixture is when there are too many parts of air and too little fuel. For example 15.7:1. This means that there is more air than the recommended number. And this situation will cause symptoms that we are going to cover later on in the article.

While the second one is the rich situation. This means that the car is either injecting too little air into the system or too much fuel. For example, a 13:1 can be considered to be a rich ratio.

It is worth noting that the mixture fluctuates. For example, when you start the car, it normally runs rich since the engine demands more gas on startup when the engine is cold. But as soon as the engine warms up, the mixture should get close to the 14.7:1 numbers.

Still, there could be some problems with certain components inside of the engine that can spoil this air to fuel ratio permanently. So, what are the causes of this situation? Let’s elaborate on them next.

## Causes For Bad Air To Fuel Ratio

Now as we learned what is the air to fuel ratio and what it represents. Now it is time to cover the causes of why it runs out of balance.

Knowing the causes will help you out when it comes to diagnosing and sorting out this problem in your car. So, let’s go through all of the causes briefly before we elaborate more on the symptoms of the bad air to fuel ratio.

### 1. Clogged Air Filter

The first and most common cause of poor air to fuel ratio is a clogged air filter. The air filter is before the intake manifold as you know.

The filter has the task to purify the air before it goes into the intake. So, when people skip the change of this filter, the filter gets clogged up.

Once the filter gets clogged up, it will prevent the air to get inside the intake. So, the car will start to suffocate and run rich. The ratio will be not even close to the recommended numbers and you will have poor vehicle performance.

This is why whenever your car doesn’t run and you suspect that the mixture is bad. You should check this air filter first and replace it if necessary. This will hopefully solve your problem with the air to fuel ratio.

The next probable cause for bad air to fuel ratio is a problem with the MAF sensor. The MAF sensor is the sensor that measures the flow of air that goes into the intake manifold.

This sensor is usually located before the throttle body on the intake hose. This sensor unfortunately can get clogged up and cause trouble like in our case with the air and fuel mixture.

So, when this sensor starts to malfunction, you will be notified with a check engine light and it will be up to you to dive deeper and start diagnosing the problem with an OBD2 scanner. If you deem the sensor is faulty, you can replace it and this will hopefully solve your problem with the air to fuel ratio. If not, check the other probable causes.

The next possibility for the bad air to fuel ratio problem is the MAP sensor. The MAP sensor is a manifold absolute pressure sensor. So, what this sensor does?

Well, this sensor has a similar role to the MAF sensor. This MAP sensor is mounted on the intake manifold and basically, it measures the vacuum inside of the intake.

Then this sensor sends the data to the PCM and the PCM adjusts the air to fuel ratio. You can find this type of sensor on gasoline engines (if you’re interested, do check out our guide on what color is gasoline) that were produced before the year 2000, or in modern turbocharged petrol or diesel engines.

Modern naturally aspirated engines only run MAF sensors. So, if you have a MAF sensor on your car and the air to fuel ratio is messed up, it is definitely worth checking this sensor out because this sensor could be the root of your problem.

### 4. Bad Air To Fuel Ratio Sensor

The next probable cause for this problem with the bad air to fuel ratio is the sensor itself that is in charge of this task. Which is also known as an O2 sensor.

Every car has at least two O2 sensors. One of these sensors is mounted before the catalytic converter and is known as an upstream sensor while the second one is mounted after the catalytic converter. And this sensor is known as a downstream sensor.

Both of these sensors are important but the one before the catalytic converter plays a huge role when it comes to adjusting the air to fuel mixture. But what do these O2 sensors actually do?

Well, their main task is to measure the exhaust gases. More precisely to measure the level of oxygen inside of the exhaust. Once they measure this gas, they send this data to the PCM and the computer adjusts the air to fuel ratio on your car. Simple as that.

And what happens when these sensors are not functioning is that they will send bad readings to the computer or no readings at all. So, in this case, the computer can only guess what to do. And you will get the check engine light along with the bad air to fuel ratio. Your car will behave very strangely and will produce symptoms. More on these symptoms will elaborate later on in the article.

### 5. Vacuum Leaks

The next cause of bad air to fuel ratio is probably the vacuum leaks. Vacuum leaks in fact are the most notorious cause of a lean air to fuel mixture on your car. So, why is this the case?

Well, there are a ton of vacuum systems on a car. Like the EVAP system, the PCV system, EGR valve. All these systems can break and cause unmetered air to enter the combustion and ruin the mixture.

Also, in addition to this, there are other causes. More precisely issues with the intake manifold. Modern cars have plastic manifolds and on some cars, they are really cheaply made.

And over time they start to develop cracks and start to leak air into the intake. Something which is not particularly good to happen. Also, another thing to add is the intake manifold gaskets. These rubber gaskets can also fail and cause unmetered air to enter into the combustion and cause a lean situation.

Finding these leaks can be tricky. They are usually tracked with a smoke machine or with the help of soapy water. People spray the hoses and fittings and if there is bubbling, you know where to look. Now let’s cover the lean vs rich symptoms in the following chapters before we learn how you can fix the air to fuel ratio.

## Rich Vs Lean Symptoms

As we covered the causes, now it is time to focus on the symptoms of bad air to fuel ratio. Let’s cover all of the different causes of this situation.

We will cover both of these situations in two separate chapters so you don’t confuse yourself since the symptoms can be rather different for both situations. So, let’s get into the symptoms.

### Lean Air To Fuel Mixture Symptoms

Let’s first cover the symptoms that you will experience when you are having a lean air to fuel ratio. This means that when you have more air than fuel in the system. What can you expect in these cases?

Well, this lean air to fuel ratio is most often associated with the check engine light. In this situation, you will likely get a check engine light and a P0171 code. Or possibly a different code that will indicate a lean mixture.

Next are the symptoms when you have performance issues. Namely rough idle. The engine will shake and idle poorly whenever this is the case and you have a lean state.

Other symptoms include engine stutter and hesitation. The engine will also jerk and will accelerate quite slowly. You will feel like you lost a ton of power from the engine when the air to fuel ratio is lean. Overall, these are the most common symptoms associated with the situation when the engine runs lean. Now let’s cover the rich situation symptoms.

### Rich Air To Fuel Mixture Symptoms

Now let’s cover the rich air to fuel ratio symptoms. What symptoms can you expect in this situation when the air to fuel mixture is rich?

Well, the check engine light will be there, that’s for sure. The check engine light will hide some codes that probably are P0172 or P0175. Also, codes concerning the catalytic converter like P0420 or P0430 can be present as well. It is up to you to diagnose the problem and try to sort it out.

Other symptoms when your car will run rich include high fuel consumption. This is quite understandable since there will be a ton more fuel that is injected and this fuel will not burn completely.

This will attribute not only high fuel consumption but also a smell of gasoline coming from the exhaust that will be accompanied by black smoke.

In some extreme cases, your car could also backfire because of this poor air to fuel mixture. Backfiring is a situation when the fuel ignites inside of the exhaust. So, be prepared for this outcome.

Also, other symptoms such as engine misfires, and poor engine performance will be present as well when you have this bad air to fuel ratio. The run even though will have too much fuel, this fuel will not be able to burn completely and create power and cause an opposite situation.

The fuel will start to stick to the spark plug and cause them to foul. In addition, the rich situation will make your catalytic converter go bad as well. The fuel will stick and clog up the honeycomb structure (for more insight, check out our guide on what is a catalytic converter and what does it do and how does a catalytic converter work, as well as what is a catalytic converter). This is why you need to diagnose the problem with the air to fuel ratio and that’s what we are going to cover next.

## How To Diagnose And Fix A Bad Air To Fuel Ratio?

So, what you can do in order to diagnose this bad air to fuel in your car? Let’s explain in detail what should be your plan for attack.

Presumably, you drive a modern car that has an OBD2 port, in this case, you will need to plug an OBD2 scanner and see what type of codes your engine is producing. There are codes that start from P0170 that indicate issues with the air to fuel mixture.

There you will be able to learn if your car is running lean or rich. Also, in the next step, once you learned what kind of condition you have, you need to see if you have any specific codes, that will indicate a problem with a specific sensor. Such as the MAF, O2 sensor, or a problem with some of the vacuum systems.

Then the next step will involve you diving deeper and diagnosing the problems. For this, you will need a multimeter. This multimeter will help you bench test electric components like the sensors and see if they are working well or not.

Also, you will need to check for vacuum leaks if you have a lean air to fuel mixture and no OBD2 codes that indicate problems with the sensors. For this, you will need to improvise a smoke machine or get soapy water and hunt these vacuum leaks.

In addition to this, inspecting the spark plugs might be another good way to determine if you have rich or lean air to fuel ratio. If your car runs lean the spark plugs will be blistered and white. If you run rich the spark plugs will be black and will smell of gas. Overall, that’s all you need to know to diagnose a bad air to fuel ratio.

## Cost To Fix The Air To Fuel Ratio

Now as we learned what is the air to fuel ratio and how we can diagnose this problem with a lean or rich mixture. Now let’s elaborate more on the costs involved in this type of work. How much can you expect to pay when you are dealing with these issues?

Well, this depends much on the root cause of the problem. Let’s say that you have a bad sensor. Something like a bad MAF or a bad O2 sensor. For this repair, you can expect to pay somewhere between \$250 and \$450 to fix this problem.

If you have a vacuum leak or a bad air filter, on the other hand, you can expect to pay less than \$200 to sort out this problem.

## Air/Fuel Ratio and Stoichiometry Facts:

1. Engines need air, fuel, and spark to generate power, and the right mixture of air and fuel is necessary to optimize engine performance.
2. Air/Fuel Ratio (AFR) measures how many parts of air are mixed with each part of fuel, and it is essential to have the correct ratio for the engine to function correctly.
3. Stoichiometric Ratio (Stoich) is the ideal AFR at which the mixture burns entirely during combustion.
4. The Stoichiometric ratio or Stoich is different for different fuel types, such as gasoline, E85, and Methanol.
5. Lambda (λ) is another way to measure the air/fuel mixture, represented by the Greek symbol λ, which is calculated as AFR divided by stoich.
6. A lean mixture has not enough fuel, while a rich mixture has too much fuel.
7. An AFR higher than stoich or a lambda value higher than 1 represents a lean mixture, while an AFR lower than stoich or a lambda value lower than 1 represents a rich mixture.
8. O2 Sensor data from the ECU or an Air/Fuel Ratio Gauge can be used to monitor the air/fuel mixture.
9. The air/fuel mixture significantly impacts engine performance, economy, and engine damage.
10. The optimum AFR for maximum power, drivability, and idle depends on the engine’s specific requirements and can be adjusted based on the engine’s application.

## Conclusion

In this article, we have covered quite a bit when it comes to the air to fuel ratio. First, we learned what this ratio is and why it is so important. Then we covered the types of AFR.

After that, we learned the main causes for this problem with bad AFR on your car. After that, we learned the symptoms of this problem and how you can diagnose bad AFR on your car.

## F.A.Q

### What Does AFR Stand For

AFR stands for air to fuel ratio. This is the recommended ratio for your engine. For a gasoline engine, the recommended AFR is 14.7:1. This means that there should be 14.7 parts of air and 1 part of fuel. These are the numbers for gasoline engines. Diesel and other types of fuels can have different ratios.

### What Does Running Lean Mean

When a car is running lean refers to the situation when the air to fuel mixture is bad. In other words, there are too many parts of air and too little fuel. An example of this ratio is 15.7:1. In this case, the air to fuel ratio is not 1.47:1 but slightly bigger. And this could cause a lean mixture.

### What Does Running Rich Mean

When a car is running rich, it means that there are more parts fuel and fewer parts air in the AFR mixture. If the air to fuel mixture is 12.3:1 it will trigger a check engine light and a code that your car is running rich. This will also be noticeable by the smell of gasoline coming from the exhaust.

### How To Fix Air To Fuel Ratio

Fixing this problem can be rather tricky because you need to diagnose the right reason for this problem. The most common cause of this issue is the O2 sensor. But still, you need to perform proper diagnostics and troubleshooting to determine what could be the real issue.

### How To Check Air/Fuel Ratio On Carburetor

You can check the AFR ratio on these cars with a special O2 sensor that is mounted on the exhaust. This tool is known as the FAST Air Fuel Ratio Meter. This tool is really popular when it comes to tuners on classic motors because it shows you the AFR ratio all the time on a small screen.