You will commonly hear people referring to their vehicle as either running rich or running lean. This refers to the ratio of air and fuel that are being utilized together to power the engine of your vehicle. The combustion ratio that your engine produces will determine whether your car is running lean or running rich.
It is generally preferable that your vehicle is running neither lean nor rich, and instead utilizing the optimal air-to-fuel ratio to ensure that your engine is running healthy and smoothly. Running lean means that your vehicle’s engine is receiving too much air and not enough fuel to run optimally. Running rich means the opposite, meaning that your engine is currently receiving more fuel than air.
Neither of these engine conditions is preferable. This guide will contain all of the information that you need to identify is your vehicle is running lean or rich and detail some of the damage that these conditions can cause for other parts of your vehicle.
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Causes Of Running Lean
There are a few different circumstances that could cause your vehicle’s engine to receive more air than fuel, causing it to run lean. It is generally more difficult to determine when your engine is running lean than when running rich.
Clogged Fuel Filter
A clogged fuel filter is one of the most common causes for a vehicle to run lean. The fuel filter is a vital component of your vehicle that keeps debris from entering your engine through your fuel tank. The fuel filter is a component that will need to be regularly replaced.
All of the debris that the fuel filter collects will eventually hinder the passage of usable fuel to your engine. When this happens, you will need to replace the filter with a new one. A new fuel filter will cost you around $45, depending on the type of vehicle you have and where your purchase it.
If you change the fuel filter yourself, these are the only costs that you will incur. If you take your vehicle to a mechanic to have the fuel filter replaced, you can expect to pay around $125 in total for your replacement. The typical labor cost for a fuel filter replacement is around $65, depending on the type of vehicle you own.
You can save money by purchasing your own fuel filter online. The table below contains some of the most popular online vendors and their prices for fuel filters. You can use this table to find the cheapest option for your vehicle.
| Vendor | Avg. Fuel Filter Price |
| Walmart | $35 |
| Autozone | $42 |
| Pep Boys | $36 |
| Amazon | $37 |
| eBay Motors | $36 |
Clogged Fuel Injectors
If your fuel injectors have become clogged with debris, this can cause your vehicle to run lean. Unlike the fuel filter, the fuel injectors can become clogged or damaged by even small amounts of debris. When this happens, your air-to-fuel ratio will become suboptimal and you could risk further damage to your vehicle.
If your fuel injectors have become damaged and are causing your vehicle to run lean, then you will need to replace them. You can expect to pay around $180 for the parts alone needed for the replacement. Labor costs associated with fuel injector replacement average at around $150. This will cause your fuel injector replacement to total around $330, depending on the type of vehicle that you own.
Keep in mind that it is not advised that you attempt to replace the fuel injector yourself. Fuel injector replacement is a complicated process that is best carried out by a mechanically trained professional.
You can save money by purchasing your own fuel injector online. The table below contains some of the most popular online vendors and their prices for fuel injectors. You can use this table to find the cheapest option for your vehicle.
| Vendor | Avg. Fuel Injector Price |
| Walmart | $165 |
| Autozone | $188 |
| Pep Boys | $176 |
| Amazon | $167 |
| eBay Motors | $190 |
Faulty Fuel Pump
The fuel pump of your vehicle is where many air-to-fuel ratio issues can arise. If your fuel pump has become clogged with debris or has become damaged in some way, this can cause your vehicle’s engine to not operate optimally.
If your fuel pump is merely clogged, it is likely that a mechanic could clear the fuel pump of debris to get it working properly again. If your fuel pump is faulty, it will, unfortunately, need to be replaced. A fuel pump replacement part cost between $155, depending on the type of vehicle that you own.
If you attempt to change the fuel pump yourself, these are the only costs that you will incur. If you are planning on taking your vehicle to a mechanic for your fuel pump replacement, you can expect to pay an additional $375 to $570 in labor costs.
You can save money by purchasing your own fuel pump online. The table below contains some of the most popular online vendors and their prices for fuel pumps. You can use this table to find the cheapest option for your vehicle.
| Vendor | Avg. Fuel Pump Price |
| Walmart | $145 |
| Autozone | $167 |
| Pep Boys | $156 |
| Amazon | $165 |
| eBay Motors | $170 |
Symptoms Of Running Lean
When your engine is running lean you will notice that it starts to produce noticeably more heat than it normally does. You will also notice poor performance in your vehicle and trouble starting your car in some cases.
If you notice that your car is not accelerating properly and overheating frequently, you should check the fuel system in your engine to ensure that you are not running lean. Running lean for too long can cause serious damage to your vehicle.
Causes Of Running Rich
There are a few different circumstances that could cause your vehicle’s engine to receive more fuel than air, causing it to run rich. You can determine that your car is running rich by paying attention to your gas mileage, acceleration, and the smell of gasoline.
Faulty Oxygen Sensor
The oxygen sensor is responsible for delegating the air-fuel ratio for your engine. It works with your vehicle’s engine control unit to signal to the fuel injectors to adjust the ratio of air and fuel to your engine. When the oxygen sensor in your vehicle goes bad, it will have trouble properly adjusting these levels in your vehicle.
You can expect to pay around $255 for a new oxygen sensor and an additional $150 if you choose to have it installed at a mechanic. You can save money by purchasing your own oxygen sensor online. The table below contains some of the most popular online vendors and their prices for oxygen sensors. You can use this table to find the cheapest option for your vehicle.
| Vendor | Avg. Oxygen Sensor Price |
| Walmart | $224 |
| Autozone | $245 |
| Pep Boys | $235 |
| Amazon | $255 |
| eBay Motors | $265 |
Clogged Air Filter
A clogged air filter will also cause issues with the air-to-fuel ratio of your vehicle. Your air filter will collect debris over time and will need to be replaced regularly to ensure proper functionality. This can lead to poor engine performance and potential engine overheating.
The air filter itself will cost around $35. It is a rather simple process to replace the air filter in your vehicle, so if you attempt to do it yourself, these are the only costs that you will incur. If you choose to have your air filter replaced by a mechanic, then you can expect an additional $40 for labor costs associated with your replacement.
You can save money by purchasing your own air filter online. The table below contains some of the most popular online vendors and their prices for air filters. You can use this table to find the cheapest option for your vehicle.
| Vendor | Avg. Air Filter Price |
| Walmart | $18 |
| Autozone | $22 |
| Pep Boys | $25 |
| Amazon | $32 |
| eBay Motors | $28 |
Faulty ECU
The Engine Control Unit, or ECU, is responsible for maintaining the proper air-to-fuel ratio of your engine. If your ECU is malfunctioning, it will cause your fuel injectors to not function correctly, causing your vehicle to run rich.
ECU replacements can be expensive. You can expect to pay around $1800 for the part alone. An additional cost of $450 can be expected in labor costs. You can save money by purchasing your ECU online. The table below contains some of the most popular online vendors and their prices for ECU units. You can use this table to find the cheapest option for your vehicle.
| Vendor | Avg. ECU Unit Cost |
| Walmart | $1800 |
| Autozone | $1780 |
| Pep Boys | $1950 |
| Amazon | $1670 |
| eBay Motors | $1756 |
Symptoms Of Running Rich
One of the main symptoms of your vehicle running rich is your fuel economy being noticeably worse. Because of the excess gasoline being used by your engine due to the improper air-to-fuel ratio, this will cause you not to get the normal amount of miles per gallon out of your vehicle.
Another common symptom of running rich is the smell of gasoline in the cab of your vehicle. If you are smelling gasoline in your vehicle while driving, then it is likely being caused by your vehicle running rich.
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Final Thoughts
This guide has provided you with all of the information you need to identify if your vehicle is running rich or lean. You can use the information in this guide to help you resolve these issues and find the cheapest possible replacement parts for your vehicle available.
I’m William Guzenski, ASE certified master automobile technician & automotive expert. I love to attend race events and car shows throughout the country. I also loves to travel 40-foot motorhome, exploring abandoned mines and ghost towns. I’m currently building another car for Bonneville Salt Flats and will be campaigning a drag car at several events.