How To Fix Engine Running Rich: What To Do

In order for your car to start, there needs to be a balanced ratio of fuel and air during combustion. When the ratio is off and the car is getting too much fuel entering the engine at combustion, it is known as the car running rich.

If your car is running rich, it will run poorly, get less fuel mileage, and idle roughly. In this article, we’ll cover the reasons your car could be running rich and 6 ways to fix your car when it’s running rich.

Signs Your Car Is Running Rich

Your car needs to run a ratio of 14.7 parts air to 1 part fuel in order to properly start. When the ratio is off, it allows too much fuel to enter the engine at combustion. This excess fuel doesn’t get the opportunity to correct burn-off and leads to your car running rich.

How can you tell if your car is running rich? First, if your car has an onboard computer, pull the codes. The error code P0172 means “gases rich in gasoline”, or your exhaust is heavy with unburnt fuel. It knows this because the ECU monitors your vehicle emissions.

As we stated before, your car will begin to run poorly and get less fuel mileage if it’s running rich. You may notice a strong smell from the tailpipe, like rotten eggs. The smell is the excess unburnt fuel that the catalytic converter couldn’t burn off.

You may notice your car has started to idle roughly, and the RPM gauge may show erratic readings. Also, you may notice unnecessary vibrations from your car when it used to idle smoothly with no vibration.

Reasons Your Car Is Running Rich

Replace MAF Sensor

Car mass airflow sensor four pin connection.

A faulty mass airflow sensor, or MAF sensor, will cause your car to run rich. The MAF sensor measures the air entering your engine and determines the amount of air to fuel mix is needed after each combustion. A blocked or faulty sensor will cause your car to run rich. The MAF sensor will pick up dust over time, but it can be cleaned and reinstalled as long as the sensor hasn’t failed.

Carbon Deposits on Spark Plugs

Spark plugs in an old engine.

Sometimes, when your car is running rich, it’s evident by looking at the spark plugs. Carbon deposits will form on the end of the spark plugs, and soot can move to other engine parts and cause damage to them.

The carbon deposits form from the unburnt fuel, which contains impurities that will clog your catalytic converter. If that happens, you’ll have to dismantle, clean, or replace your catalytic converter.

Faulty Oxygen Sensor

Oxygen sensors are located in the exhaust pipe of your car. They detect the air to fuel mix at the time of combustion, and if the mix is irregular, they will tell the ECU to correct the mix. If you have faulty oxygen sensors, they won’t send the correct information to the ECU, resulting in your car running rich.

Faulty MAP Sensor

A close-up photo of a manifold absolute pressure sensor or MAP sensor.

Your manifold absolute pressure or MAP sensor relays information on the pressure inside the intake manifold to the ECU. Over time, this sensor wears out and sends incorrect data to the ECU. When that happens, your car will run rich, and you’ll experience low fuel economy and low engine performance.

Faulty Fuel Pressure Regulator

A fuel pressure regulator under a hood.

Your car’s fuel pressure regulator keeps the fuel pressure level. If additional fuel pressure is required, the regulator sends more fuel to the engine. The fuel pressure regulator affects the combustion process and may cause the engine to lose power. Other signs are engine misfiring, losing acceleration, and black smoke coming from the exhaust pipe.

Faulty ECU

An engine control unit.

Another reason a car can run rich is due to a defective ECU. The ECU, or engine control unit, controls the air and fuel mix in the combustion chamber, and a faulty ECU won’t correctly regulate the mix.

Intake Temperature Sensors

Because a cold engine uses more fuel than an engine at the correct temperature, the intake temperature sensor determines how much fuel needs to go to the engine based on temperature.

Intake Air Temperature sensor.

Your car will run rich with unregulated fuel amounts if the intake temperature sensor fails. This sensor is often installed inside the MAF sensor and cannot be replaced separately, so the MAF and Intake temperature sensors will need to be replaced.

How Do You Fix Your Car That Is Running Rich?

First, if you notice a gas smell coming from the oil dipstick, or an occasional knocking sound, you need to have a mechanic find the root cause of your car running rich. If not, critical engine complications can start to occur. You can start troubleshooting the problem, however, several of the issues that cause your car to run rich will require a mechanic to repair if it turns out to be the problem.

Check Air Duct Flap

The air duct flap acts as a choke for the engine. When you start your engine, the air duct should be partially open. It will fully open once the engine is warmed up. If it doesn’t fully open when the engine warms up, you need a mechanic to repair this issue.

Check Vacuum Hoses and Lines

If the vacuum lines and hose pipes aren’t properly connected, they will leak and your car will run rich. If they are incorrectly connected and leaking, you’ll hear a hissing sound from the air leak.

Change Spark Plugs

Worn-out spark plugs don’t create sparks. When that happens, gas doesn’t burn correctly and it leads to high carbon emissions. Your engine will feel as though it’s running rough, with a high and vibrating idle. You should change out spark plugs if they show any signs of carbon buildup, or soot, on them.

Because soot can move from the spark plugs to other parts of the engine, it’s important that the spark plugs be changed when there is any sign of carbon deposits on them.

Change 02 Sensor

The oxygen sensor relays the air to fuel mix to the ECU, which corrects any error in the ratio. A faulty oxygen sensor won’t send the ECU correct data, and that results in the mix ratio not being corrected prior to combustion, leaving unburnt fuel deposit impurities in your catalytic converter and other vital engine parts. Because the 02 sensor is in the exhaust pipe, this is a replacement best left to your mechanic to perform.

Clean MAF Sensor

The mass airflow sensor in your car measures air entering the engine and decides the amount of air to fuel mix that is needed after combustion. Over time the MAF sensor can pick up dirt and dust, and that can hinder its performance. You can remove the MAF sensor and clean it, then reinstall it. If your car is running rich because of a dirty MAF sensor, cleaning and reinstalling the sensor should fix the issue.

Change MAP Sensor

The manifold absolute pressure or MAP sensor relays information on the pressure inside the intake manifold to the ECU. By design, the MAP sensor wears out over time. Once it becomes faulty, it won’t send correct data to the ECU and instead, the air and fuel mix will be off causing your car to run rich.

Typically, you’ll see low fuel economy and sluggish engine performance when the MAP sensor fails. The MAP sensor will need to be replaced by your mechanic to ensure the correct information is relayed to the ECU.


Although there are several reasons for your car to run rich, most of the symptoms are the same. If you experience low fuel economy and sluggish performance from your car, your car may be running rich. Some other indicators are the smell of gas from the oil dipstick, occasional knocking sound, a drop in acceleration power, and black smoke from the exhaust pipe.

Although it may start off as a small problem that simply feels like rough idling, your car running rich can quickly become a big issue if carbon deposits are able to make it into your vital engine components.

Any sign of carbon build-up on spark plugs should be immediately handled by replacing the spark plugs and finding the root cause of the car running rich. Once the root cause is isolated and the problem is repaired, your car will go back to running like normal, no longer getting bad fuel economy or idling roughly.

Back-lit car control panel showing idle.

Remember, to start troubleshooting, pull the code from your car’s onboard computer. The error code P0172 means “gases rich in gasoline”, or your exhaust is heavy with unburnt fuel. It knows this because the ECU monitors your vehicle emissions, and they will be full of fuel. This is why one of the symptoms of a car running rich is black smoke coming from the exhaust, and the smell of sulfur coming from the exhaust due to unburnt fuel.

If you have any questions about your car running rich, or any of the solutions we have listed in this article, please contact us.

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About Matthew Webb

Hi, I am Matthew! I am a dedicated car nerd! During the day, I am a journalist, at night I enjoy working on my 2 project cars. I have been a car nerd all my life, and am excited to share my knowledge with you!

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