6 Symptoms of a Car Engine Running Rich

The internal combustion chamber of an engine requires there to be just the right amount of fuel and air mixed together before ignition. The engine control unit (ECU) is the main computer of the vehicle which oversees all the actions related to the engine. It controls the air flow sensors, oxygen sensors, oxygen intake, and fuel injectors.

If the computer or any one of these components were to malfunction, it could possibly cause the wrong amount of fuel to be injected into the combustion chamber. In situations where there is too much fuel injected into the chamber and not enough room for oxygen, this would be known as having a “rich fuel mixture”.

Signs Your Car is Running Rich

A rich fuel mixture in the internal combustion chamber is never a good thing. It will cause a variety of problems for the engine. The symptoms of these problems will be easy to spot if you simply know which ones to look for.

Below are the top 6 symptoms that you will experience if you have a rich fuel mixture in your engine. With today’s fuel prices, getting this issue resolved should be a high priority for most.

1) Strong Fuel Smell

car smells

The most telltale sign of a vehicle that’s running rich is smelling gasoline while the engine is running. While it may seem like an annoyance, it’s actually quite dangerous if the fumes make their way into the vehicle cabin while driving.

2) Bad Gas Mileage

When you have too much fuel being injected into the combustion chamber, your miles per gallon rate is going to decrease. Then you will find yourself going to the gas station more often to fill up your gas tank since you are essentially wasting fuel.

If you want to get your gas mileage to where it should be, you have to solve your rich fuel mixture problem ASAP.

Related: 13 Tips to Save Money on Fuel

3) Check Engine Light

check engine light shine

Likely the easiest symptom to spot will be a “Check Engine” light in your dashboard. If this light were to illuminate, it will mean there is some type of issue going on with your vehicle. It won’t necessarily mean that you have a rich fuel mixture, but that can be one possibility.

Using a good code reader, scan your car to see if there is an error code that comes up, such as P0172. In many cars, this code indicates that there is too much unburnt fuel in the exhaust gases that are leaving the combustion chamber.

4) Failed Emissions Test

emissions testing

A rich fuel mixture will form an abundance of carbon emissions from your engine. You may notice this problem if you take your car to get an emissions test and your vehicle fails.

Even if you live in a state which doesn’t require a regular emissions test, you should still get it tested if you experience any of the other symptoms to help with diagnosing the issue.

5) Rough Idle

engine idle speed

A rough idling engine can be caused by many things, but a rich fuel mixture is one of them. This symptom will be noticeable because your car will often be shaking or vibrating as your idling. Plus, as the engine runs, there will be a very rough feel to it.

Pay attention to the tachometer as you’re driving normally because the needle shouldn’t be bouncing around all over the place when the engine is running.

6) Weak Performance

car won't accelerate

The balance of fuel and air must be present before they are mixed together and ignited. If you have too much fuel and not enough air, then it will reduce the performance of your engine.

You may still get an ignition from the spark and have steady compression, but the overall performance of the vehicle will be worse. You will notice this the most as you’re driving.


A rich fuel mixture is a problem that cannot be postponed for too long. On top of the health risks and added expense of using more gas, think about the components of your engine.

An abundance of fuel in the combustion chamber is going to cause these components to get extra hot and will quickly wear out expensive components like O2 sensors and catalytic converters.

If these conditions continue for too long, it could very well cause permanent damage to your engine and require hundreds or even thousands of dollars in repairs. It is so much easier to just address the fuel abundance problem in the early stages so don’t postpone it.


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