Engine Running Rich Causes & Symptoms

Car Running Rich

Car engines run through the combustion of air/fuel mixture. Spark plugs are used to provide the necessary spark for ignition. This will then move the pistons and crankshaft.

However, inefficiencies can cause more fuel than required to be supplied to the engine. When this happens, we say the engine is running rich.

Having a car that is running rich means you are spending a lot on fuel. So it is necessary to fix your engine running rich as soon as possible.

7 Causes of Engine Running Rich

  1. Faulty MAF Sensor
  2. Faulty O2 Sensor
  3. Faulty MAP Sensor
  4. Faulty Engine Coolant Temperature Sensor
  5. Bad Intake temperature sensor
  6. Faulty Fuel Pressure Regulator
  7. Bad Fuel Injector

It could be a lot of causes why an engine is running rich because there are so many parts in an engine that can affect the fuel mixture.

Here is a more detailed list of the most common causes when your engine running rich.

Faulty MAF Sensor

Intake And Maf Sensor

A faulty MAF sensor is the most common cause of an engine that is running rich.

The MAF sensor calculates the air, which is going into the engine, and then calculating the air-fuel mixture that should be added. If this is dirty or failed, it will cause the engine to run too rich or too lean. 

If the MAF sensor is faulty, it will calculate the wrong amount of air entering the engine and adding too much or too little fuel.

Faulty O2 Sensor

Oxygen O2 Sensor Location E1609865834690

The O2 sensors are located on the exhaust pipe to sense the air-fuel mixture from the previous combustion.

If the O2 sensor is getting information suggesting a lean mixture, it will tell the engine control unit to add more fuel during the next combustion and vice versa.

If this is faulty and telling the engine control module to add more fuel, even if the air-fuel ratio is good – it may cause a rich fuel mixture. A faulty O2 sensor can cause the engine to run too rich.

Faulty MAP Sensor

Map Sensor

In some cars, they have a MAP sensor instead of the MAF sensor. There are also cases when you can have both a MAP and MAF sensor.

The MAP sensor calculates the air-fuel mixture based on the air pressure in the intake manifold. If you have a MAP sensor, it’s absolutely worth checking this part.

Diagnosing the MAP sensor is quite easy with a diagnostic tool since you can check the pressure it shows when the engine is off, which should be the same pressure as our air pressure.

Faulty Engine Coolant Temperature Sensor

Coolant Temperature Sensor

When the engine is cold, the engine needs more fuel to run properly. It is the engine coolant temperature sensor‘s job to measure the coolant’s temperature to identify when it should add extra fuel to the engine.

If the engine coolant temperature sensor is faulty, you may get a mixture that is too rich.

Faulty Intake Temperature Sensor

Intake Temperature Sensor

The intake temperature sensor calculates any additional fuel that should be added or restricted based on the temperature of the air entering the engine.

The intake temperature sensor is often installed inside of the MAF sensor and can not be replaced separately.

Faulty Fuel Pressure Regulator

Fuel Pressure Regulator

A faulty fuel pressure regulator will cause the fuel pressure to get too high or too low. This can cause a mixture that is too rich.

You will also want to check the vacuum hose to the fuel pressure regulator to ensure there are no leaks around it.

Faulty Injector

Fuel Injector Location

The injectors are the ones handling the amount of fuel entering the engine. If one injector is not flowing as it should or is stuck open, it might cause a rich mixture in your engine.

7 Symptoms of an Engine Running Rich

  1. Check Engine Light
  2. Fuel smell from exhaust
  3. Constantly refilling your gas tank
  4. Poor engine performance
  5. Black smoke from the exhaust
  6. Sooty spark plugs
  7. High carbon monoxide content

If you think that your engine is running with a rich mixture, there are some things you should check to confirm this.

Here is a more detailed list of the most common causes when your engine running rich.

Check engine light

Check Engine Light On Dashboard E1609869927250

When the fuel to air ratio is high, you will have the check engine light coming on.

The engine control module controls all the sensors, and if one sensor in your car will misfunction, it will light up the check engine light on your dashboard.

Fuel smell from exhaust

Bad Car Smell E1609777166642

If excess fuel is going to the combustion chambers, it means that some of it will not be fully ignited.

The catalytic converter has a way of removing some of this fuel, but it will find its way to the exhaust system when it is in excess. Unburnt fuel smells like rotten eggs.

Constantly refilling your gas tank

Refuel Car

One of the symptoms of an engine running rich is that you do not get the right gas mileage. This is because the car does not need all the fuel being supplied. However, it is normal to spend more on gas during winter or when carrying heavy loads.

Poor engine performance

Slow Acceleration Car

For your car’s engine performance to be normal, there has to be the right amount of fuel/air mixture. The assumption is that if there is an overflow of fuel, then the car will move faster. This is not the case as the excess fuel does not get combusted.

When you have problems with the air/fuel ratio, then you will experience low car performance. Also, you will notice that anytime your car is idle, the RPMs keep moving erratically.

Black smoke from exhaust

Black Smoke From Exhaust

When your engine is running rich, it will cause bad emissions. A rich air-fuel mixture will create black smoke, which will then come out from your exhaust pipe.

If it looks by your exhaust pipe like you have a diesel engine, but you don’t – then it is really time to check the air-fuel mixture.

High carbon monoxide content

Car Emission Test

Carbon monoxide is a dangerous exhaust gas. The catalytic converter works overtime to remove any traces of carbon monoxide in the exhaust. When your car’s engine is running rich, it means that you are producing more gas.

This can be dangerous when you are in an enclosed room with poor ventilation. You also risk not passing state-sanctioned emissions tests.

Sooty spark plugs

Sooty Spark Plug

If your engine is running rich, the spark plugs accumulate some black deposits at the bottom. This prevents them from operating efficiently. The soot will find its way to other engine parts, further causing damage.

The unburnt fuel eventually finds its way to the catalytic converter, and due to the number of impurities, it will clog it. With time, you will be forced to dismantle it and replace it.

Engine Running Rich Diagnosis

Car Diagnostics

Diagnosing an engine that is running rich is really not that simple. It often requires some diagnosis skills if you do want to waste money on just replacing parts.

This is how a professional would do it, and you might need some extra tools to make it.

  1. Connect an OBD2 Scanner and check for related trouble codes. If you find any other trouble code regarding another sensor, start your diagnosing at the sensor.
  2. Check the live data values of the O2 sensor. Does it show that it is decreasing the fuel amount all the time? Then it is probably not any fault with this sensor.
  3. If you have access to one emission control tester or an external air-fuel meter, connect it, and check the actual air-fuel meter. If it shows that the fuel mixture is lean, while the O2 sensor is telling us that the engine is rich and removing fuel – there is a problem with your O2 sensor, and it needs to be replaced.
  4. Check the values of all the temperature sensors like the coolant and air temperature.
  5. Check the values of the MAF sensor or MAP sensor if you have one. Replace if faulty.
  6. Check the fuel pressure and ensure that the pressure is not too high on idle or on acceleration. Check the fuel pressure regulator or the vacuum hose to it if it is too high.

Written by: Magnus Sellén

Founder, owner & main author of Mechanic Base. I have been repairing cars for more than 10 years, specialized in advanced diagnostics & troubleshooting. I have also been a drifting driver and mechanic for over 7 years.