Engine Running Rich – Symptoms, Causes & fixes

rich mixtureInternal combustion engine 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.

Typical gasoline engines use air to fuel ratio of 14.7:1.

However, inefficiencies can cause more fuel being supplied to the engines. 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. Modern engines now come with onboard computers that regulate the fuel/air mixture. This is done having sensors on the fuel injectors, airflows, and the emission points.

Causes of a Rich Mixture

When your car is running rich you need to check for defective oxygen sensors, mass airflow sensors, ECU, clogged air filters and open injectors. A closer examination of each will reveal the root cause for the blockage. 

If you think that your engine is running rich, the easiest way to diagnose this is with an OBD2 Scanner, to find out if there are any stored trouble codes.

However, if you did not get any trouble codes from your engine control unit, here are a few parts that are often causing rich mixture in your car engine.

1. Faulty O2 Sensor

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 of rich mixture, it will tell the engine control unit to add less fuel during the next combustion and vice versa. A faulty O2 sensor can cause the engine to run too rich.

2. Faulty MAF Sensor

The MAF sensor is calculating the air-fuel mixture that should be added by measuring the amount of air which is entering the engine. If this one is dirty or failed it will cause the engine to run too rich or too lean. 

3. Faulty MAP Sensor

The MAP sensor is calculating the air-fuel mixture based on the air pressure in the intake manifold. The MAP sensor is often installed instead on the MAF sensor in some cars. If you have a MAP sensor, it’s absolutely worth checking this part.

4. Faulty Engine Coolant Temperature Sensor

When the engine is cold, the engine needs more fuel to run properly. This is the job for the Engine Coolant Temperature sensor to measure the temperature of the coolant to know when it should add extra fuel to the engine. If the engine coolant temperature sensor is faulty, you may get a too rich mixture.

5. Faulty Intake Temperature Sensor

The intake temperature sensor is calculating 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 fitted inside of the MAF Sensor.

6. Faulty Fuel pressure regulator

A faulty fuel pressure regulator will cause the fuel pressure to get too high or too low. This can cause a too rich mixture, you do also want to check the vacuum hose to the fuel pressure regulator to make sure there are no leaks around it.

7. Worn out spark plugs

The spark plugs themselves are not causing rich mixture. However, they could cause misfires which will result in the rich mixture when the O2 sensors are not working as they should. Spark plugs are critical for providing that essential charge required for combustion. But, if the spark plugs are faulty then you will have incomplete fuel burning. You should change your spark plugs during your next service time.

An engine running lean is the reverse of the one running rich. In this instance, there is more air than fuel being supplied to the combustion chambers. Just like the engine running rich you will need to check your fuel injectors for any clogging. This can lead to less fuel being supplied.

The fuel pump helps generate the right pressure for fuel to flow through the car’s internal system. However, the air filters do get clogged. Modern cars now have oxygen sensors that monitor the amount of air being supplied to the engine. This information is collected by the sensors and transmitted to the onboard computer and from the air/fuel mixture is adjusted.

If the sensors collect dirt them they cannot remit the right information hence a reduction of fuel supplied. In other instances, the onboard computer itself can be damaged leading to faulty readings. A car engine has a set of air hoses and with time this does leak leading less air getting to the combustion chambers. You should have these inspected occasionally as they are under a lot of pressure from the excess engine heat.

Symptoms of an engine running rich

car running richIf you think that your engine is running with a rich mixture, there are some things you should check to confirm this. Here are the most common signs of a rich mixture.

Check engine light

When the fuel to air ratio is high you will have the check engine light coming on. If you have a car with an onboard computer, you should look out for error P0172. The oxygen sensor, mass air flow sensor, and the manifold absolute pressure sensor all will send information to the ECU that there is an excess of fuel going to the combustion chambers. A closer inspection of the exhaust gases will reveal more fuel being present.

Fuel smell from exhaust

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 these fuels but when it is in excess it will find its way to the exhaust system. Unburnt fuel smells like rotten eggs.

Constantly refilling your gas tank

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

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

When you have problems with the air/fuel ratio then you should experience low car performance. In addition, you will notice that anytime your car is idle the RPMs keep moving erratically. This can be accompanied by unnecessary car vibrations.

High carbon monoxide content

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 of the 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

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.


Your car needs the right air/fuel ratio to function effectively. If the fuel is too much then the engine is termed to be running rich. This can cause you to spend more on fuel while getting low mileage. A closer inspection of the various oxygen and fuel injector sensors could reveal dirt accumulation. You will also notice that the exhaust gases have a pungent smell. Your car cannot pass gas inspections until you solve the problem.

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