car running rich

Engine Running Rich – Symptoms, Causes & Fixes

In Engine by Magnus Sellén8 Comments

Internal combustion 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.

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

However, inefficiencies can cause more fuel to be 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 by having sensors on the fuel injectors, airflows, and the emission points.

Causes of a Rich Mixture

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 possible causes of a 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 suggesting a 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 calculates the air-fuel mixture that should be added by measuring the amount of air which is entering the engine. If this is dirty or failed, it will cause the engine to run too rich or too lean. 

SEE NEXT:  What is Piston slap? - Information & Fixes

3. Faulty MAP Sensor

The MAP sensor calculates the air-fuel mixture based on the air pressure in the intake manifold. An MAP sensor is often installed instead of an 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. It is the job for the engine coolant temperature sensor to measure the temperature of the coolant to identify when it should add extra fuel to the engine. If the engine coolant temperature sensor is faulty, you may get a mixyure which is too rich.

5. Faulty 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 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 mixture which is too rich. You will 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 do not cause a rich mixture. However, they could cause misfires which will result in a 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 one running rich. In this instance, there is more air than fuel being supplied to the combustion chambers. Just like when the engine is 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 can 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 the air/fuel mixture is adjusted.

SEE NEXT:  Subaru Head Gasket Problems & Symptoms

If the sensors collect dirt then they cannot remit the right information; hence, a reduction of fuel is 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 these can leak, leading to 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 rich

If 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 will all 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 this fuel 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.

SEE NEXT:  Types of Replacement Engines Described (Remanufactured, Used, Rebuilt, Crate)

When you have problems with the air/fuel ratio then you will 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 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 there is too much fuel, 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.

8 thoughts on “ Engine Running Rich – Symptoms, Causes & Fixes ”

  1. Thank you, Magnus! You made an easy to understand article for someone who doesn’t know a ton about cars. Now I know how to talk to my mechanic about this problem. Thanks again!

    1. thanks a lot for the information.
      please note that i have a JKU jeep 2012 and have a heme engine installed and the car keep giving me this error rich fuel p0172 and p0175, i have done all of what you have said but unfortunately i am still getiing those codes.
      the only thing i have not done is check the fuel pump.
      I even changed the gas cap, replaced the
      1-MAP sensor,
      2-coolant sensor
      3- PCM
      4- Purge valve
      5- coils
      6-spark plugs
      7-checked the catalytic converters
      8-replaced oxygen sensor
      9- the next step is to custom programming for the idle as i get a lot of vibration when idle and the jeep only get this code after i drive about 10 miles then i stop for some time while on idle then i get those codes.
      under the hood it is very very hot and the fuse box get also very hot
      any idea what could it be

  2. I have a 1991 Firebird 5.7 TPI.And I have the following problem.When I start the car it ideals rough,running rich that I can smell the fuel,And it smokes because to much fuel. I have replaced the following parts
    2.Fuel regulator
    3.spark plugs injectors air filter
    6.New coolant sensor
    7.New vacuum hoses
    8.But still the same problem.
    9.Ran a scan test to get trouble codes no trouble code from ECM

  3. I have a 2005 honda pilot I recently started having problems with air and fuel mixture…
    When starting sometimes its hard to that it’s on will run maybe a minute or two than all hell brakes loose…truck shake spits sputter smoke..smells a lot of gas and black droplets on driveway..try to give it gas bogs down then shuts off and will not start for at least 20mins or more…
    What do you think???

    1. This seems like the same problem I’m having with my truck. I have a 2009 Dodge Nitro R/T 4.0L engine. It burns gas extremely bad (was getting 19-20mpg now done dropped to a terrible 8, yes 8mpg!) and leaves a black spot behind it coming from the exhaust pipe. It also has a rough idle and vibration coming from the steering wheel. Now it’s done started backfiring. I have no clue what’s wrong with it.

  4. Thanks for the in-depth article. I have a Ford Expedition 2007 with 114000 miles on it. I changed the plugs and coils at the prescribed 100K, but since then, spark plugs tips have been breaking off. I initially bought Motorcraft spark plugs and coils through Amazon. After changing them out, the vehicle ran very rough, so it took it into the nearest Ford dealer. They said one of the plugs was defective and replaced it. Some months later, the vehicle showed the same symptom, so I had the engine computer read. It read misfire, so I took it into a local mechanic who said that it was the wrong spark plug. It’s tip, just the electrode, had broken off. I assumed at this point, that I had bad plugs from Amazon. A few months later, another plug tip broke, so I replaced the remaining plugs I had purchased from Amazon and replaced them with plugs I bought from my nearby auto supply shop. A month or so later, I had the same symptom and one of the new plug tips had broken. I took the broken plug to the nearby Ford dealer to confirm that the plug was the correct one. He did note that there seemed to be more carbon buildup on the broken plug than normal. Do you have any idea what might be causing these plugs to break? It seems to be occurring in different cylinders after at least 1000 miles after installation.

  5. Due to very high demand and a high amount of comments, you might have trouble getting your comment answered by me. If you want to get fast answers from a certified automotive technician you should ask your questions here: Ask A Mechanic

Leave a Comment