A vehicle’s fuel mixture refers to the ratio of fuel and air in the combustion process. When your vehicle is consuming too much fuel and not enough air, your vehicle is said to be running “rich.”
On the other end of it, when your vehicle is consuming too much air and not enough gas, your vehicle is said to be running “lean.”
This article will show you the most typical signs that your vehicle is running rich, so you can get the problem fixed and not waste fuel on inefficient combustion (as if gas prices weren’t high enough).
Common Symptoms of an Engine Running Rich
#1. Check Engine Light.
When an engine runs rich, chances are good that you may see the check engine light illuminate on your dashboard.
Using an OBD2 scanner, you may retrieve stored diagnostic trouble codes (DTC) like P0172, P0175, and P2097.
All of which are related to excessive gasoline in the exhaust gases exiting combustion chambers.
The ECU (engine control unit) gets input from many sensors, like oxygen sensors, the manifold absolute pressure sensor, and the mass air flow sensor.
These sensors monitor the air-fuel mixture, and when they detect an issue, they enable the check engine light.
#2. Poor Fuel Economy.
If you seem to be filling the gas tank more often than usual, you may have a rich-running engine. A rich condition causes the car to burn more gasoline than is necessary for proper engine operation.
Remember that colder temperatures usually cause engines to run slightly richer than warmer temperatures, so a measured decrease in fuel economy during winter is usually acceptable.
However, a noticeable decrease, in summer or winter, in fuel economy could be indicative of a rich-running engine that should be addressed.
#3. Strong Fuel Odor.
A strong fuel odor, especially from the exhaust, is another common indication that the engine is running rich. If the combustion process isn’t burning all of the excess fuel, it escapes out the exhaust.
You may also notice that the smell of rotten eggs, a lot of the time, means a catalytic converter issue. This component could be working too hard, and as a result, stop working, when it’s constantly exposed to excess fuel, leading to a rotten egg smell.
The expensive nature of catalytic converter replacement is as good a reason as any not to ignore a rich-running engine. What may be a relatively easy fix may turn into a costly repair.
#4. Poor Engine Performance.
You would think that when an engine is receiving too much fuel, you would have better performance. You would be wrong, and here’s why.
To achieve optimal engine performance, it requires a balance of fuel, air, spark, and compression is required. When an engine receives too much fuel, it doesn’t stay balanced, and too much fuel leads to incomplete combustion, whereby the fuel can’t burn enough or even completely.
You may also notice that your vehicle feels less responsive when you hit the accelerator, or when it has trouble holding or maintaining speed, like when you go uphill.
The aforementioned performance hit happens because the unburned fuel is preventing the engine from being able to combust the fuel, and thereby not letting the vehicle achieve its full performance potential, and actually reducing power.
#5. Rough Engine Idle.
Many people will feel a rough or uneven idle when they experience a rich condition. This means that, depending on the severity of the over-fueling, your vehicle may vibrate noticeably at idle, even when it’s not moving.
You may see your tachometer moving up and down when it’s stationary, instead of a steady RPM value. Depending on the severity of the rough condition, the rough idle may be felt while driving, essentially a constant vibration.
Again, the excess fuel is preventing the engine from returning to a regular combustion cycle, which leads to these uneven vibrations, making the driving experience less smooth and less comfortable.
#6. Black Smoke from Exhaust.
One of the most noticeable symptoms of running rich is black smoke from your exhaust. This is again due to the excess of fuel not burning completely in the combustion chamber, thus the dark gray or black smoke coming from your tailpipe.
Some exhaust is normal on cold starts, but consistent dark smoke (which thickens when accelerating) means your engine is getting more fuel than it can use effectively.
This symptom is not only alarming as an engine problem, but it could also cause some embarrassment from the people surrounding you on the road.
#7. High Carbon Monoxide Emissions.
It would be negligent if we did not mention that a rich running engine produces carbon monoxide and runs significantly higher than normal. Every gasoline engine produces some carbon monoxide, but excessive CO is bad and could even cause a vehicle to fail state emissions testing.
It is also important to mention that not only does carbon monoxide, but it is customary for hydrocarbons to emit much higher amounts in a rich fuel mixture.
Not only do these higher emission values damage the environment, but the emissions also create health concerns because they are often expelled in the cabin area and could be inhaled along with the regular air.
This air quality concern with harmful exhaust fumes justifies higher emission level limits not being allowed.
#8. Engine Misfires.
Misfires are when there is an improper ignition of fuel in one or more cylinders. You might feel this as stuttering or hesitation when you accelerate. You may also be able to detect rough/intermittent engine operation. In extreme cases, the engine may shake or temporarily lose power.
Misfires are caused because excessive fuel can foul out spark plugs, degrading their ability to produce the spark that is necessary for proper combustion. If the misfires are diligently ignored, this could develop into worse damage to the engine over time.
#9. Hard Starting (Especially When it’s Warm).
An engine running rich can lead to starting problems sometimes, especially when the engine is warm. This is because the excess of fuel can “flood” the engine, which reduces the ability of the spark plugs to ignite the rich fuel mixture.
You may start to notice that it’s relatively easy to start up your car in the morning, but when it’s been running for a while, it may not want to turn over or start again. You will notice when you attempt to start your vehicle that many times there is a distinct smelling scent of fuel.
If you notice this symptom, try to wait a few minutes before you attempt to start the car again, which gives some of the excess fuel time to evaporate.
#10. Clogged/Fouled Up Parts.
One of the results of running rich is clogged or fouled-up parts. Over time, being a vehicle owner, this will happen. The two commonly fouled-up components are spark plugs and the catalytic converter.
In the worst cases, spark plugs might receive black soot (carbon deposits) on them, and this will interfere with their ability to provide a proper spark.
The catalytic converter also wants to reduce harmful emissions, but excess fuel could choke it out and eventually clog/degrade the internal honeycomb.
The excess fuel could create a backfiring condition if the catalytic converter cannot process excessive fuel in extreme situations.
Reasons for an Engine that is Running Rich
Faulty Oxygen Sensor
An oxygen sensor measures the amount of oxygen in the exhaust to help the engine control module (ECM) adjust the air-fuel ratio. If the sensor is defective, it can send incorrect information to the ECM, which may use too much fuel.
How to Fix: Replace the defective oxygen sensor. In most cases, a diagnostic trouble code will be stored that tells you which O2 sensor has failed.
For instructions on how to locate the sensor, you can use your specific model/service manual, or many freely available online resources, and perform the replacement according to standard practices (if you choose to do it yourself).
Faulty Fuel Injectors
Fuel injectors are sometimes clogged or stuck open, which causes them to inject too much fuel. Clogged fuel injectors can have other causes, such as wear, debris, or electrical issues.
How To Fix: If the fuel injectors are clogged, you may be able to clean them using a fuel system cleaner (like these). Stuck or disassembled fuel injectors should be replaced.
Malfunctioning Mass Airflow Sensor
The mass airflow (MAF) sensor determines the volume of air coming into the engine. If the MAF sensor is reporting lower air volume than what is actually coming in, the ECM will compensate by injecting more fuel.
How To Fix: You can clean the MAF sensor by using MAF sensor cleaner spray. If cleaning does not work, you will need to replace the MAF sensor.
Replacing the MAF sensor typically is not complicated. It will be located between the air filter and the throttle body. You will need to unplug the electrical connector and remove the sensor from its place, then install the new one.
Vacuum Leaks
Pesky vacuum leaks can allow unmeasured air to enter the engine and subsequently affect the air-fuel ratio. Leaks can sometimes be very easy to fix, but checking the location of the leak is probably the worst part.
How to Fix: Look over the vacuum lines for cracks or loose connections. Replace any damaged lines and make sure all connections are tight. It may be very difficult to find a leak that is taking place in the intake manifold or other components, but a smoke machine can help you find these leaks.
Bad Coolant Temperature Sensor
The coolant temperature sensor needs to give the operating temperature of the engine, so the ECM knows what temperature it needs to inject fuel. A defective coolant temperature sensor can make the ECM think the engine is cooler than it is, which may cause it to inject more fuel.
How to Fix: Replace the coolant temperature sensor. You will need to drain some coolant, locate the temperature sensor (it’s usually near the thermostat housing) unplug the cool temp sensor, remove it, replace it with the new one, and refill with coolant and properly bleed the cooling system.