10 Common Causes of an Over-revving Engine
A vehicle’s fuel mixture refers to the ratio of fuel to air in the combustion process. If there is too much fuel and not enough air, the car is considered to be “rich”. On the other hand, if there is too much air and not enough fuel, the vehicle is considered to be “lean”.
In this article, we’ll show you the most common signs that your car is running too rich, so you can fix the problem and avoid wasting fuel (because gas prices are high enough as they are).
Common Symptoms of an Engine Running Too Rich
1) Check Engine Light Comes On
When your engine over-revs, the check engine light will often come on in your dashboard. An OBD2 scanner can show you stored diagnostic trouble codes like P0172, P0175, and P2097. These are all related to excess gasoline getting into the exhaust gases leaving the combustion chamber.
The Engine Control Unit (ECU) receives data from various sensors, including the oxygen sensor, manifold absolute pressure sensor, and mass air flow sensor. These devices monitor the air-fuel ratio, and if they detect a problem, the ECU will turn on the check engine light.
2) Poor fuel economy
If you have to fill up your gas tank more often than usual, your engine may be running too rich. In this condition, your vehicle burns more fuel than necessary to operate properly.
Keep in mind that a slight drop in fuel efficiency is normal in the winter, as cooler temperatures naturally cause your engine to run a little richer. However, a significant drop in fuel economy, regardless of the time of year, may be a sign that your engine is running rich and warrants attention.
3) Strong Fuel Smell
A distinctive fuel smell, especially from the exhaust, is another common sign that your engine is running rich. This happens because excess fuel is not completely burned in the combustion process and escapes through the exhaust system. You may also notice a rotten egg smell, which is usually an indication of a problem with your catalytic converter. Continued exposure to excess fuel can overload this component and eventually cause it to fail, resulting in that distinctive smell.
The high cost associated with replacing a catalytic converter is reason enough not to ignore that your engine is overloaded. What may be a relatively inexpensive solution now could become a costly repair later.
4) Poor Engine Performance
You might think that an engine with too much fuel would perform better. But it’s not. Here’s why: Optimal engine performance requires a precise balance of fuel, air, compression, and spark. Too much fuel in the mixture upsets this balance. The extra fuel can’t be burned completely, causing incomplete combustion.
Your car may feel sluggish when accelerating or have difficulty maintaining speed, especially when driving uphill. This loss of power occurs because unburned fuel impedes the combustion process, preventing the engine from operating at its full potential, and actually reducing overall performance.
5) Rough Idling
Many people may experience a rough or erratic idle when their engine is running too high, meaning the vehicle may vibrate noticeably when not moving.
Keep an eye on your tachometer when the car is stopped; the RPMs may fluctuate erratically instead of remaining constant. This idling unevenness may also be noticeable at times while driving, and you will feel a constant vibration. Excess fuel can disrupt the engine’s normal combustion rhythm, causing irregular vibrations and reducing a smooth and comfortable driving experience.
6) Black Smoke From the Exhaust
One of the most visible signs of an engine running rich is black smoke coming from your exhaust. This again, occurs because the excess fuel isn’t burning completely in the combustion chamber. Dark gray or black spoke coming out of your tailpipe is the result.
While some exhaust is normal, especially on cold starts, consistent dark smoke (thicker when accelerating) is a clear indicator that your engine is receiving more fuel than it can efficiently use. This symptom not only signals an engine problem but can also lead to some embarrassment from those around you on the roads.
7) High Carbon Monoxide Emissions
Even worse than smoke, a rich-running engine produces much higher levels of carbon monoxide than normal. While all gasoline engines emit some carbon monoxide, excessive amounts can be dangerous and often cause a vehicle to fail state emissions testing.
It’s important to note that increased carbon monoxide isn’t the only concern; hydrocarbon emissions also rise significantly with a rich fuel mixture. These elevated emission levels are not only bad for the environment, but they can also pose health risks, especially if exhaust fumes enter the vehicle cabin.
8) Engine Misfires
A misfire occurs when fuel fails to ignite properly in one or more cylinders. It may feel like a stuttering or hesitation when accelerating, or a rough, irregular engine operation. In severe cases, the engine might shake noticeably or lose power momentarily.
Misfires happen because the excessive fuel can foul spark plugs, making it difficult for them to create the spark needed for proper combustion. If simply ignored, persistent misfires can lead to more serious engine damage over time.
9) Difficulty Starting the Engine (Especially When Warm)
An engine running rich can sometimes lead to starting problems, particularly when the engine is already warm. This can happen because the excess fuel can “flood” the engine, making it harder for the spark plugs to ignite the overly rich fuel mixture.
You might notice that your car starts easily first thing in the morning but struggles to turn over after it’s been running for a while. This symptom is often accompanied by a strong fuel smell when you attempt to start the engine. If you experience this issue, it’s best to wait a few minutes before trying to start the car again, allowing some of the excess fuel to evaporate.
10) Clogged/Fouled Up Parts
Over time, an engine running rich can lead to clogged or fouled components. The most common victims are spark plugs and the catalytic converter.
Spark plugs can become coated with a dry, black soot known as carbon deposits, which affects their ability to create a proper spark. The catalytic converter, tasked with reducing harmful emissions, can become overwhelmed by the excess fuel. This may lead to clogging and degradation of its internal honeycomb structure.
In severe cases, you might experience backfiring as the catalytic converter struggles to process the excess fuel.
Causes of an Engine Running Rich
Faulty Oxygen Sensor
An oxygen sensor measures the amount of oxygen in the exhaust and helps the engine control module (ECM) adjust the air-fuel ratio. A malfunctioning sensor can send incorrect data, causing the ECM to add too much fuel.
How to Fix: Replace the faulty oxygen sensor. A diagnostic trouble code will typically be stored and indicate which O2 sensor has failed. Locate the sensor using a model-specific service manual or various free online resources and follow standard replacement procedure (if going the DIY route).
Damaged Fuel Injectors
Fuel injectors can become clogged or stuck open, leading to excess fuel delivery. Wear, debris, or electrical issues can also cause injector problems.
How to Fix: Clean the injectors using a fuel system cleaner (like one of these) if they’re clogged. Stuck or damaged injectors will need to be replaced.
Malfunctioning Mass Airflow Sensor
The mass airflow (MAF) sensor measures incoming air volume. If it reports lower airflow than actual, the ECM will inject too much fuel to compensate.
How to Fix: Clean the sensor using MAF sensor cleaner spray. If cleaning doesn’t work, replace the sensor. Replacement is typically as simple as locating it between the air filter and throttle body, unplugging the electrical connector, unscrewing the sensor, and installing the new one.
Vacuum Leaks
Pesky leaks in the vacuum system can cause unmetered air to enter the engine, disrupting the air-fuel ratio. Often easy to fix, finding the spot of the leak is usually the hardest part.
How to Fix: Inspect vacuum lines for cracks or loose connections. Replace damaged lines and make sure all connections are tight. Use a smoke machine to detect hard-to-find leaks in the intake manifold or other components.
Failing Coolant Temperature Sensor
This sensor helps the ECM determine the engine’s operating temperature. A faulty coolant temp sensor can make the ECM think the engine is colder than it is, causing it to add extra fuel.
How to Fix: Replace the coolant temperature sensor. Drain some coolant, locate the sensor near the thermostat housing, unplug it, remove the old sensor, and install the new one. Refill the coolant and bleed the system.
