Engine Running Lean (Causes & Symptoms)

An engine that runs lean may sound like a good thing due to reduced fuel consumption, but it can actually damage your engine. Here are the symptoms and causes of a lean running engine

Engine Running Lean

When the engine runs lean, you know it’s not getting enough fuel. This lack of fuel creates many types of performance problems that become more noticeable as the situation gets worse. 

In this article, we look at the signs that your engine is running lean. We also discuss the causes, so you can avoid these problems in the first place. 

What Does it Mean When the Engine Runs Lean?

If the engine runs lean, it isn’t getting enough fuel. To someone that doesn’t understand car engines, it might seem that this is a good thing, possibly even save some money on gas. However, allowing the engine to run lean can lead to serious damage.

When the engine doesn’t get the right amount of fuel, it has to work harder to perform. Left unchecked, the problems only get larger, eventually leading to irreparable engine damage. 

Engine Running Lean Symptoms

The most prominent engine running lean symptoms include lower power output and stalling. It can also lead to trouble starting the engine, issues with the spark plugs and an illuminated Check Engine Light.

Here is a more detailed list of the symptoms of a lean running engine:

1. Poor engine performance

Slow Car Acceleration

The first sign that the engine is running lean shows itself in the everyday performance. There will be a significant drop in the power output of the engine, resulting in a reduction of acceleration.

Fuel injectors must push enough fuel into the cylinder to ensure maximum power output. If there isn’t enough fuel getting to the cylinders, you will see issues with the responsiveness of the car. 

2. Stalling 

Engine Stall

As the problems get worse, you might have trouble keeping the engine running. It’s going to sputter and sound like it’s dying right before it stalls.

In some cases, you may be able to step harder on the accelerator to force more fuel into the system. However, it’s only a matter of time before the vehicle leaves you stranded. 

3. Trouble Starting Engine

Difficulty Starting Car

The engine cannot run if the fuel is withheld. That’s precisely what happens when the engine runs lean. At first, you might notice the performance issues and stalling, but the trouble gets even worse.

Eventually, the car won’t start at all. If you have been experiencing the other symptoms and it turns into having trouble getting the motor turned over, you might have a situation that’s causing it to run lean. 

4. Contaminated Spark Plugs

Spark Plug Misfires

Most people don’t examine the spark plugs, but if you did, you would notice they look different when the engine runs lean. Spark plugs are meant to get dirty and worn.

If you pull one out and it looks brand new or white, it’s an indication that the engine isn’t getting enough fuel. In this case, clean-looking plugs aren’t a good thing. 

5. Check Engine Light

Check Engine Light On Dashboard E1609869927250

When the Check Engine Light comes on, you know something is wrong. However, it can be tough to discern the meaning because this light illuminates for hundreds of reasons. 

When the ECU senses that something is wrong, a trouble code is set and the light comes on. With a code scanner, you can determine if the problem is related to a lean-running engine or something else, so you can take quick action. 

Engine Running Lean Causes

The most common reason an engine is running lean is due to a clogged fuel filter, failing fuel pump or clogged fuel injectors. It can also be a faulty sensor like a bad O2 sensor or MAF sensor sending the wrong information to the ECU.

Here is a more detailed list of the possible causes of engine running lean:

1. Clogged Fuel Filter

The fuel filter is responsible for removing contaminants and dirt from the gas. Over time, the grime and debris build up in the filter, eventually leading to a clog. 

If fuel can’t get through the filter, it won’t make its way to the engine. That’s why it’s important for you to change the fuel filter as part of your regular maintenance. 

2. Failing Fuel Pump

The fuel pump sits in the tank and it’s responsible for pushing the gas into the engine. If the pump fails to work, fuel cannot make its own way into the combustion chamber, causing the engine to run lean. 

Fuel pumps aren’t part of the regular maintenance and can be difficult to replace. That’s why it’s important to keep your tank always more than ¼ full to ensure the pump gets the care it needs. 

3. Clogged Fuel Injectors

If you are regularly changing the fuel filter, the injectors are being cared for. However, when debris and contaminants get into the injectors, trouble is about to follow.

Even the smallest bit of dirt can cause an injector to malfunction. You can try to use a fuel injector cleaner, but if that doesn’t work, it’s time for a replacement. This can be a hefty bill you don’t want to pay. 

4. Bad O2 Sensor

The oxygen sensors tell the ECU how much fuel needs to be added to create the perfect balance with air. However, when the sensors malfunction, the wrong information can be sent to the ECU.

The ECU could trick the vehicle into sending too little fuel because of this faulty data. However, changing out the oxygen sensors is often a simple task. 

5. Faulty MAF sensor

The MAF sensor is measuring the amount of air entering the engine and if it’s faulty it will send the wrong information to the ECU.

This can either lead to a lean or rich mixture, but in most cases a lean mixture.

Engine Running Lean vs. Rich

We covered that a lean-running engine means it isn’t getting enough fuel. This means the combustion chamber contains too much air, creating an imbalance. The symptoms of this problem result in sluggish performance, stalling and even trouble starting the car. 

How is this different from when the engine runs rich? A rich-running engine means that too much fuel and not enough air are in the combustion chamber. In this case, you might be able to start the car but will likely experience a decrease in fuel economy and a strong smell of gas. Over time, this problem will cause the catalytic converter to fail, which is an expensive repair. A dirty air filter is the most common cause for an engine running rich and is a simple thing to fix. 

In either situation, it’s best to repair the problem causing the imbalance right away. Otherwise, you may find yourself with more costly and troublesome issues down the road. 

RELATED: Engine Running Rich (Causes & Symptoms)

Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.