Engine Lean Causes & Symptoms

When the engine runs lean, you know it’s not getting enough fuel. Lack of fuel causes many performance problems, which get more severe the worse it gets. 

We will be looking at signs your engine may be running low. The causes are also discussed so that you don't get into these issues. 

How does it look 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. Leaning the engine can cause severe damage.

When the engine doesn’t get the right amount of fuel, it has to work harder to perform. The problems will get worse if they aren't addressed. This can eventually lead to engine failure. 

Engine running lean symptoms

Engine running lean is characterized by lower engine power and stalling. You may also experience problems with your engine's ignition, spark plug problems and an illuminated Check Engine Light.

Below is a list that explains the signs and symptoms of an efficient running engine.

1. Poor engine performance

A decrease in engine performance is usually the first indication that your engine may be running less efficiently. The engine's power output will drop significantly, which can lead to a decrease in acceleration.

For maximum power output, the fuel injectors have to pump enough fuel into each cylinder. If there isn’t enough fuel getting to the cylinders, you will see issues with the responsiveness of the car. 

2. Stalling 

You might find it difficult to keep the engine running as the problem gets worse. It’s going to sputter and sound like it’s dying right before it stalls.

You may be able push the accelerator harder to get more fuel in some situations. However, it’s only a matter of time before the vehicle leaves you stranded. 

3. Problems starting the engine

Without fuel, the engine will not run. That’s precisely what happens when the engine runs lean. The performance and stalling issues may initially be obvious, but then the problems get 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

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 can get dirty or 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. Verify Engine Light

The Check Engine Light will turn on when you suspect something is not right. It can be difficult to determine the cause of this check engine light, as there are hundreds of possible causes. 

The ECU will set a trouble code if it senses something is not right. You can use a code scanner to determine if your problem is caused by a lean-running motor or another issue so that you can swiftly take corrective action. 

Motor Running Lean Causes

An engine running lean most often is caused by a blocked fuel filter, failing fuel pumps or clogged Fuel Injectors. A faulty sensor, such as a bad O2 or MAF sensor that sends the wrong data to the ECU, can cause the engine to run lean.

This is an in-depth list of possible reasons for engine running lean.

1. Blockage of the Fuel Filter

It is responsible to remove dirt and contaminants from gasoline. The filter can become clogged over time as dirt and grime build up. 

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. Failure Fuel Pump

The fuel pump sits in the tank and it’s responsible for pushing the gas into the engine. The pump can fail, and the fuel will not make it into the combustion chamber. This causes the engine to be 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

The injectors will be well taken care of if you regularly change the fuel filter. Trouble can occur when contaminants and debris get in the injectors.

An injector can fail if it gets even the slightest bit of dirt. 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. Sensor for Bad O2

The ECU's oxygen sensors inform it how much fuel is needed to achieve the ideal balance of air and fuel. If the sensors are not working properly, an error can occur and the ECU may receive the incorrect information.

Because of the faulty data, an ECU can trick the vehicle to use too little fuel. It's often easy to replace the oxygen sensors. 

5. Sensor malfunctioning in the MAF

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.

It can lead to either a rich or lean mix, but it is most often a lean mixture.

Engine Running Rich vs. Engine Running lean

We covered that a lean-running engine means it isn’t getting enough fuel. It means that the combustion chamber has too much air. This can lead to a slow performance, stalling or even difficulty starting your car. 

This is different than when an engine runs heavy. Rich-running engines are those that have too much fuel or not enough air in their combustion chambers. This will cause the engine to not start but it may also result in a decreased fuel economy, increased gas smell and reduced performance. The catalytic converter will eventually fail. This is costly repair. An engine that is running too rich is most likely to have a dirty filter. This is easy to repair. 

In either situation, it’s best to repair the problem causing the imbalance right away. If you don't, it could lead to more serious and costly problems later on. 

RELATED: Engine Running Rich (Causes & Symptoms)