Car Running Lean: The Best Way To Fix It

A car running lean can be frustrating, particularly if the problem has progressed to the worst-case scenario. Fortunately, you can easily fix it by reading the DTC codes and replacing or repairing any faulty component. Car Running Lean Check out the details below, including the car running lean meaning and causes of the lean-running condition. We will also mention common symptoms so you can diagnose the issue before it gets out of hand.

How To Fix a Car Engine That Is Running Lean?

To fix a car engine that is running lean, read the DTC codes using an OBD2 scan tool. Once you identify the troubled part, repair it or install a new one. You may also need to do a vacuum-leak test to identify a leaking point and do the necessary repairs. Running lean means that the engine isn’t getting sufficient fuel. In other words, there’s excess air in the combustion chamber, which creates an imbalance. For the engine to run smoothly, it requires the correct air/fuel ratio. If that doesn’t happen, you’ll experience lean or rich running. The most common consequence of a lean-running engine is poor performance. On the other hand, an engine running rich implies too much fuel in the combustion chamber and insufficient air. A dirty air filter is the most notorious cause of this condition. While you can start and drive the car easily, a rich-running state often leads to ruined fuel economy. A rich-running engine can damage the catalytic converter if left unchecked for long, leading to a costly repair. The best way to repair a car engine that’s starved of fuel is to address the possible cause of the problem. There are three main causes of a car running lean:
  • Problems within the fuel system, such as a bad fuel pump
  • An air seepage
  • Faulty sensors
With that said, we will now take you through the details of how to fix a car running lean. You’ll need an OBD2 scan tool to accurately diagnose and repair the problem.
  • Replace Defective Fuel System Components

As mentioned, a lean condition results from insufficient fuel in the engine. Therefore, the first suspect becomes the fuel system. The system comprises the fuel filter, pump, and injectors.
Your engine’s filter is there to remove contaminants from fuel. Over time, dirt and grime accumulate in the filter, causing it to clog. A clogged fuel filter means that the oil won’t pass through effectively. To fix that, you must replace the filter. Replacing Defective Fuel System Components A faulty fuel pump will cut down fuel pressure. The low oil pressure will reduce the fuel flow rate through the fuel injectors, eventually triggering a lean state. A bad pump will also require a replacement. Lastly, you must check the injectors for clogging. If you haven’t changed your filter for a long time, likely, the accumulated dirt has also blocked the injectors, which is affecting the oil pressure (a dirty filter will also trigger a running rich condition). You can remove and clean the clogged fuel injectors yourself (or send them to a fuel injector cleaning service) or buy new and replace them. Take note that you should replace the fuel filter routinely to avoid encountering problems, such as the lean running condition. Experts recommend changing the filter every second time you’re swapping oil.
  • Fix a Possible Air Seepage (Vacuum Leak)

One of the common spots where air leakage can happen is the intake manifold. That’s where the air passes from the outside to the engine. The boost pipes or manifold may be cracked or gaskets damaged, leading to air seepage. An air seepage means air entering the engine unmonitored by the mass airflow sensor. That will cause the engine air fuel ratio to become lean. To fix air seepage, you’ll first need to perform a vacuum leak test to determine where the leak might be. The “how long can an engine run lean?” has crossed many people’s minds. Take note that the poor state causes the engine to heat up too quickly. Continuing to drive when the engine is too hot can cause irreversible damage.
  • Replace Faulty Sensors

Any sensor that measures fuel pressure and airflow, including the coolant temperature sensor, can cause a lean state if it malfunctions. However, you need to focus on the MAF sensor, exhaust gas recirculation (EGR) sensor, and oxygen (O2) sensors. Replacing Car Faulty Sensors The MAF sensor measures the air entering the engine and sends the result to the onboard car’s computer (PCM – powertrain control module). The PCM uses this information to calculate the amount of fuel needed to achieve a correct fuel mixture. Thus, if the sensor fails, it will probably send inaccurate information to the computer, which may create a lead state. An EGR sensor measures the amount of unburnt air entering the exhaust system so that the onboard computer can recirculate it for further combustion. Suppose the sensor is trapped in an open position. In that case, it will trigger symptoms similar to an air seepage as it will allow excess exhaust to return to the engine. On the other hand, the oxygen sensors monitor the amount of oxygen going through the exhaust system. The PCM uses the data to determine how long the injectors must remain open and how much fuel to inject. A faulty oxygen sensor will send inaccurate data, which may force the engine to run lean. Take note that when any of these sensors start failing, the PCM will trigger the check engine light. You’ll need to use the scan tool and the Diagnostic Trouble Codes (DTC codes) to identify and fix the problem. Also, sometimes you can find yourself in an “car running lean no codes” situation. If that happens, you can try to reset the PCM and see if it’s the one to blame.

How To Know When Your Engine Is Running Lean?

To know when your engine is running lean, observe the tell-tale symptoms of a lean state. If your car engine experiences a lean condition, you’ll experience sluggish performance and a stalling engine. The worst part is that your car may have trouble starting and will illuminate the dashboard warning light.
  • Sluggish Performance

You’ll notice a significant difference in the engine’s power output and reduced acceleration. That occurs when fuel injectors fail to push enough fuel into the cylinders due to being blocked or a malfunction on another vital component. Typically, the combustion process is the one that produces power to propel the vehicle. If the engine runs lean, the process is interrupted, meaning the car won’t move as fast as expected.
  • Stalling Problem

As the issue progresses, you may notice that your engine has trouble running. It may sputter and produce other weird engine running lean sounds. Stalling Problem of Car Sometimes you may prevent the stalling by stepping the gas pedal harder, but eventually, the vehicle will fail to keep going.
  • Start-up Trouble

An engine can’t run without fuel or the correct fuel mixture, which happens when it’s lean. At first, it will only be a performance and stalling issue. But as the problem progresses, the engine will eventually fail or struggle to start.
  • The Check Engine Light Turns On

A lean state could result from malfunctioning components, including the fuel system and sensors. And whenever something goes wrong within the engine, the PCM will notify you by triggering the warning light. The light is usually accompanied by a trouble code, making it easier to diagnose the problem using a code reader. The DTC codes will help you determine if the trouble is an engine running lean damage or something else, then act accordingly.
  • Clean Spark Plugs

How often do you examine the spark plugs? If the engine works fine, the plugs should become dirty and appear worn out. However, their look is different when the engine is deprived of fuel. Cleaning Spark Plugs A plug that looks new even when it’s months old may indicate an engine is not receiving enough fuel. As mentioned, if the fuel in the engine burns as expected, it should leave a residue on the spark plugs. For those asking how a car running lean smells, note that it’s uncommon to notice any smell. Usually, the rich-running condition is the one people notice easily, accompanied by black smoke from the tailpipe.

How Can a Loose Spark Plug Lead to a Car Running Lean?

A loose spark plug can lead to various symptoms, affecting engine performance. When a spark plug is not securely tightened, it can result in uncontrolled air-fuel mixture combustion, causing the engine to run lean. This condition leads to decreased power, reduced fuel efficiency, and potentially engine misfires. If you experience any of these loose spark plug symptoms explained, it’s crucial to address the issue promptly to avoid further complications.


You’re now informed about what running lean engine means, its symptoms, and, most importantly, how to fix it. Here’s a summary:
  • A lean state (opposite of running rich) means that there’s not enough fuel in the engine’s combustion chamber.
  • Causes of the condition include a faulty component within the fuel system, air seepage, and defective sensors.
  • To fix the problem, you’ll need an accurate diagnosis and repair or replacement of the defective part.
  • Common symptoms of lean fuel include reduced engine performance, stalling, and problem starting.
We now leave you to use the knowledge you have received to fix your car. Don’t hesitate to call an expert if you need help.  
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