Diagnosing an engine running rich is often pretty easy. But diagnosing an engine running lean can be a bit more difficult. But just because it’s more difficult to figure out the cause doesn’t make it less serious.
If you never take the time to fix an engine running lean, you could cause unrecoverable engine damage and cost yourself thousands of dollars!
So, what is a mechanic saying when they tell you your engine is running lean, and what can you do about it? Keep reading as we unpack it all.
What Does “Running Lean” Mean?
When a mechanic tells you your engine is running lean, they are referring to a lack of fuel reaching the engine. While we know you might think this is fine, as you’ll save a little money on fuel, an engine that is running too lean is a very serious condition that should be addressed immediately.
If your engine is not getting enough fuel, you will notice performance issues, but the bigger problem will be explosive!
Your engine is operating under too much stress. The fuel-to-air ratio is not correct, and your engine is working much harder than it needs to for each cycle. And it is only a matter of time before your added stress creates a much larger problem.
Symptoms of a Car Running Lean
How do you identify whether your vehicle is running lean? With a problem somewhere you might notice, however, there are a few other things to watch out for so you don’t cause damage, potentially to your engine!
#1. Lack of Power and Poor Performance.
Even if you can get your vehicle in to started, if the engine is running lean, you will see poor performance and decreased power. It is about more than throwing fuel in the vehicle to maximize performance.
The fuel injectors have to “squirt” the appropriate amount of fuel into the cylinder each cycle to use as much fuel as possible for maximum efficiency and power.
If the engine is running lean, you probably will notice its lack of acceleration and responsiveness than what you had before.
The reason is simple; in either a diesel- or gas-powered vehicle, the explosion of the fuel needs to generate the force that drives the piston back up.
The explosion of explosion drives the piston back up, and is where your engine generates its speed or power. If the engine does not have enough fuel, the explosion is not as big, generating a slower return ceiling of the piston.
#2. Hard Starting Car.
For your vehicle to run, the engine needs fuel. Without fuel, you won’t be going anywhere. So it is a common symptom of an engine that’s “running” lean that it won’t run at all!
#3. Clean or White Spark Plugs.
When everything is functioning correctly, it gets dirty. When you pull one of your spark plugs, and it’s white or looks brand new, you probably have a problem. Normally, you’d think you’ve got faulty spark plugs if they’re dirty, but that’s not always true.
The fuel should be burning, and the burning will leave some deposits on your spark plugs. The older your spark plug is, the more deposits should be present.
#4. Check Engine Light Is On.
Manufacturers load modern engines with various sensors. And when those sensors detect something abnormal, they send a signal to the ECU, and your ECU triggers the check engine light to signal to you that something is wrong.
It probably won’t say you’re running lean or anything, but it will give you a better idea of what is going wrong.
Maybe you have a check engine light for bad O2 sensors or low fuel pressure. Either way, with your check engine light, you can follow a direction at least!
#5. Stalling Engine.
So maybe you get your vehicle running, but because it’s running lean, you are going to have trouble keeping it running. It will sound horrible, and it will sputter and die. This will be worse when idling.
It is easier to keep the engine running when you are adding more fuel to the system with your foot on the accelerator pedal. But you still don’t get the power output you need.
Common Causes of a Car Running Lean
While identifying that your engine is running lean is an important first step, knowing the cause is just as important. Many issues can lead to a lean engine operation, but some are more common than others.
Here is a list of common causes and how you can fix them yourself!
#1. Clogged Fuel Filter.
Your fuel filter does all the dirty work, and we mean dirty. This filter cleans out all of the dirt, grime, gunk, and everything else that has gotten into your fuel tank. As the filter is burned through, it slowly builds up this gunk and reduces the amount of fuel that is allowed to pass through.
As the filter will let fuel pass through perfectly fine, the junk cavitates or resides in the filter, and your fuel can no longer get past that. This is one of the major reasons you should regularly change your fuel filter.
#2. Clogged Fuel Injector.
Another reason you should be replacing your fuel filter regularly is to protect your fuel injectors. Fuel injectors are finicky, stubborn beasts, and unlike fuel filters that can endure a lot of foreign gunk in your gasoline, fuel injectors will ruin your day, as you need your fuel injectors to get fuel into your combustion chamber.
Once they are clogged, they will not let enough fuel pass through. If this happens, you will need to clean your injectors, and if you are unlucky, have them replaced.
Cleanings are relatively cheap, but replacements aren’t cheap.
#3. Bad Fuel Pump.
If your engine is running lean, it’s because you aren’t getting enough fuel into the combustion chamber. Getting the proper fuel into the combustion chamber all starts at the fuel pump.
If it’s clogged or isn’t able to be able to pull enough fuel out of the tank anymore, the end result will be low fuel pressure and a lean-running engine.
If you have to replace your fuel pump, it won’t cost you much, but it’s not cheap, and it’s not easy to do if you don’t have the right equipment.
This certainly is a leading cause for a lean-running engine, but it’s not what you are hoping for since it will usually cost you $400 – $600 to replace.
#4. Bad Oxygen Sensor.
Due to having so many outputs controlled by sensors in modern vehicles, one of the most common reasons for a lean-running engine is that you have a bad oxygen sensor or a bad air-fuel sensor.
Your vehicle’s ECU receives input from the oxygen sensors to modulate the amount of fuel to each cycle. If the faulty oxygen sensor is misreading excessive emissions, then it’s sending messages to the ECU to send less fuel into the combustion chamber!
No matter how little fuel it sends, it’s always reading excessive emissions since it is broken.
The good news is that replacing oxygen sensors is generally easy enough for anyone with average mechanical skills. If you’re running and have the problem of a lean running engine, this is certainly the problem I hoped to encounter.
How long can an engine run lean?
An engine can run lean (too much air and not enough fuel in the air/fuel mixture) for varying periods, depending on the severity, before damage to the engine begins:
- Mildly Lean – An engine can run mildly lean for an extended period, often 100,000+ miles, before having major issues. Mildly running lean is usually just lost power, a rough idle, and then eventually fouled spark plugs over time. It is an incremental wear, though.
- Severely Lean – Highly lean mixtures can cause major engine issues almost instantly, along with overheating, backfiring, seizing, and melting pistons within minutes or hours. Lean extremes well over 18:1 ratios can start melting and destroying critical engine parts immediately (worst case).
- Moderately Lean – At a moderate lean state, like around 16:1 air/fuel ratios, the engine runs rough, much quicker, and pre-detonation can occur. Piston and valve temperatures rise quickly, and irregular wear occurs for internal components. This sort of moderate lean condition starts wearing rings, bearings, and valve seals within 15,000 to 30,000 miles.