Engine Running Lean – Too Much Air, Not Enough Fuel?

Every internal combustion engine for the last 150 years works on a similar principle, as there are two key ingredients to its functioning… Some sort of fuel (today, it would mostly be gasoline or diesel), as well as fresh air to combust with. Those, along with some compression to pressurize the ignition, and a spark or light of sorts to catch it on fire. But, what happens then, with an engine running lean?

In automotive terms, and especially concerning internal combustion engines, there are two that you have to remember. This would be “rich” and “lean”, both of which signify the mixture of the fuel and air inside your combustion chamber. And, both of which, if left to ignite (either rich or lean) on their own for long, isn’t ideal. So, an engine running lean isn’t optimal, though it’s not as catastrophic.

“Lean” would be the opposite of “rich”. Whereby, your air-to-fuel ratio contains far too much air and not enough fuel in the mix. With there not being enough fuel to burn, which is the primary source of power for any engine, you’ll certainly notice a drop in performance. That goes alongside challenges in your car’s driveability, and compromises elsewhere on reliability, with an engine running lean.

• What Is AFR?
• Causes
• Symptoms
• Side Effects
• Diagnosis
• Solutions
• Final Thoughts

What Is An Air-To-Fuel Ratio (AFR), Anyway?

To ensure thorough and complete combustion, every engine has an air-to-fuel ratio. This goes even with an engine running lean, as it too has an air-to-fuel ratio, but is somewhat skewed. And AFR means the amount and more specifically, the ratio, of air and fuel being mixed together inside an engine for combustion to occur. In most vehicles, the ideal ratio follows the stoichiometric value of 14.7:1.

In other words, that’s 14.7 parts (or grams) of air that’s needed to burn with each 1 part (or gram) of fuel. An engine’s central command module, the ECU or ‘engine control unit‘ (aka the ECM for ‘engine control module’, or PCM for ‘powertrain control module‘) carefully manages this ratio. It does so via a network of sensors, first by measuring the rate of airflow (volume), air pressure, temperature, etc.

In doing so, your ECU knows how much air is intaking into the engine. Now, it could carefully control the amount of fuel to inject (through electronic fuel injectors or EFI), to even out the air-to-fuel ratio. The sensors include the mass airflow (MAF) sensor, oxygen (O2) sensor, manifold absolute pressure (MAP) sensor, barometric air pressure (BAP/BARO) sensor, intake air temperature (IAT) sensor, and more.

Combined, the ECU now has a good idea of much fuel to pump into the engine. For example, let’s say that there are 29.4 grams (14.7 x 2) of air that’s bound for the combustion chamber. The engine knows that it should correspondingly inject 2 grams of (gasoline) fuel for a complete burn. And, it will do so to provide as much performance as possible, with minimal emissions and maximal fuel economy.

Is Air-To-Fuel Ratio The Same For Every Vehicle?

A common misconception held by many is that the air-to-fuel ratio is fixed, across all vehicles around the world. However, this is far from the truth, as AFR actually varies from engine to engine. That so-called “ideal” 14.7:1 ratio most applies to gasoline-powered cars. And even then, some gas engines might be tuned from the factory (by the automaker, to OEM specs) to have a unique AFR, instead.

The stoichiometric value of 14.7:1 is only theoretical and is a baseline figure that automakers would try to get close to. But otherwise, they’ll fine-tune these engines with their own AFRs that suit what cars they’re building. Since we can’t evaluate each vehicle on Earth, we could at least compare those AFRs in commonly-used fuels that are available for cars today. And yes, this includes biodiesels mixes:

• Gasoline – 14.7:1
• Diesel – 14.5:1
• Methanol – 6.47:1
• Ethanol – 9:1
• Butanol – 11.2:1
• Propane – 15.67:1
• Methane – 17.19:1
• Hydrogen – 34.3:1

For any combustible fuel, the principle remains the same, then. Besides that, we also have to look at the engine it runs on:

• Spark Ignition (SI) Engines – This mostly applies to gasoline engines, where combustion is attributed to a spark from the spark plugs. Gas engines can run with an AFR as low as 6:1 (very rich) to as high as 20:1 (very lean). As the operation of the engine changes (acceleration, deceleration, etc.), the AFR changes, too.
• Compression Ignition (CI) Engines – On the other hand, this applies to diesel engines, where you need to rely on compression to heat up the air-fuel mixture. Diesel engines have an AFR between 9:1 (very rich) to 70:1 (very lean). Unlike gasoline engines, diesel engines aren’t as sensitive to AFR and could run even while overly rich or lean.

Does The Air-To-Fuel Ratio Remain Constant?

Here too, it’s surprising to learn that the engine doesn’t force the AFR to run in its most ideal ratio. In fact, it varies throughout the day, as your engine carefully flip-flops between rich and lean nearly all of the time. Quite simply, it’s difficult to maintain a perfect air-to-fuel balance. Therefore, when you start your car up in the morning, the ECU might default to running the AFR in an ‘open loop’.

In an ‘open loop’, the AFR remains fixed, is left unmanaged by the ECU, and tends to run on a richer mixture (a lot more fuel than air). This is necessary, as most of the sensors that the ECU relies on to inform itself of the AFR hasn’t yet warmed up or is operational. It might take a few minutes for this to happen. All the while, you’ll notice higher emissions and fuel consumption in an ‘open loop’.

Once those sensors (MAF, O2, MAP, BARO, IAT, etc.) are finally up and running, the ECU can start to take measurements of oncoming air. Subsequently, balancing the amount of fuel it actually needs to inject into the engine. This is what we call a ‘closed loop’ environment. Here, the engine will carefully flip-flop (more on that later) the air-to-fuel ratio to match the engine’s needs and its current state.

With most modern cars, those abovementioned sensors have been designed to warm up quicker. It’s to make sure that your engine can spend as little time as possible in ‘open loop’, before transitioning to a ‘closed loop’. With the latter, the ECU has more granular control over the AFR. For example, and where necessary, it can pump in less fuel to create an engine running lean to lower your MPG.

How Does Your Air-To-Fuel Ratio Vary?

So, how does a ‘closed loop’ affect and vary your engine’s air-to-fuel ratio, then? Throughout a day’s worth of driving, the ECU seamlessly alters the AFR at will. In differing scenarios, it’ll look something like this:

• Starting Up (Rich – As Low As 9:1) – When you’re just about to start the engine, most of its internals will naturally be quite cold. For now, more fuel needs to burn inside the engine to bring them to life, thus leading it to run very richly for a few moments.
• Warming Up (Rich – Around 12:1) – As the engine is starting to warm up and left to idle, you’re still in the process of heating up much of the engine to its operational temperature. More fuel is required for a few more minutes, while the engine still runs relatively rich.
• Accelerating (Rich – Between 11:1 To 13:1) – When you’re on the throttle pedal to accelerate hard, it would prompt more air to enter the engine. Consequently, it also requires more fuel to burn with. It still runs quite rich, as low as around 11:1 at full throttle, or closer to 13:1 with modest input.
• Under Load (Rich – Around 12:1) – With more load on your car, the engine of course has to work that much harder. For example, towing a trailer, going up a steep incline, or ferrying a lot of passengers. With the need to create more power, your engine will start running richer.
• Steady Cruising (Ideal – Around 14.7:1) – This is when your engine is biased more towards that ideal AFR of 14.7:1. Here, your engine is already warmed up, under steady load, and without any excessive input on the throttle.
• Decelerating (Lean – As High As 17:1) – As you’re slowing down, you’re most likely keeping your foot off the throttle pedal. In this regard, the engine isn’t creating any power, so no fuel is burned other than what’s needed to keep the engine running. You’re met with an engine running lean.

What Causes An Engine Running Lean?

An engine running lean can be caused by a myriad of faults within your car, both electrical as well as mechanical in nature. In all, the reasons for an engine running lean include:

• Clogged fuel filter, which is filled to the brim with impurities and gunk, that it’s impeding circulation and supply of fuel into the engine.
• Bad or clogged fuel injectors, and similarly, could be blocked quite easily by debris and contaminants that prevent it from injecting fuel into the engine.
• Faulty fuel pump, which should otherwise be responsible for ensuring that sufficient fuel is supplied to the engine. Now, it’s unable to pull any fuel from your gas tank at all.
• Leaking fuel lines or a bad fuel pressure regulator, as this could cause a loss of fuel pressure, as well as cut the supply of much-needed fuel from the engine.
• Bad oxygen (O2) sensor, as the ECU relies on it to know how much unburnt O2 there is in the exhaust fumes. If it fails or is measuring falsely, it might prompt the ECU to pump too little fuel, instead.
• Vacuum leaks near the intake manifolds, which might force unmetered outside air to start rushing into the engine, thus leading to an influx of excess oxygen.
• Broken exhaust gas recirculation (EGR) system, which could lead to far too many exhaust fumes to be pumped back into the combustion chamber for a secondary burn.
• Bad or dirty mass airflow (MAF) sensor, as it could incorrectly report to the ECU on air volume and airflow rate. This leads the ECU to falsely supply too little fuel.
• Carbon build-up on the intake valves and ports, as this might block off or impede the flow of fuel into the combustion chamber.

What Are The Symptoms Of An Engine Running Lean?

So, how can you tell if you have an engine running lean? Well, there will be an impact on your car’s performance and driveability, along with these other common symptoms:

• Check engine light (CEL) appears on your dashboard, as the ECU has an abundance of sensors that’s likely able to sense an engine running lean before you even get started. Should one of the sensors mentioned earlier have failed or if there’s a fuel delivery issue, the CEL will light up.
• Difficulty starting up your car or getting it cranking, as there’s likely too little fuel in the engine to heat it up or to maintain each combustion cycle.
• Poor performance and hesitation or struggling while accelerating, as the engine isn’t getting enough fuel needed for more power and responsiveness.
• Shuddering, sputtering, engine cutting in and out, or stalling while driving. This, once again, is due to there not being enough fuel to burn to keep your engine going. You might still be able to compensate by pressing harder on the throttle pedal, but it’s still not enough.
• Rough idling (akin to sputtering and shuddering), as the engine is still devoid of the fuel it needs. You might also notice the RPMs bouncing up and down.
• Spark plugs are still clean or lack any burn marks. They naturally dirty up with repeated combustion cycles, but as there is never enough fuel with an engine running lean, the spark plugs still look fresh. This is the opposite of overly rich mixtures, where that’ll leave scoring and carbon on the plugs.
• Your engine takes longer to heat up or get up to its operating temperatures. With less fuel to burn, each ignition is smaller and less explosive, thus without substantially heating up the engine.

What Are The Side Effects Of An Engine Running Lean?

So far, we already know that an engine running lean will lead to difficulties with performance and driveability. Your car isn’t as sprightly, nor could it maintain sufficient speed and power for long. Plus, you’ll have trouble even getting it going. Nevertheless, at least an engine running lean (when it’s compared to rich) won’t cause a substantial increase in tailpipe emissions or fuel consumption.

With that in mind, are there any other consequential side effects of an engine running lean? Let’s say you’re planning on repairs, but would prefer to put it off for a bit longer… Is this a mistake? Well, in a word, yes. It’s a bad idea to keep your engine running lean for too long. Although, it’s still not as bad as an engine that’s running overly rich. The latter could have grave consequences for the engine.

Still, this doesn’t mean that you should ignore an engine running lean. It too could leave permanent scarring and internal damage, if not fixed in time. The side effects include:

• With less fuel inside the combustion chamber, it deprives the engine of fluids for internal lubrication. This could accelerate wear and tear as there’s now more friction with the engine’s moving parts.
• Engine seizures, due to an imbalance in internal temperature. The pistons might expand rapidly and grind on the cylinder walls. Or, the connecting rods could bend, while the crankshaft arm might snap off. Either way, it’ll require a thorough (and expensive) engine rebuild or outright replacement.
• A build-up of “dry spots” in the combustion chamber where there’s a lack of lubrication (from the fuel). This could cause inner moving parts to grind on each other, leaving friction marks and burning. Your engine might still run okay after this, but not optimally so.

How Can You Diagnose An Engine Running Lean?

Owing to the sheer number of possible points of failure that could cause an engine running lean, it’s challenging to diagnose it. However, there are techniques that you can practice to at least get close to narrowing down what’s caused your engine to run lean:

1. First, we can start by grabbing an OBDII diagnostics tool, and scanning the ECU for error codes. This is applicable if you’ve seen the check engine light come up. OBD codes can massively cut down on time spent for diagnosis by immediately pinpointing what’s wrong. Is there a faulty sensor, vacuum leak, or bad fuel system somewhere?
2. Once the OBD scanner has found the faulty component, you can visually inspect it. In particular, and this relates to sensors, see if there’s any fraying or damage with the wiring. Is there corrosion, open- or short-circuit, or a loose connector? On top of that, you can try unplugging the sensors (MAF, O2, MAP, BARO, etc.) to see if it’s been clogged up with debris or if it’s faulty.
3. You can then diagnose the fuelling and intake systems. Start by pulling out the fuel injectors, and see if there’s any clogging there. You could also try swapping around fuel injectors, to see if one or more of them have failed at once. After that, make sure there isn’t a vacuum leak by the intake manifolds. While you’re here, look around to make sure there isn’t a fuel leak somewhere.
4. If all you have is clogged-up or dirty sensors, fuel injectors, and whatnot, they can be cleaned up. It’s cheaper and simpler than having to replace them entirely. You could purchase special MAF sensor cleaning solutions (or perhaps just use an electronic parts cleaner). There are also plenty of fuel injector cleaners, too.

How Can You Solve An Engine Running Lean?

As we’ve discovered already, an engine running lean can be remedied by patching up a leaky fuelling system, faulty sensors, or a vacuum leak. As mentioned, some of these components could be fixed or cleaned (for debris or clogging). But if the parts have failed altogether, it needs replacing:

• Vacuum Leaks – If it’s the intake manifold’s gaskets, then it’ll cost you between $200 to $350. If the entire manifold needs replacing, you could be looking at a cost of around $300 to $700. Should the vacuum hoses be at fault, replacing them will set you back $250 to $400. Meanwhile, an EGR valve will cost you between $350 to $500.
• Fuel Filter – A new fuel filter will cost around $100 to $200. You may be able to get it for cheaper, at under $100.
• Fuel Pump – Replacing this is expensive, between $200 to upwards of $1,000. It’ll mostly depend on how accessible the pump is, upping or lowering the labor costs heavily.
• Fuel Injectors – If you’re unable to clean it, a replacement costs between $800 to over $2,000. It could sometimes be as low as $200 for just a single injector, but mechanics typically recommend swapping them all out at once.
• Fuel Lines – Depending on how significant the leakage is, it may cost you as little as $30 for a minor repair. Or, up to $120 for heavier leaks.
• Oxygen (O2) Sensor – On average, you’re looking at $50 to $300, depending on its accessibility. But for severe cases or based on the make and model of your car, it might run up to $500.
• Mass Airflow (MAF) Sensor – In some cases, you can get away with paying as little as $100. Hard-to-reach MAF sensors might drive up the labor costs though, closer to $300 or more.

Final Thoughts

That then is a great place to round up our thoughts on an engine running lean. For the most part, an engine running lean isn’t as damaging or catastrophic to your car as it running overly rich. The latter could result in serious internal wear. Not to mention spewing out ample pollution, and impacting the fuel economy adversely. On the other hand, an engine running lean isn’t as bad, but it’s still not ideal.

You’ll surely begin to notice a drop in performance, poor acceleration, and a loss of power. And, that includes difficulties in your engine to even turn over, and subsequently, keep it running. While it might not seem serious at first, ignoring a lean-running engine for too long could still be damaging. At least fixing it (no matter how costly or complex) is still way cheaper than letting it ruin the engine.

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