Rich, Lean or Stoichiometric

When speaking about the lean and rich mixtures within an internal combustion engine, the term is referring to the fuel to air mixture. When optimal, this mixture demonstrates a ratio of 14.7 parts air to 1 part fuel. The perfect mixture will result in all the oxygen and all the fuel being burned since the amount of air introduced determines the amount of fuel that is consumed in the process. This ideal ratio for the air to fuel mixture is called the stoichiometric ratio.

Of course, this ‘perfect’ pressure can’t always exist in a working engine, which means it’s common to see a mixture that is a little on the richer side, with a higher fuel amount.

What it Means to Run Rich:
Rich Mixture

A rich mixture is more common and not necessarily detrimental to your engine. In the case of a slightly rich mixture, you may notice reduced engine efficiency and worse fuel economy, but the rich mixture will generate more power and actually burn cooler. You may also notice, in this case, that there is a distinct sulpher or ‘rotten egg’ smell that is omitted from the exhaust, and/or black smoke.

The causes of a rich mixture within your internal combustion engine include:

Everyday causes: extremely cold weather, high load on the engine, acceleration
A dirty air filter
Bad airflow sensor
Faulty electronic control unit (ECU) which is responsible for controlling a series of actuators on an internal combustion engine to ensure optimal engine performance.
Injectors that are stuck in the open position
A cooling system that is retrofitted where the thermostat is removed and the fan is running direct)
Poor engine coolant temperature (ECT) sensor
Faulty oxygen sensors
If Your Vehicle is Running Lean

What it Means to Run Lean:
Lean Mixture

A lean mixture occurs when there’s a higher concentration of air to fuel than there should be. When this occurs, your engine will likely still run but will result in jerking motions within the mechanics of the combustion engine which in turn leads to damage to the engine, such as burned valves.

Alternatively, the causes for your engine running lean are likely one of the following:

A vacuum leak
Faulty pump regulator
Poor fuel pressure
Insufficient fuel
The EGR valve is stuck in the open position
Why It’s Important to Maintain the Correct Ratio
Your investment – your vehicle – should be maintained to the point where running rich or lean doesn’t occur often. It should be noted that sometimes vehicle owners will even adjust the ratio of fuel to air in order to maximize mileage or performance, but this should be done carefully. Generally speaking, maintaining the stoichiometric ratio ensures longevity and quality performance within your vehicle.

On a short-term basis, either of these scenarios (running rich or running lean) can result in a condition that causes noticeably low fuel economy or, even more seriously, damage to your engine. Most modern vehicles do have indicators that clearly state if the system is too rich or too lean, but it’s important to keep an eye out for the various elements of the vehicle that can also demonstrate that something isn’t performing as it should be.

Here's why lean engines don't run hot (byStephen Edelstein)
Plenty of myths surround the workings of internal-combustion engines. One is that an engine running lean—meaning too much air is going into the cylinders—tend to run hot. Jason Fenske of Engineering Explained busts that myth in this video.

The ideal ratio of gasoline to air for combustion is 14.7:1, meaning 14.7 parts air to one part gas by mass. A lean mixture contains more air than that, more than can actually be used in combustion. The opposite of a lean ratio is a rich ratio, which has less than 14.7 parts air and thus too much fuel.

Temperatures actually tend to peak with that ideal ratio, decreasing both when an engine is running lean and when it's running rich, Fenske said.

High temperatures also correspond to high levels of nitrogen-oxide (NOx) emissions, one of the main pollutants created by internal-combustion engines, Fenkse noted. The chemical reaction that creates NOx occurs at high temperatures, he said.

Why does running lean or rich cause temperatures to drop? Because of the leftovers. Excess fuel in a rich mixture transitions from liquid to gas, cooling the cylinder as it does. Excess air from a lean mixture won't be burned in combustion, so its presence lowers cylinder temperature. This is the basic principle behind the exhaust-gas recirculation systems some automakers use to reduce NOx emissions, Fenkse noted.

So where did the myth that lean engines run hot come from? Fenske believes people may be confusing engines that run "less rich" with those that run genuinely lean.

When tuning for power, engines are typically set up to run rich, Fenkse said. Again, "rich" is anything with less than a 14.7:1 air:fuel ratio. As the mixture gets closer to that ideal ratio, temperatures tend to increase, along with the chance of knock. The higher risk of knock closer to the ideal ratio, compared to richer mixtures that are further away from that ratio, may be the source of the confusion, Fenske said. Generally, the closer the mixture is to 14.7:1, the hotter the combustion temperatures.