If you've ever attempted to diagnose a rough engine idle, loss of power, or sudden drop in fuel efficiency, then you may have run across the concept of metered air. This concept can seem foreign if you aren't familiar with the workings of an internal combustion engine, but it is crucial to the behavior of modern vehicles. Many common problems in modern cars can be traced back to an issue with unmetered air entering the system, so understanding what this means and how it can affect your car is a critical part of diagnosing many problems.
The Importance of Air to Internal Combustion Engines
Air is one of the four critical ingredients than any internal combustion engine needs to function. Air and fuel combine in your engine's combustion cylinders, where they are compressed and ultimately ignited by an arc from your spark plugs. Without air, your fuel cannot burn, and your engine would be unable to produce any power. Air is so crucial that many mechanics flippantly refer to internal combustion engines as glorified air pumps that use sophisticated engineering to pull air from the atmosphere into the combustion chambers.
There's more to efficient combustion than simply dumping air and fuel together, however. For fuel to burn completely, the ratio of air to fuel that enters your engine's cylinders must be perfectly balanced. This balance is known as the stoichiometric ratio, and for gas engines, it is 14.7 parts air to 1 part gasoline. Your car's computer attempts to maintain this balance most of the time, although it occasionally allows the engine to run rich (more fuel than ideal) under specific circumstances.
So… About That Metered Air?
With fuel-injected engines, it is easy for a vehicle's computer to control the amount of fuel that enters the combustion chambers. Modulation of the butterfly valve in the throttle body controls the amount of air that can enter the engine, but it is more difficult for the computer to know exactly how much air is present. Since the density of air can vary with altitude and atmospheric conditions, your car must be able to monitor the actual mass of air entering through the intake. Measuring this air is the job of your mass airflow sensor (MAF), and metered air refers to air that has passed through this sensor. In other words, metered air is air that your car's computer has already detected.
The Dangers of Unmetered Air
Unfortunately, vacuum leaks create an opportunity for unmetered air to enter your car's engine. As you might have guessed already, unmetered air has snuck into the system behind the MAF. Your car's computer is unaware that this extra mass of air is present, so it is unable to account for it when injecting fuel into the combustion chamber. This intrusion results in a fuel mixture with an excess of air, which is known as a lean condition. Lean conditions can cause a range of drivability issues, including poor acceleration, rough idling, or even stalling.
Lean conditions will often trigger check engine lights, but this may not always be the case. If your car is running poorly, don't wait for an error code before having it diagnosed. Correcting a lean condition early on will save you money by restoring your vehicle's fuel economy and possibly prevent a costly breakdown on the side of the road.
For more information on unmetered air or the problems it can cause, contact an auto repair shop.