What Makes an Engine Run Lean?

An engine requires a very precise mixture of fuel and air: ideally, about 14 parts air to 1 part fuel. The ratio can typically be as low as 10-to-1 for performance applications, or as high as 16-to-1 for maximum fuel economy. But most engines will run from about a 12- to 15-to-1 ratio of air to fuel. A "lean" condition is one where there's either too much air or not enough fuel in the mix. This is the opposite of a "rich" condition, in which there's too much fuel and not enough air. Most engines are calibrated to run slightly rich -- about 13-to1 -- under cruise conditions; a rich mixture makes for a cooler and more stable fuel burn, which prevents "detonation" and keeps the engine from self-destructing.

Lean conditions often happen because there's not enough fuel for the amount of air going in, so a malfunctioning fuel system is a prime suspect when the engine runs lean. A clogged fuel filter can reduce both fuel delivery and fuel pressure; low fuel pressure will reduce the fuel flow rate at the fuel injectors and cut the amount of fuel available in the float bowl in a carburetor. Either way, you're looking at a deficit of fuel and a lean condition.

Oxygen sensors are used to monitor the amount of oxygen in the engine's exhaust. Your computer uses information from the oxygen sensors to tell the fuel injectors how long to stay open, and thus how much fuel to inject. If the oxygen sensor malfunctions, it can send the computer incorrect information, and send the engine into a lean condition. Bad oxygen sensors will almost always trigger a check-engine light on anything made since 1996, but not all O2 sensors do the same job. Only the first set of oxygen sensors -- the ones before the catalytic converter -- directly monitor the engine. The second O2 sensor monitors the converter.

The mass airflow -- MAF -- sensor monitors and tells the on-board computer how much air is entering the engine. The MAF sensor uses a heated wire hanging down into the intake system to monitor airflow. Air flowing over the sensor wire cools it down by a certain amount, and the computer uses that information to determine how much air is going in. MAF sensors will malfunction over time, often because a layer of dirt and grime builds up on the sensor wire. The grime coating insulates it like a sweater, so the computer thinks there's less air going in than there is. These sensors are usually easy to clean, and spray-on MAF sensor cleaner solutions are available at most auto parts stores.

Almost any sensor that monitors airflow or fuel pressure can cause a lean condition. This includes not just the O2 and MAF sensors, but the manifold air pressure -- MAP -- sensor, intake air temperature sensor and even the sensor that monitors the exhaust gas recirculation system. An EGR stuck in the open position will act just like a massive vacuum leak, allowing excess air from the exhaust to re-enter the engine in an uncontrolled manner. Any of these should trigger a check-engine light.

Legitimate vacuum leaks aren't as common as they used to be, but they still happen. Vacuum leaks happen anywhere intake manifold vacuum has a chance to pull air in from the outside. Any number of hoses and lines could leak, but so could loose air intake hoses and leaking intake manifold gaskets. The old mechanic's trick is to spray ether starting fluid in short bursts at the suspected vacuum leak. If a vacuum leak is present, the engine will suck the starting fluid in, smooth out, briefly increase rpm and run properly for a few seconds. Be careful with this though -- starting fluid is extremely flammable, and doesn't get along well with electrical sensors and connections.