The purpose of this article is to answer previous questions on this topic and provide additional information on this subject. 1. When a car is running lean is it getting too much air and not enough fuel, or the opposite? When an engine is running too lean, it is not receiving the correct fuel/air mixture for it to perform properly. A lean condition means the engine is receiving too much air and not enough fuel. Not all, but for most vehicles, the normal air to fuel ratio for optimum performance is approximately 13 parts air to 1 part fuel. When this ratio decreases to approximately 10 parts air to 1 part fuel the engine will run rich. When the fuel/air mixture reaches a ratio of approximately 16 parts air to one part fuel the engine will run lean. This is just a general guideline and is not applicable to all vehicles. The specific fuel/air mixture requirements are established by the configuration of the motor and the fuel supply system incorporated. Several factors affect these requirements such as fuel injectors versus a carburetor. The air induction system plays a key role in this process as well. Turbocharged and supercharged engines require a precise fuel air mixture to achieve an increased performance level. Can a dirty fuel filter cause a car to run lean? Yes, a dirty or clogged fuel filter will restrict fuel flow and lower the available fuel pressure resulting in lean operation. The recommended interval for replacing a fuel filter on a car, truck or SUV can vary from 20,000 to over 100,000 miles based on the year, make and model. The location of the fuel filter and the complexity level of replacing it will also be vehicle specific. You should consult the specific technical manual for the automobile prior to attempting the removal and replacement process. In some circumstances, it may be quick and easy, by removing a couples clamps with a screwdriver. Other applications may require special tools to disengage the connectors. Most important, you must know if the system is pressurized and how to relieve the pressure prior to removal. What if fuel pressure is good? The fuel pressure can be perfect and you can still have an engine that is running lean. The fuel injectors could be clogged restricting fuel flow to the cylinders. You could also have a defective mass airflow sensor, O2 sensor or EGR valve. Intake leaks will also affect the fuel/air mixture without affecting the fuel pressure. The initial fuel pressure is just the starting point for the combustion process and several conditions and components affect the overall function of the fuel supply system. Dirty, clogged or defective fuel injectors prevent the fuel from being delivered to the combustion chamber even when the fuel pressure is correct. If a carburetor is the fuel distribution source, it can be defective affecting the fuel/air mixture. The correct fuel pressure may be affected by air induction issues due to faulty components providing too much air to the mix. Vacuum leaks and faulty emissions components can result in lean operation as well. Could bad plugs cause the same condition? Not normally, bad plugs will usually cause an engine to run rich due to insufficient combustion. It is very common for just the opposite to happen, lean operation will damage the spark plugs and replacing them will not correct the fuel/air mixture issue. The most common method used to determine if an engine is running too lean is to perform a visual inspection on one or more of the spark plugs. The more plugs you examine is better to properly diagnose this issue. When the fuel/air mixture is correct the spark plugs will be clean, dry and sightly tan in color. If the motor is running too rich the plugs will be soaked with fuel, dark brown or even black in color. When the issue is due to a lean fuel/air mixture the plugs will be white and chalky in appearance. You may also notice that the size of the spark plug gap has increased. In extreme cases the porcelain on the inside of the plug may be cracked. Note: If the engine has a carburetor, a stuck float or vacuum leaks will cause the motor to run lean in some circumstances. Why is immediate attention important? The damage that can result from operating an engine running too lean can be very extensive. Several internal engine components are subjected to a higher level of heat than normal. Fuel is also very important to the lubrication process. The intake valves, exhaust valves and the heads can be damaged. Pitting of the pistons or more severe damage can occur as well. Unfortunately, This situation can destroy an engine if it is not corrected in a timely manner. Note: This article is just an information source to help you understand the concept of a lean running motor. You should consult with the specific technical manuals for your vehicle. I would recommend for this issue and any other automotive malfunctions or trouble codes that you start by reviewing the tech bulletins on your vehicle for known issues.