Answer: "Dirty" is actually a misnomer. Rarely are injectors clogged with dirt. Rather, they are usually clogged or restricted by a buildup of fuel varnish deposits. This reduces the amount of fuel that the injector sprays, which in turn may cause the engine to run lean and misfire, hesitate or stall. A fuel injector is nothing more than spray nozzle. With mechanical injectors, a spring loaded valve allows fuel to squirt out of the nozzle when line pressure overcomes spring tension that holds the valve shut. With electronic injectors, a spring-loaded solenoid pulls open a pintle valve or ball type valve when the injector is energized by the computer. This allows the pressurized fuel in the fuel rail to flow through the injector and squirt out the nozzle.

Problems can occur even with a slight buildup of deposits. Because the injector orifice is so small, it doesn't take much crud to restrict the flow of fuel or to disrupt the spray pattern. For good combustion, the injectors must produce a fine cone-shaped mist of fuel vapor. Wear or deposits in the nozzle can create "streamers" of liquid fuel that vaporize and burn poorly. This, in turn, can cause hesitation, emissions and performance problems.

Injector cleaning: The cure for a set of clogged injectors is cleaning -- or replacement if they're too badly clogged to respond to cleaning. Injectors are expensive to replace. Injectors should only be replaced as a last resort. If your injectors are clogged, they can be cleaned with pressurized solvent, or removed for off-car cleaning. There are also fuel tank additives that claim to clean clogged injectors, but the cleaning such products do is usually minimal. So save your money and put it towards a professional cleaning.

There are do-it-yourself on-car injector pressure cleaning kits that are similar to the equipment professionals use. But some of these kits can be tricky or even dangerous to use. Our advice is to let a professional do it.

On-car injector cleaning involves feeding solvent under pressure into the injector fuel rail or supply line. The concentrated solvent passes through the injectors and loosens and washes away the accumulated varnish deposits. The results are usually good, and make a noticeable difference in idle smoothness, emissions and fuel economy.

If your injectors are really clogged and fail to respond well to on-car cleaning, off-car cleaning using special fuel injection cleaning equipment would be the next logical option. Some of this equipment is designed to "reverse" flush the injectors so any debris that's trapped inside the injector or above the inlet screen will also be removed. Off-car cleaning also allows a mechanic to observe the spray pattern of the injectors to make sure there aren't any streamers or problems. Off-car cleaning is more expensive because of the labor involved to remove the injectors, but the results are usually better.

Keeping injectors clean: The best way to minimize or eliminate the need for injector cleaning is to use a quality brand of gasoline that contains sufficient detergent to prevent varnish buildup. Most brand name gasolines today have enough detergent to do this. As a rule, premium grades usually contain a somewhat higher concentration of cleaners. You can also use fuel tank additives to keep your injectors clean. Such products really aren't necessary if you're using quality gasoline. But if you're buying the cheapest gas you can find, using an additive might be good insurance.

Answer: For high mileage vehicles, replacing the fuel filter annually for preventative maintenance is a good idea for two reasons. By the time a vehicle is six or seven years old, there can be a fair amount of rust and debris in the fuel tank. Rust can be formed by moisture and condensation, and debris can get into your tank anytime you add fuel. So changing it on a periodic basis can help minimize the risk of plugging. Most newer vehicles do not have a specified interval for replacing the fuel filter. In fact, some even have filters that supposedly never need to be changed. But any filter can plug up if enough rust or debris gets sucked into the fuel inlet. Gasoline is supposed to be filtered at the pump. But it sometimes isn't. What's more, the fuel you put in your tank may be contaminated with water from leaky underground storage tanks, improperly mixed alcohol blends, or even watered-down by an unscrupulous operator who's trying to make a fast buck. So there are no guarantees. Replacing the filter periodically for preventative maintenance is simply insurance that reduces the risk of it plugging up. Given enough time, every filter will eventually reach the end of its service life. Even the filter in a brand new vehicle right off the showroom floor is at risk if the owner happens to get a dirty tank of fuel.

Filter inspection: If you don't want to replace the filter unnecessarily, you can remove it and check it to see if it passes air easily (More information on replacing fuel filtersis available). If the filter creates any significant resistance when you blow through it, it needs to be replaced.

Answer: The starter seems to work fine, and all noises seem normal. The main problem is simply the engine won't run. As you continue trying to start the engine, you notice the starter begins to sound different and seems to be working more slowly. This is not the same problem you started with. By continually trying to start the engine, you are wearing down the battery. The engine won't start because of some other reason beside the battery. Some causes of this problem may have been evident for a short time. You may have noticed that the engine didn't start as quickly as it did before. Normally, this condition suddenly appears and is a total surprise. Probable Causes: 1. You ran out of gas. 2. The fuel filter is plugged with dirt. 3. The fuel pump quit working. 4. Too much gas went into the engine. 5. There is a problem with the ignition system 6. The engine's timing belt or chain is broken.

Answer: You notice black smoke coming from the exhaust when you start your car. The smoke may still be there after the car is warmed, but it may be less noticeable. The smoke may be accompanied by engine idling problems. This problem normally develops over time, and the amount of smoke indicates the seriousness of the problem. The probable causes are: 1. If you have a carburetor, the carburetor choke may be stuck closed. 2. The fuel injectors may be leaking. 3. The air filter may be clogged. 4. There may be an ignition problem.

Answer: Whenever you run the engine and are not moving, you notice an awful smell from the exhaust. The smell is like that of rotten eggs. Not only do you notice this, so does everyone around your car. You may also have noticed that your gas mileage has been worse lately. The probable causes are: 1. There is a problem with your electronic engine control system. 2. You have an ignition problem. 3. Your fuel pressure regulator is bad. 4. The engine may have mechanical problems. 5. The engine is running too hot.

Answer: Whenever you press on the gas pedal, you hear a popping from the exhaust. For the most part, the engine seems to run fine. However, you have noticed your gas mileage has gotten worse. The louder the popping noise, the worse the problem. The probable causes are: 1. You have a vacuum leak. 2. One or more of your fuel injectors are leaking. 3. There is a hole or leak in the car's exhaust.