"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.
Injectors come in a variety of styles. Early Bosch style injectors have a pintle valve and are the ones most prone to clogging. In 1989, General Motors introduced its new "Multec" style injectors which have a ball valve design and are claimed to be more resistant to clogging. Other injectors have a disc-valve design that is also said to resist clogging.
The truth is ANY injector can clog. Nobody's injectors are immune to this kind of problem, but some are obviously better than others.
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.
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. New domestic injectors sell for $60 to $100 each, with new import injectors fetching $125 to $175 each. 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.
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.