After finding clogged injectors, the real challenge is what to do about them.

Fuel injectors have always been a fairly high-tech device, even though their function and operation is fairly simple. Functionally, an electric fuel injector is nothing more than a solenoid valve, and all it needs to operate is just the right voltage and current at the right time -- no problem for a computer. Mechanical injectors are poppet valves, and the fuel pressure and flow that operates them also is easily controlled by a computer. But before modern materials and computer-controlled production equipment were available, it wasn't even possible to make gasoline fuel injectors as we know them today. If you think about what it takes to make those tiny passages, intricately shaped pintles, precision-ground valve seats and closely calibrated springs, it's easy to see that injectors are a very complex mechanical device. It's also easy to see how little it takes to clog those tiny passages. In fact, given enough time in service, almost every injector will eventually fail, often due to clogging. Today we'll look at how and why injectors clog, and some options for dealing with them.

The cause

Gasoline is actually several different components that have different boiling points. Along with a blend of extremely volatile and less-volatile hydrocarbons, there is a complex package of additives to reduce corrosion, aging and fuel system icing, along with detergents to control deposits. Alcohol or ethers are used to increase the octane rating, and the whole mixture is adjusted regionally and seasonally to reduce EVAP emissions and hot-start problems. It all adds up to a somewhat unstable mixture, and over time it will naturally separate into lighter and heavier components. Unless the gas is in a sealed container, the lighter components evaporate, leaving behind the heavier 'ends' to glom onto any surface they contact. Those heavy ends are the nasty stuff you find in the carburetor when the lawnmower won't start after sitting all winter. The separation/evaporation process happens a lot faster when the gasoline is exposed to heat without ignition. Under the right conditions, the separation can happen in just a few minutes inside a fuel injector. The conditions that enable the fuel, heat and small passages to combine in just the right way are a little complicated, but as it turns out, quite common and predictable.

Since 1991, a company called Southwest Research Institute (SwRI) has been conducting tests to learn what causes deposits on fuel injectors. They have developed a driving cycle that "produces the most consistent fuel injector fouling," a cycle they call 15/45. The vehicle is run at 55 mph on a dyno for 15 minutes, then shut down for 45 minutes. Repeating this cycle, they can build up enough of a deposit to reduce the fuel flow through an injector by 10 percent in an average of only 2,000 miles, and sometimes as little as 350 miles. The reason SwRI stops the engine for only 45 minutes is so they can repeat the cycle. In the real world, anytime a car is driven just long enough to fully warm up and then shut down long enough for a full hot-soak (45 minutes or more), the cycle is complete.

In his book Bosch Fuel Injection and Engine Management, Charles Probst explains how this heat promotes injector clogging. With the engine running, the injector tip is cooled by fuel flow and intake airflow. When the engine is shut down and hot-soak begins, the injectors absorb heat from the engine. Fuel system residual pressure keeps the gas that is inside the injector stable, but in the little pintle chamber below the valve seat, the lighter ends evaporate, leaving the heavier ends to form a deposit on the pintle. The heat then bakes the deposit, making it hard. Cars that reach high underhood temperatures during hot-soak, such as those with turbochargers, are more susceptible to injector clogging. In the SwRI tests, injectors two and three, the hottest positions, clogged more often than one and four.

The standard industry definition of a clogged injector is one that suffers a 10 percent flow reduction as measured on a special test rig. In the real world, small deposits that cause only a slight flow reduction can still degrade the spray pattern, causing driveability and emissions problems. A poor spray pattern also promotes deposits on the intake valve. Once an injector is partially fouled, it's more likely to leak after engine shut-down, which also makes it more likely to foul even more.

The prevention

Injector heat-soak has been understood for a long time, and the key to preventing injector clogging is to minimize the heat-soak. In certain markets, the first fuel- injected Datsun 260Z was equipped with special fans and air ducts to cool the injectors after the engine was shut off. While some cars still use these today, most just run the electric radiator fans after shutdown to reduce underhood temperature as quickly as possible. This is not so easy in warmer climates. Most vehicles with electric cooling fans simply slave the fan to a coolant temperature switch. Some use an underhood ambient temperature switch, like the simple one-wire switch mounted an inch above the fuel rail on the early Volkswagen Corrado.

Injector design also has evolved to reduce the effects of heat-soak. After many years in production, Bosch added a 'chimney' to their electric injectors, a nozzle tip extension that also forms the seating flange, so the injector doesn't directly contact the hot engine. It's standard on all the injectors they make today, and many older cars can be retrofit with the newer type. In General Motor's (GM) Central Port Injection (CPI) system, the injectors are mounted in the fuel metering assembly in the intake plenum. With the actual metering device isolated from the hottest parts of the engine, flow reduction due to clogging is less likely. The poppet valves also are check valves, so fuel pressure is maintained at both sides of the electric injector even when the engine is shut off. The poppet valves mounted near the intake port are still subjected to heat-soak, but the tips are fixed spray nozzles, not moving pintles. A spray nozzle offers less opportunity for clogging because most of the area downstream of the valve seat is open, not contained inside a tip like a pintle design. GM's Multi-tech II, Ford's Deposit Resistant Injector (DRI) and every other injector introduced during the last 10 years have all been designed to reduce the possibility of clogging due to heat-soak.

We noted earlier that gasoline has detergent additives to help control nozzle deposits, and they are effective. Without those detergents, some of the other components in gasoline would form gummy deposits throughout the fuel system. Using only high-quality fuel -- which does not necessarily mean high-octane premium -- is the first step in preventing clogged injectors. This is easy to do here in North America, but it's no guarantee. SwRI used 107 different gasolines in their tests, and almost 15 percent of these caused injectors to clog to a 10 percent reduction in flow.

The most commonly recognized way to prevent clogged injectors is the same way a lot of other engine problems can be prevented: avoid repeated short trips. For many car owners, that's simply not possible, so it helps to make a more extended drive at least once between fill-ups. Occasionally switching to a different brand of gasoline also can help, as deposits left by one brand of gas may be cleaned away by another. Injector cleaners added to the fuel tank have been proven effective at keeping injectors clean when they're used regularly as preventive maintenance, but there's little agreement on how often such products should be used.

The search

When it comes to diagnosing faulty injectors, there are two basic challenges: determining that there really is an injector flow problem, and finding which one it is. An injector with reduced flow can produce driveability symptoms that look like a vacuum leak or a leaking exhaust gas recirculation (EGR) valve, or several other types of problems. It almost always causes an emissions test failure. The first clue is usually a rough idle and hard starting when cold. If an injector's flow is significantly reduced, that cylinder will run lean and the PCM will command a richer mixture because it reads the O2 in the exhaust for all cylinders (or all in that bank). The resulting imbalance will cause a rough idle and probably high CO and HC numbers. If the problem has been going on for some time, there may be significant intake valve deposits that can cause similar symptoms, and excessive combustion chamber deposits that usually cause high NOX numbers. If the flow is not significantly reduced but the spray pattern is poor, this also will produce similar starting, driveability and emissions symptoms, especially when the engine is cold.

Before starting your search for bad injectors, it's a good idea to get a little history of the car. Is it typically used in a way that mimics the 15/45 drive cycle? Did the problem occur suddenly, especially after a recent fill-up, or was it more gradual? If the driver hasn't noticed a decline in performance but the car failed an emissions test, a cylinder balance test is in order.

Injector balance testing in the engine has been common in the diesel business for a long time. They simply look for the 'cold' exhaust runner to find the dead cylinder. If you can reach the exhaust manifold with a temperature probe, or see it with a non-contact temperature measuring gun, this can be a quick-and-dirty way of at least identifying a lean cylinder. If you can reach the connectors, you can do a balance test by disconnecting one injector at a time, looking for one that makes the least difference in idle speed. It may be necessary to disconnect the IAC motor and temporarily increase the base idle speed, but on some cars this may also set a code.

Probably the best way to find an injector with low flow is with special test equipment, either connected to the PCM or directly to the injectors. When working through the PCM, the test equipment will shut off each injector one at a time and report the difference in power contribution. Other equipment allows you to pulse each injector one at a time with the engine stopped and measure the drop in residual fuel pressure. Injectors are considered properly balanced if the pressure drops are within about 5 percent of each other. Even if the injectors don't behave the same way electrically -- different coil resistance and/or current draw -- so long as the fuel flows match, the injectors will work well together. For the pressure drop test, the injectors should be removed from the engine to avoid sending raw fuel down the exhaust pipe to the catalytic converter. This would also allow you to view the spray pattern and measure the amount of fuel flowing through each injector for an extended period of time. Although they each take a different approach to this test, all the major tool companies sell the equipment needed for injector balance testing.

The cure

After you've determined an injector is clogged, you have three options: You can clean them in place, remove them for cleaning or replace them. Most people's first choice is to add injector cleaner to the fuel tank and see what happens. Looking again at the SwRI tests, after an injector experienced a 10 percent flow reduction in the 'dirty-up' drive cycle, they switched to a different gasoline with a different additive package to test its ability to clean up the injector. They learned that keeping injectors clean is not difficult, but getting clogged injectors clean while driving is not easy. Injectors that had clogged early in the test cycle were more likely to be cleaned when the fuel was switched. But when it took a long time to foul the injector, it also took a long time to clear it. In the real world, this means that detergents and aftermarket injector cleaners added to the gas tank do help prevent deposits, but once firmly in place, removing those deposits quickly will require a more aggressive approach. This doesn't mean the gas tank additives won't help, but a clogged injector usually causes driveability or emissions problems that must be fixed right away.

The most common method for cleaning injectors in the shop without removing them is to run the engine on a very powerful cleaning solvent supplied through the fuel rail from a separate fuel tank. Probably the simplest set-up for this is a hose that connects to the engine side of the fuel supply and return lines, forming a closed loop. Through a service port in this hose, another hose is connected to a canister containing the cleaning solvent. The canister is pressurized with shop air and the engine is run on the cleaning solvent until it's gone and the engine stalls. This equipment is available from several tool companies with a variety of accessories, but probably the most useful accessory is another hose that loops the fuel tank side of the supply and return lines together. By short-circuiting the fuel system like this, the engine can be run without disconnecting power to the fuel pump, which may set a code on some cars. There also is more sophisticated equipment that replaces the pressurized canister with a machine that includes a pump, gauges, regulators and, on some, a cleaning solvent reservoir. These machines can be easier to use on a greater variety of fuel systems, even diesels, and usually include testing and diagnostic capabilities either built-in or available as accessories. These machines also can be expensive, so the decision to buy one will be based at least partially on how many injector-cleaning jobs your shop performs. For most shops, this shouldn't be a problem. Older cars, city cars or cars that idle for extended periods are the most likely candidates, and the results can be dramatic.

Regardless of the delivery system, it's the cleaning solvent that really does the job. There are many different cleaning solvents, but those designed for use in the shop all have one thing in common. They are all very aggressive, and prolonged exposure to non-metal components in the fuel system will probably cause serious damage. Unless the label states that the solvent should be added to the fuel tank, these solvents must not be allowed to reach the fuel tank through the return system. Similarly, injector cleaning machines also can be damaged by using the wrong solvent. Some people have been successful at using different (read: less expensive) solvents in their injector cleaning machines, but the risk of damage far outweighs the potential savings. The safest way to use these machines is with the solvent sold by or recommended by the machine's manufacturer.

Running a cleaning solvent through the engine does more than just clean injector deposits. It also reduces deposits on the intake valves and combustion chamber. A port-injected engine will have at least some deposits on the back side of the intake valve, formed partly by the same heavy ends in gasoline that clog an injector. If an injector has reduced flow, a poor spray pattern or is leaking, or if the engine is ingesting oil through the intake tract, the engine is more likely to have significant intake valve deposits. If those deposits are bad enough, they can affect emissions and driveability just as much as a bad injector does. So cleaning the injectors while they're in the engine has the added benefit of cleaning intake valves too, but that might still only be part of the job. Unfortunately, the detergents in gasoline that help keep injectors and intake valves clean also tend to promote combustion chamber deposits that accumulate around the piston above the top ring. This material is much harder than deposits on the valves and injectors. Combustion chamber cleaners that are added to the fuel tank are effective, but those meant for shop use work much faster. Again, it's the solvent that does the job, and sometimes it's a special solvent that's used either before or after cleaning the injectors.

If you already have the injectors out of the engine, your options for cleaning them are limited. Ultra-sonic cleaning tanks have been around for years, and they are effective. Some innovative technicians have developed methods for operating the injector with a pulse tester while flushing cleaning solvent through it. Usually this produces good results, but it's difficult to tell if your cleaning efforts have been successful until the injectors installed on the engine again. If you already have the injectors out, a better option might be to send them out for cleaning, or even replace them with new or reconditioned units. There are companies on the Internet that offer injector cleaning services and sell reconditioned injectors at prices much lower than new ones. They usually recommend replacing injectors in sets, so they can be flow-matched.

Most of the OEM and aftermarket injector manufacturers expect their sales to increase over the next several years, partially as a result of the increasing average of cars and light trucks in the U.S. fleet. Since they expect you'll be seeing more faulty injectors, it makes good business sense to have the tools, knowledge and resources for dealing with them. Even though you probably won't see a lot of clogged injectors, they're too expensive to replace without confirming there is a flow problem. Some of the companies who make the tools and diagnostic equipment also offer training materials, and that's always a wise investment.