Fuel Injector Cleaning - Unclogging a Common Condition, Larry Carley, Underhood Service, December 2000

We’ve all seen the classic symptoms of dirty injectors: lean misfire, rough idle, hesitation and stumbling on light acceleration, a loss of power and higher hydrocarbon (HC) and carbon monoxide (CO) emissions. And the older the vehicle, as a rule, the worse the driveability symptoms.

It doesn’t take much of a restriction in an injector to lean out the fuel mixture. Only an 8% to 10% restriction in a single fuel injector can be enough to cause a misfire. When this occurs, unburned oxygen enters the exhaust and makes the O2 sensor read lean. On older multiport systems that fire the injectors simultaneously, the computer compensates by increasing the on time of all the injectors, resulting in an overly rich fuel condition in the rest of the cylinders. HC emissions shoot up because of the misfire, and CO emissions rise because the computer is overfueling the engine.

All vehicles are vulnerable to injector clogging, but the ones that are most vulnerable and most likely to experience such driveability and emission problems are older vehicles with pintle-style multiport injectors.

In the early pintle-style injectors, a solenoid in the top of the injector lifts the pintle valve up off its seat. Pressure in the fuel line then forces fuel to spray out through the nozzle. The shape and orifice size of the nozzle determine how much fuel flows through the injector and the shape of the spray pattern. Most pintle-style injectors are designed to produce a cone-shaped spray pattern. But if fuel deposits accumulate in the nozzle area, it can restrict fuel delivery and break up the spray pattern causing a lean fuel condition and many of the problems just mentioned.

Where do the deposits come from? Mostly from the fuel itself. Gasoline is a mixture of many different hydrocarbons, including oilfins which are heavy, waxy compounds. The heavier the hydrocarbon, the more energy it yields when it burns. Oilfins give gasoline more bang for the buck (actually more like two bucks these days). Anyway, when the engine is shut off, the fuel residue in the injector nozzles evaporates leaving the oilfins behind. Because the engine is off, there is no cooling airflow through the ports and no fuel flow through the injectors, so heat bakes the oilfins to form hard varnish deposits. Over time, these deposits can build up and clog the nozzle.

The formation of these deposits is a normal consequence of engine operation, so detergents are added to the fuel by gasoline refiners to help keep the injectors clean. The trouble is, the early pintle-style injectors are hard to keep clean because of the design of the nozzle.

To make matters worse, the build up of deposits can quickly outpace the cleansing efforts of the fuel detergent if the vehicle is used primarily for short trip driving. Every time the engine is shut off, the heat soak interval that follows in essence cooks the injectors and bakes on fuel deposits. This can lead to a rapid buildup of deposits, especially in multiport injectors where the injectors are located close to the cylinder head. On four-cylinder engines, the #2 and #3 injectors are in the hottest location and tend to clog faster than the end injectors on cylinders #1 and #4. The same applies to the injectors in the middle cylinders in six- and eight-cylinder engines. The hotter the spot, the more vulnerable the injector is to clogging from heat soak. Throttle body injectors are less vulnerable to heat soak because of their location high above the intake manifold plenum.

Injector clogging also can be accelerated by the type of fuel used. Though all major brands of gasoline contain adequate levels of detergent to keep most injectors clean under normal use, some cut-rate brands of gasoline do not (you get what you pay for). What’s more, some detergents are more effective than others. But most detergents are not strong enough to remove heavy accumulations of deposits once they’ve become baked in place. The type and amount of octane-boosting additives and oxygenates (MBTE, ethanol, etc.) in the fuel, as well as other contaminants such as sulfur, can also affect the rate at which deposits form. Microscopic particles of rust, scale and dirt from inside the fuel system itself can also add to the deposit problem, especially in older vehicles that have seen a lot of miles.

Service Options
When you encounter an engine that has the classic symptoms of clogged injectors, you have three options: clean the injectors in place, remove the injectors for bench cleaning on special equipment designed for this purpose or replace the injectors if they fail to respond to cleaning.

It’s important to remember that dirty injectors may not be the only condition that’s causing the driveability or emissions problem. The same oilfins that form varnish deposits in the injectors also can contribute to the formation of carbon deposits on the intake valves, pistons and combustion chambers.

If an engine has a lot of miles on it, it will probably suck some oil past the valve guide seals and down the guides. The rings may be passing some oil too. Oil that comes down the guides will usually form thick, carbon deposits on the intake valves as well as the pistons and combustion chambers. Dirty valves can obstruct airflow and interfere with proper fuel atomization, contributing even more to a lean fuel condition and hesitation/stumbling under light acceleration.

If the cylinders are full of carbon as a result of oil burning or short trip, stop-and-go driving, the increase in compression will increase the risk of preignition and detonation. So if the engine pings and knocks under load, it’s a sure sign the cylinders as well as the injectors need cleaned.

Another component that may also need to be cleaned to remove fuel varnish is the throttle body. Fuel vapor rising up through the intake manifold can accumulate and vaporize around the throttle plate and air bypass circuits, causing a change in the idle air/fuel mixture. Sometimes you can see the deposits, and sometimes you can’t. Either way, cleaning the throttle body and intake tract also may be necessary to fully restore engine performance, idle quality and emissions. An aerosol cleaning solvent works well here.

Benefits of Injector Cleaning
Injector cleaning is a service that delivers very real benefits to your customer. In most cases, the vehicle owner will feel a significant improvement in engine performance and idle smoothness after a thorough cleaning of the injectors and fuel intake system. Cleaning also should produce an improvement in fuel economy, and lower emissions - which can help the vehicle pass an emissions test. And if the intake valves and cylinders also need cleaned, spark knock should be a thing of the past.

The benefits realized by injector cleaning will obviously vary depending on the condition of the injectors prior to cleaning and how badly they are clogged. Really dirty injectors will obviously show more of an improvement than ones that have only a light accumulation of deposits. But either way, the injectors should be restored to like-new performance.

Injector cleaning can be promoted as a preventive maintenance service, especially if a vehicle has a history of injector clogging. Most older high mileage vehicles will benefit from periodic injector cleanings. Ball and disc-style injectors in newer vehicles are less vulnerable to clogging as are injectors in "returnless" EFI systems that do not recirculate fuel from the engine back to the fuel tank. Even so, after 30,000 to 50,000 miles of driving (especially city driving), almost all injectors will benefit from cleaning.

Regular cleaning every year or so may seem like an unnecessary expense to some, but for urban motorists who seldom get out on the open highway, it can help maintain engine performance and reduce urban air pollution.

Unfortunately, in most cases, injectors are not cleaned until they’re showing signs of clogging. Fortunately, most injectors respond well to cleaning, provided the cleaning is thorough enough to remove all the deposits.

On-Car vs. Off-Car
The benefits of on-car cleaning are:

1. It’s relatively easy to do;

2. Saves your customer money;

3. It cleans the intake valves and cylinders.

With on-car cleaning you don’t have to remove the injectors (which can be a real chore on some engines), and you don’t need any special off-car cleaning equipment. Once the equipment is hooked up, the cleaning cycle takes only about 10 minutes - and you can usually tell immediately if your cleaning job did the trick.

On-car cleaning also is an affordable add-on sale that can tack some much-needed profit onto a simple tune-up. If you charge $59 for an on-car cleaning, you can more than double your profit. If the cylinders also need a decarbon treatment, that’s an additional charge and even more profit.

When doing the cleaning procedure itself, you must use pressurized equipment to feed the solvent directly into the fuel rail while the engine is running. This means you either have to disable the fuel pump and plug the fuel return line, or install a U-tube so the fuel will recirculate right back to the tank. Disabling the fuel pump can set a fault code on some cars, requiring you to clear the code after the job is done.

Easy as it is, there are some limitations with on-car injector cleaning. One is that badly clogged injectors may not pass enough solvent during a normal cleaning cycle to be thoroughly cleaned. Some baked on deposits can be really hard to remove even with a strong solvent, requiring you to prolong or repeat the cleaning process. And if on-car cleaning fails to restore injector performance, you’ll end up having to remove them anyway to replace or clean them.

Another limitation is that you may have to do some additional tests to confirm the injectors responded to your cleaning efforts. A test drive may be needed to see if the driveability symptoms have been eliminated, or you may have to check emissions to make sure HC and CO levels are back to normal. A power balance test is another way to confirm engine performance and check for weak cylinders (there should be less than a 10% power variation between cylinders). An injector pressure drop test will tell you if the injectors are flowing evenly or not.

There may be some risk to the vehicle’s fuel system when using concentrated solvent to clean the injectors in place. Most equipment suppliers say to disconnect and plug the fuel return line so solvent does not circulate back to the fuel tank. Strong solvents may attack rubber and plastic components in the fuel pump, regulator and fuel lines, creating additional problems you or your customer don’t need.

Back in 1991, General Motors issued a technical service bulletin (#913126E) cautioning service technicians not to clean its "Multec" fuel injectors. The GM bulletin said cleaners containing methanol or other strong solvents may damage the injector’s coil wire insulation and accelerate wear in the injector ball and seat. The bulletin also said cleaning is usually no value with this style of injector. Even so, many shops clean Multec injectors successfully with no problems at all.

On-car injector cleaning also involves some risk to the person who’s performing the service. You have to disconnect pressurized fuel lines, make sure there are no fuel leaks and feed high pressure solvent (which is just as flammable as gasoline) into the engine while the engine is running. Safety precautions always should include eye protection, making sure there are no open sources of ignition (sparks) nearby and avoiding direct exposure with the cleaning solvent.

Some experts recommend replacing the spark plugs after doing an on-car injection cleaning or decarbon treatment. The residue that’s loosened and washed away by the solvent may increase the risk of plug fouling. Changing the oil and filter also is a good idea with a cylinder decarbon treatment because some of the solvent will get past the rings and end up in the crankcase.

Off-car injector cleaning, by comparison, has its own advantages and drawbacks. It’s a more expensive procedure because of the labor involved to remove the injectors (which can be considerable on some applications), and it requires special equipment, which can cost from $4,000 up to $8,300. Most shops charge anywhere from $25 to $35 per injector for off-car cleaning - which makes it a harder sell, but is still a lot cheaper than replacing a set of clogged injectors.

On the plus side, off-car injector cleaning can often restore dirty injectors that fail to respond to on-car cleaning. The cleaning process is longer (typically 30 to 45 minutes), and most machines have an ultrasonic bath that can dislodge even the most stubborn deposits.

Some machines also reverse flush the injectors, which provides an added measure of cleaning.

Another benefit of using off-car cleaning equipment is that the injectors can be flow tested after they’ve been cleaned. The injectors are typically mounted on a test manifold and energized to spray solvent into clear graduated cylinders. By comparing the volume of fuel delivered, it’s easy to see if all the injectors are flowing evenly. There should be less than 5% to 7% variation between injectors (some performance engine builders aim for 1% or less variation between injectors!). If an injector isn’t passing as much liquid as its companions, it may respond to additional cleaning. And if it fails to respond to additional cleaning, there’s no guesswork about which injector needs replaced.

Flow testing also allows you to compare the actual flow rate of each injector to factory specifications. If the flow is within specifications, you know the injector should perform properly when it is reinstalled back in the engine. Flow testing is also a good way to make sure the injectors are the right ones for the engine (one or more injectors may have been previously replaced by someone else).

What’s more, an off-car flow test on the cleaning equipment allows you to see each injector’s spray pattern. If it looks normal, you know the injector will perform properly. If you see streamers in the spray pattern, or if the cone isn’t the right shape, you know additional cleaning is needed or the injector needs replaced.