Fuel Injector Maintenance

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Fuel injection systems replaced the carburetor as a more reliable means of supplying fuel to the engine while reducing emissions and improving fuel efficiency. Fuel injection problems can result in excessive fuel consumption, poor engine performance, and eventual physical damage to critical engine components.

About Your Vehicle’s Fuel Injection System

Carburetor History

Most vehicles manufactured in the United States before 1990 are equipped with a carburetor. The carburetor provides the correct mixture of air and fuel to the engine. The carburetor is a mechanical device that uses vacuum to draw fuel from the fuel system into the engine.

The carburetor is made up of a tube with an adjustable throttle plate (controlled by the accelerator pedal) that controls the flow of air through the tube. A narrowing in the tube (called the venturi) causes vacuum to be created as air flows across the tube. This vacuum causes fuel to be drawn into the carburetor at a controlled rate. The fuel is then drawn through a machined hole in the venturi (called a jet) and is “fed” to the combustion chambers (cylinders).

Carburetors work best either at “idle” or at “full throttle.” Under other driving conditions, such as rapid acceleration or driving under a heavy load, carburetors often respond to the engine’s changing air/fuel need too slowly. This results in higher emissions and reduced fuel efficiency.

The shortcomings of the carburetor became even more evident when catalytic converters were introduced in response to tougher emissions standards. The catalytic converter is a device that is installed downstream of the exhaust manifold to help oxidize (burn) the unburned and partially burned fuel left over from the combustion process.

The vehicle’s computer checks the efficiency of the catalytic converter by monitoring the converter with two oxygen sensors. One sensor is located before (upstream) of the converter and one is located after (downstream) the converter. Catalytic converters require precise control of the air/fuel mixture for proper operation.

Oxygen sensors monitor the amount of oxygen in the exhaust, and the vehicle’s computer uses this information to adjust the air/fuel ratio in real-time. This process is known as closed loop control. This level of control was not possible with the mechanically-controlled carburetor, and resulted in the incorporation of fuel injection systems in all vehicles manufactured today.

Overview of the Fuel Injection System

A fuel injector is an electronically-actuated electromagnetic valve mounted on the engine’s intake manifold. Its function is to “inject” (supply) fuel to the combustion chambers (cylinders) during vehicle operation. The fuel pump supplies pressurized fuel to the fuel injectors through the fuel rail. Operation of the fuel injectors is accomplished by the vehicles computer.

The first fuel injection systems are known as throttle body fuel injection (TBI) systems (or as single-point (SFI) or central fuel injection systems). These systems incorporate electrically controlled fuel-injector valves into the throttle body, and provide a measured amount of fuel to the engine, where it is distributed through a “spray bar” to each cylinder in equal amounts.

The next generation of fuel injection systems are known as multi-port fuel injection (MFI) (also called port, multi-point or sequential fuel injection). In these systems, a fuel injector is associated with each cylinder, allowing for more accurate fuel metering and quicker response.

Fuel Injection System Operation

When an injector is energized, an electromagnetically-operated plunger opens, and allows pressurized fuel to exit through the injector’s nozzle. The shape and design of the nozzle causes the fuel to exit the injector as a fine mist (in a process called atomization). This provides an “even” distribution of fuel in the air/fuel mixture, and ensures complete and proper burning of the fuel during combustion.

The amount of fuel supplied to the engine is determined by the period of time each injector remains “on” or “open.” A fuel injector is energized and de-energized (pulsed) one or more times during 720° (two full revolutions) of crankshaft travel. The period of time the injector is energized is called the pulse width, and is controlled by the vehicle’s computer. The ratio of fuel injector “on” time to “off” time varies depending on the current engine needs. This ratio is known as the “duty cycle.”

The vehicle’s computer must monitor a number of operating parameters to determine the correct amount of fuel to supply to the cylinders (the correct pulse width for the injectors) in response to changing driving conditions. The computer accomplishes this by monitoring the status of various sensors located throughout the engine, and adjusting the injector pulse width according to a set of pre-programmed algorithms for each combination of returned values. The primary sensors associated with fuel injector control are:

• Mass Air Flow (MAF) Sensor – This sensor tells the computer the mass of air entering the engine.
• Manifold Absolute Pressure (MAP) Sensor – This sensor the pressure of the air in the intake manifold to determine the amount of power the engine is producing (the greater the amount of air that enters the engine, the lower the manifold pressure).
• Coolant Temperature Sensor – This sensor tells the computer when the engine has reached its proper operating temperature.
• Voltage Sensor – This sensor monitors the electrical system voltage. If the voltage begins to drop (generally an indication of a high load condition), the computer adjusts the vehicles idle speed.
• Throttle Position (TPS) Sensor – This sensor monitors the position of the throttle valve (which determines the amount of air entering the engine). Information from the TPS allows the computer to respond quickly to changes (caused by pressing or releasing the accelerator pedal), and increase or decrease the fuel rate, as necessary.
• Engine Speed Sensor – This sensor monitors the current speed of the engine (see ENGINE RPM in Volume 1 for more information). Engine speed is a primary factor used to determine the correct pulse width for the fuel injectors.
• Oxygen (O2) Sensor(s) – A vehicle may be equipped with one or more O2 sensors. These sensors monitor the amount of oxygen present in the exhaust gas so the computer can determine how “rich” or “lean” the air/fuel mixture is and make adjustments to the air/fuel mixture as needed.

Troubleshooting Fuel Injection Problems

Fuel injection problems can result in excessive fuel consumption, poor engine performance, and eventual physical damage to critical engine components. Physical symptoms associated with fuel injection problems can include:

• Idling problems (high idle, low idle or rough idle)
• Loss or lack of power
• Poor fuel economy

Accurate diagnosis of injector problems is essential before replacing parts to rule out other possible causes of performance problems. Damaged wiring, a defective connector or computer circuit problem can result in an inoperative injector. Air leaking past the rubber O-ring seal at the base of an injector can also result in symptoms similar to those of a clogged injector.

Diagnosing fuel injection problems involves a thorough inspection of the vehicle’s fuel system, along with related sensors and actuators. These checks include visual and functional inspections to determine the serviceability of fuel system components.

• Gasoline: Be sure the gasoline used is appropriate for both the vehicle and the local environmental conditions (gasoline made for high-temperature environments will not perform well in cold locations).
• Fuel Filter: Be sure the fuel filter is clean and in good condition. Be sure the fuel filter is replaced regularly according to the vehicle manufacturer’s recommendations. A dirty fuel filter can restrict fuel flow and result in a “lean” air/fuel ratio.
• Air Filter: Be sure the air filter is clean and in good condition. Be sure the air filter is replaced regularly according to the vehicle manufacturer’s recommendations. A dirty air filter can restrict air flow and result in a “rich” air/fuel ration.
• Fuel Lines: Check for evidence of leaks, restrictions, or other damage that might restrict fuel flow. A vacuum gauge can be used to check fuel system pressure and aid in diagnosing a leaking fuel system.
• Fuel Injectors: Be sure the fuel injectors are clean and in good physical condition. Dirty fuel injectors will restrict the flow of fuel to the cylinders. A fuel injection pressure tester can be used to check fuel injection system pressure.
• Functional Inspections: Follow the vehicle manufacturer’s instructions to check the performance of the vehicle’s fuel pump, Mass Air Flow sensor, Manifold Absolute Pressure sensor, Engine Coolant Temperature sensor, Throttle Position sensor, Engine Speed sensor and oxygen sensor(s), and ensure all test results are within specification.

Causes of “Dirty” Injectors

Injectors become “dirty” or clogged through a buildup of fuel varnish in the injector nozzle as a result of the combustion process. Heavy waxy substances (called olefins and diolefins) in gasoline form deposits that gradually accumulate and restrict the injector nozzles. When this occurs, the injector's normal cone-shaped spray pattern is affected. The spray pattern may develop “streamers” that inhibit proper fuel atomization, become misdirected or “off-center”, or the fuel may fail to atomize at all.

Deposits also restrict the size of the opening and reduce the amount of fuel that's delivered. This results in a “lean” air-fuel mixture that causing hesitation, lean misfire, rough idle and similar driveability symptoms.

“Dirty” or clogged fuel injectors are generally the result of a combination of environmental and operational conditions. The following factors can result in the development of fuel injector restrictions:

• Injector soak time - Deposits don't form while fuel is flowing through the injector, but only when the engine is off. Injector soak time is generally a factor on vehicles used primarily for “short-trip” driving (typically 15 minutes of running followed by 45 minutes of hot soak). Under these conditions, significant deposits can form with less than 1,000 miles of accumulated travel.
• Injector design - Fuel injector design varies greatly between vehicle makes, models and manufacturers. Some injectors are more prone to developing deposits than others. Port-type injectors tend to be most susceptible to clogging due to injector soak time. Manufacturers are constantly working to develop less deposit-sensitive injectors, and some claim to have developed injectors that will not clog (these injectors typically use a “metered” disc rather than the conventional pintle vale). However, these types of injectors are currently used in limited applications, and do not have sufficient operating history to determine the accuracy of these claims.
• Fuel system configuration and injector tip temperature - High injector tip temperature causes the formation of fuel deposits. The greater the temperature, the more rapid the progress. Low hood lines and tightly packed engine compartments don't help.
• Environmental conditions - Injector failure occurs at a higher rate in hot, dry climates.
• EGR and PCV gasses - Deposits on injectors and valves are promoted by EGR and PCV gases present in the intake passages. When a vehicle is shut down, these gases condense onto sensitive surfaces. This condition can be a major contributor to injector clogging.
• Poor intake filtration - Leaking intake housing seals and gaskets can allow dirty air into the intake system, increasing deposit formation.
• Fuel stress - The constant heating and pumping of fuel through the pressure regulator and return line in a typical multiport fuel injection system can result in subtle chemical changes to the fuel that can contribute to deposit formation.
• Other fuel components - Gasoline is comprised of hundreds of chemical components. Any number of these components can contribute to the formation of deposits on injectors.
• Engine operating temperatures and thermal characteristics - Heat is major contributor to the formation of deposits on fuel injectors. Therefore, cooling system design and operation play an important role in minimizing heat related problems.
• Poor fuel quality - Low grade gasoline can include additional components not found in higher grade fuels. Additionally, lack of detergents in low grade fuels is a major contributor to clogging of port type injectors. After an extended period of using inadequate fuels, buildup of carbon deposits can rob an engine of peak performance.

Fuel-related problems have become even more pronounced in recent years. The same refiners that cut detergent levels have also refined fuel with higher proportions of deposit-forming residue left in it.

The need for higher octane “lead-free” gasoline coupled with a more severe refining process has increased the olefin/diolefin content of gasoline. Lack of detergents in some fuel blends is a particular problem for port type fuel injectors.

Even when refiners properly blend the fuel and include the necessary detergents, distributors may subsequently mix the high grade blend with a lower grade fuel (a practice know as “splash blending the fuel. The result is a drastic change in the volatility of the fuel, as well a degrading effect on the effectiveness of the detergents.

NOTE: If your vehicle is equipped with a port type injector system, BE SURE to use only fuels labeled “Contains Detergent.” While these fuels are usually more expensive, they will save time and expense in the future by minimizing the damaging effects of restricted injectors.

Testing for “Dirty” Injectors

Restricted or “dirty” fuel injectors are generally diagnosed by performing a power balance test and/or an injector flow test. The injector flow test is generally the more reliable of the two, and will positively identify a bad injector. The power balance text can be affected by out-of-tolerance operation in other related systems.

Use EXTREME CARE when performing a power balance test to avoid a fire. BE SURE to have a suitable fire extinguisher on hand.

NOTE: BE SURE to let the engine cool before performing a power balance test. Heated fuel present in the fuel rail can result in invalid readings. If repeating the power balance test, ALWAYS run the engine between tests, or flooding may occur.

The injector flow test involves pressurizing the fuel rail and “pulsing” each injector (using a timed “logic pulser”), one at a time, and monitoring fuel rail pressure with a pressure gauge. Fuel rail pressure will “drop” each time an injector is pulsed. If the injectors are spraying equally, the pressure drop for each injector will be roughly the same.

If an injector has failed open, it will show a greater pressure drop than the remaining injectors. If an injector has failed closed, is restricted, or is electrically damaged, it will show a pressure drop significantly less than the remaining injectors. Any injector that shows a pressure drop greater than 1.5 psi than the remaining injectors is suspect.

Fuel Injector Maintenance

Cleaning Fuel Injectors

Do fuel injectors ever require cleaning? There is a general disagreement between vehicle manufacturers, automotive maintenance professionals and even DIYers on this issue. The easy answer is that there is no easy answer.

While vehicle manufactures have logged literally thousand of hours of tests without encountering a single clogged injector, the test vehicle were run under “laboratory” conditions rather than “real life” driving conditions. The engines were never shut down, but instead were allowed to run continuously to rack up as many test miles as possible.

The fact is that fuel injectors, particularly port type injectors DO become fouled. The speed and frequency at which fuel flow restrictions develop depends on your driving habits, ambient environmental conditions, vehicle design, fuel related factors. Ultimately, dirty” injectors WILL result in performance problems and low fuel economy, and will reduce the life of the vehicle.

There are a number of commercial products and chemicals available to clean “dirty” injectors and/or to aid in minimizing the formation of deposits. These products are generally available at your local auto parts store. The type of product used, and the proper method for cleaning fuel injectors depends on the amount of contamination present.

Fuel additives can provide a “first line” of defense, by minimizing the formation of deposits in the injector nozzle. Moderately clogged injectors can be cleaned “on vehicle” by feeding pressurized solvent into the fuel rail to flush the injectors. Excessively clogged injectors are most commonly removed and cleaned “off vehicle” using specialized cleaning equipment.

Some chemicals, particularly those that are added to the fuel tank, can result in damage to the fuel system by degrading the protective coating in the tank or fuel lines, particularly if you use more than the recommended amounts. BE SURE to follow the manufacturer’s recommendation when using tank additives.

Replacement Criteria

In some cases, injectors are too badly clogged to be fully restored by cleaning. If a power balance test shows significant differences between cylinders after cleaning, or if an injector flow test reveals restricted fuel delivery or a poor spray pattern after cleaning, the bad injector(s) must be replaced.

Injector replacement is also necessary if an injector is excessively “worn” (a common condition in high mileage engines) or has “failed” electrically due to a short or open circuit. You can often use “aubicle” clues to identify a “failed” injector. A working injector (even if worn) will make a “buzzing” noise as it cycles open and shut; a “failed” injector will remain silent.

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