Fuel And Air Induction Systems Diagnosis

Electronic Fuel Injection (EFI) was introduced to the automotive world in the late fifties, when Chrysler offered the ‘Electroinjector’ system as an option on its Hemi V8. Shortly after its debut, reliability problems forced Chrysler to recall the few sys­ tems that were sold. Ironically, re­ called vehicles were retrofitted with dual four-barrel carburetors. Three decades later the tide would turn, when manufacturers abandoned the carburetor in favor of electronic fuel injection.


There are two basic types of EFI systems, including Throttle Body In­jection (TBI) and Multiport Fuel Injection (MFI). In a TBI system, one or two injectors (depending on engine size) are mounted above the throttle plates of a single or dual bar­ rel throttle body. Like a carburetor, fuel is delivered above the throttle plates and distributed throughout the intake manifold. Although TBI is more efficient than carburation, it shares the inherent problem of uneven fuel distribution common to the ‘wet-manifold’ design. With multiport injection, fuel is delivered directly to each cylinder via individual injectors, while the intake manifold is used exclusively for air induction. The injectors are mounted on the manifold so that the injector nozzles are aimed at the backside of the intake valves. This arrangement provides a significant improvement in overall engine performance over TBI.

Injector Firing Strategies

Depending on the application, the injectors in an EFI system may be fired simultaneously, in groups, or one at a time. The simplest of these control strategies is simultaneous injection. With this method, each injector is fired once every crank­ shaft revolution, which results in two shots of fuel per combustion cycle. Simultaneous injection is also used for engines equipped with single TBI. Dual TBI systems, as well as some multiport designs, use group injection. This strategy, also known as ‘bank firing’ separates the odd and even numbered cylinders into two separate groups. The injectors in each group are fired simultaneously, while the groups are fired alternately. The most effective strategy for acti­vating the injectors is known as Sequential Fuel Injection (SFI). With this” technique, the injectors are fired one. at a time in the spark plug fir­ing order. Unlike simultaneous and group injection, SFI systems require a cylinder identification signal to ini­tiate injector sequencing. This signal is typically provided by a camshaft position sensor, or a synchronization slot cut into the crankshaft sensor reluctor. All OBD II systems use sequentially fired injectors.

Speed Density vs. Mass Air Flow

Speed density and mass airflow are two techniques used for determining the amount of air entering the engine. In a speed density system, the ECM calculates airflow based on manifold absolute pressure, intake air temperature, EGR flow, volumetric efficiency and rpm. In contrast, mass airflow systems use a MAF (Mass Air Flow) sensor to provide the ECM with a direct measurement of engine airflow. Be aware that some engines are equipped with both MAP and MAF sensors. On OBD I applications with this arrangement, the MAP signal is used as a backup in the event of a MAF failure. On OBD II systems with this combination, the ECM uses the MAP signal to evaluate EGR performance.

Returnless vs. Continuous Return

Most fuel injected engines use a continuous return system. With this design, fuel is delivered from a high-pressure electric pump to a fuel meter assembly (TBI) or a fuel rail (MFI). The pump is designed to deliver fuel in excess of engine requirements. This ensures that there is more than enough fuel avail­ able under extreme loads. A regulator maintains the required level of fuel pressure under all conditions by routing excess fuel back to the tank via a separate return line. On TBI systems, the pressure regulator is part of the fuel meter assembly, while on multiport designs it is a separate component mounted on the fuel rail. The regulator on TBI sys­tems keeps fuel pressure at a constant level. In contrast, most continuous return MFI systems use a vacuum­ controlled regulator that adjusts fuel pressure according to changes in engine load.As the name implies, a returnless system has no return line, and therefore does not recirculate fuel. Instead, fuel in excess of en­gine needs is returned to the tank through a passage located within a tank mounted pressure regulator. By eliminating the return path from the fuel rail, fuel temperature is reduced, which lowers evaporative emissions. This allows the use of a smaller charcoal canister as well as a reduction in vapor purge cycles.


There are a variety of test proce­dures available for diagnosing prob­lems in the fuel delivery system. For a complete fuel system analysis, each of the following tests should be performed.

Fuel Pressure

Measuring fuel pressure should be among the initial tests performed when looking into an engine performance concern. This is because incorrect pressure is often the root cause of many driveability problems. Although the closed loop system can compensate for slight pressure irregularities, it cannot prevent the symptoms that result when fuel pressure is either too high or too low. On most vehicles, a schrader valve on the fuel rail provides a parallel test point. However, on some TBI systems, the pressure gauge must be installed in series. This involves disconnecting the fuel supply line and connecting the appropriate adapters. Once the gauge is connected, run the fuel pump until the maximum pressure is reached and record the reading. Low fuel pressure may be the result of a clogged in-tank strainer or inline fuel filter, defective fuel pump, or problems in the fuel pump electri­cal circuit (high resistance). Excessive fuel pressure can be caused by a restricted return line (continuous return systems), faulty fuel pressure regulator, or a damaged vacuum signal line (continuous return systems).

Fuel Leakdown

If fuel pressure is normal, recheck the gauge reading after 10 minutes to determine the leakdown rate. Generally speaking, the leakdown rate should not exceed 5 psi in 10 minutes. As leakdown rates become greater than this, engine crank times will increase proportionately, resulting in complaints of hard starting. An excessive leakdown rate can be caused by a faulty fuel pump check valve, a pressure regulator that is stuck open, a leaking fuel line, or leaking injectors.

Fuel Volume

Even if fuel pressure is within specifications, it doesn’t necessarily mean that the engine is receiving an adequate supply of fuel. The pressure could be the result of air pockets in the system. This is why checking fuel volume is so important when diagnosing a complaint of poor driveability. With most pressure gauges, you can check fuel volume without opening the system. Simply place the drain hose from the gauge into a clean graduated container, and then activate the fuel pump. Next, collect the fuel by holding the relief valve oil the gauge open for 15 seconds while the pump is running. Generally speaking, the pump should deliver one pint of fuel within that time, provided that the battery is fully charged and there is no excessive resistance in the fuel pump electrical circuit. The most likely cause of poor fuel volume is a clogged in­tank strainer or inline fuel filter.

Fuel Quality

In an effort to improve air quality, many areas of the country use oxygenated fuels. Oxygenates are alcohols, such as ethanol and ether, that add oxygen to the air/fuel mixture. This helps reduce carbon monoxide emissions and inhibits the formation of ground level ozone. However, if alcohol concentration exceeds about 1O% by volume, it can cause the engine to run too lean as well as damage the fuel pump. This is why checking fuel quality is sometimes necessary in order to uncover the root cause of a driveability problem. To check fuel quality, you will need a glass cylinder graduated in milliliters. Begin by filling the cylinder to the 90ml mark with fuel. Next, add 10ml of water to bring the liquid level up to the 100 ml mark. Now seal the end of the cylinder and shake the mixture thoroughly. Finally, place the cylinder on a level surface and allow the solu­tion to settle so that the liquids can separate. Since water is heavier than gasoline, the water will settle to the bottom of the cylinder along with any alcohol contained in the fuel. This is known as the ‘Water Extraction Method.’ If the fuel contains any alcohol, the water level will be greater than its initial reading of 10ml. If the new water level exceeds the 20ml mark, it indicates that alcohol concentration is greater than 10%. Under this condition, the tank should be drained and cleaned, and the fuel lines should be flushed using compressed air. In addition, a new in-tank strainer and inline fuel filter should be installed.

Injector Balance

Defective fuel injectors can cause problems ranging from a cylinder misfire to a no-start condition. A balance test allows you to pinpoint faulty injectors by measuring the amount of pressure drop that occurs as each injector is activated. Before conducting the test, allow the engine to cool, since any vapor bubbles in the fuel rail will yield misleading test results. Begin by connecting the fuel pressure gauge to the schrader valve, and the leads from the injector balance tester to the battery. Next, unplug the injector harness connectors, and then connect the tool’s test plug to an injector. With the test equipment in place, run the fuel pump until the maximum pressure is achieved, and then record the pressure reading. Now activate the balance tester, and then record the new pressure reading the moment the gauge needle stops. The difference in the two readings is the injector’s pressure drop (flow rate). Repeat this procedure for each of the remaining injectors, making sure to re-pressurize the fuel system before each test. When the test is complete, compare your results with the manufacturer’s specifications for injector drop. Injectors with a lower than normal pressure drop will cause the affected cylinder to run lean, while excessive drop will produce the op­posite effect. If injector flow rate, as indicated by the pressure drop readings, is not within the proper specifications, using the appropriate cleaning equipment may correct the problem. In most cases however, the faulty injector(s) will require replace­ment. Also, be aware that the injectors on some vehicles are not meant to be cleaned. This is because the injector windings can become permanently damaged when exposed to cleaning solutions.


The two most common injector control drivers are the ‘saturated switch’ and the ‘peak and hold’ type. With a saturated switch, maximum current is applied to the injector for the duration of the pulse. For this reason, saturated switch drivers are used to control high-impedance injectors (12+ ohms). In contrast, a peak and hold driver allows maximum current to flow just long enough to open the injector(approximately 1.5 ms). At that point, current is reduced to the level required to hold the injector open. Peak and hold drivers are used to control low­ impedance injectors, such as the type found on many TBI systems.


Waveform analysis is the most effective method available for identifying problems in the injectors and their control circuits. To perform this test, connect the ground probe from a lab scope to the negative battery terminal, and the channel probe to the control circuit (ground side) of the injector you wish to check. Set the voltage scale to read 10 volts per division and adjust the time base to 2ms per division. In a saturated switch waveform, the upper horizontal line shows the supply voltage, while the lower horizontal line represents the pulse width (injector ON-time). Supply voltage should be the same as charging system voltage, while the control signal should be within 0.1V of ground. If voltage is incorrect, look for high resistance in the power or ground circuits. Once the injector is turned off, the magnetic field surrounding the solenoid winding collapses. This induces a voltage into the coil approximately five times greater than the supply voltage. If the amplitude of the inductive spike is less than this, and the power and grounds are OK, the coil winding is probably shorted. This condition will cause the affected cylinder to run lean.In a peak and hold waveform, there are two pulses and two inductive spikes. The first pulse pulls the injector open, and will be approximately 1.2-l.8ms in duration regardless of engine operating conditions. The first inductive spike indicates the start of the current limiting pulse. Unlike the initial pulse, the current limiting pulse varies according to engine speed and load. At the conclusion of this pulse, a second inductive spike occurs. While the height of this spike may appear to be equal to or greater than the first, keep in mind that there is less voltage applied to the injector during the current limiting phase. This is because a portion of the supply voltage is dropped across a current limiting resistor inside the ECM. This voltage drop is represented by the height at which the second pulse occurs. Consequently, the second spike has less amplitude than the first.


Air induction components consist of the intake manifold, throttle body assembly, ductwork, air filter and air cleaner housing. Depending on the application, parallel air ducts may be used for silencing. On systems that use mass airflow control, the integrity of the ductwork is extremely important . This is because a leak downstream of the MAF sensor will cause the engine to draw in unmetered air. As a result, the air/fuel ratio will become too lean. When checking for air leaks on this type of system, look for improperly installed rigid ductwork and/or cracks in flexible air intake hoses. While the condition of the intake plumbing may not be critical to mixture balance on speed density systems, missing or damaged ductwork or hoses can cause other performance problems. For example, a missing duct allows the engine to draw in hot under hood air, resulting in higher combustion temperatures. This can cause spark knock as well as excessive NOx emissions.


All of us who work with computers have experienced instances when we worked a little too fast and locked up our computer. A computerized control system is similar in function to all computers, and inputs can be received faster than the ECM is capable of processing changes in input values. This is especially true with operator inputs, such as the throttle position sensor. It is possible for the driver to exercise the throttle faster than the ECM could process changes. When this occurs, the usual result is an increase in emissions. In the interest of reducing emissions to the lowest possible levels, electronic throttle controls have been developed.

Electronic throttle control offers several advantages over a conventional mechanical throttle:

  • A reduction in moving parts reduces wear, adjustments and maintenance.
  • The human factor in throt­tle control is now an input to the ECM rather than an ac­tual change in throttle angle.This helps to keep emissions as low as possible.
  • A greater accuracy in throttle control improves driveability, economy and vehicle response.

Some of the common system features will be discussed here, but you are cautioned to reference specific factory information when servicing an electronic throttle control system. At the heart of all throttle control systems is the throttle actuator. This device contains the throttle plate and the necessary motor to move the throttle plate. Some throttle actuators are duty cycle controlled , while others move in response to a voltage signal of the correct polarity. On some vehicles the ECM directly controls the throttle actuator. Other manufacturers use a separate throttle actuator module between the throttle actuator and the ECM. Use of a separate throttle actuator module was more common on older systems. On most throttle control systems, inputs to the ECM are enhanced when compared to earlier electronic control systems.For the purpose of enhanced accuracy, the throttle actuator usually contains two Throttle Position (TP) Sensors. One TP sensor will start with a low voltage that gradually rises as the throttle opens. The second TP sensor will usually have a high voltage at idle that drops as the throttle opens. The ECM uses opposing signals to calculate a much more precise measurement of throttle position than would be possible by simply directly measuring one TP sensor. The system also uses at least one Accelerator Pedal Position (APP) sensor, although the use of two sensors is much more common. Once again, two sensors with different voltage values allow the ECM to calculate a value for accelerator position that is more accurate than a single design among the various manufacturers. Many vehicles with electronic throttle control use an accelerator pedal module with integral Accelerator Pedal Position (APP) sensors. The ECM uses the accelerator position sensor input to command the throttle actuator. The APP sensors may be mounted on the accelerator pedal, or they may be part of an integral throttle assembly. The failure of one sensor will usually set a code and limit throttle operation. The failure of two sensors will usually result in the throttle actuator being disabled. In addition, some systems use additional inputs to enhance throttle control. On Mercedes Benz systems, for example, the throttle opening is closed to a predetermined value if the ECM receives a brake pedal input together with an accelerator pedal input.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

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