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Electronic fuel injection (EFI) comes in several basic varieties. Back in the mid to late 1980s, many cars were equipped with "throttle body injection (TBI). This type of EFI system uses a single throttle body with one or two fuel injectors. The throttle body sits on the intake manifold in place of a carburetor, and sprays fuel and air into the intake manifold. By the early 1990s, TBI was obsolete. Another type of electronic fuel injection system is "Multiport" injection (MPI). This type of system has a separate fuel injector for each of the engine's cylinders. Each injector sprays fuel directly into the intake valve port. This provides better cylinder-to-cylinder fuel distribution than throttle body injection systems and improves engine performance, emissions and fuel economy.

Some General Motors vehicles have a "Central Port Injection (CPI) fuel injection system, or its newer version "Central Multi-Port Injection" (CMPI). Here, a central "Maxi" fuel injector feeds fuel through individual lines to mechanical poppet injector valves at each cylinder. When fuel flows through the lines and reaches a certain pressure (37 to 40 psi), it pushes the poppet valves open and sprays out of the individual injectors.

 Most older multiport fuel injection system energize all the injectors at the same time (once every revolution of the crankshaft or every other revolution depending on the application). Most newer multiport fuel injection systems, by comparison, energize each injector individually just before the intake valve opens. This is called "Sequential" fuel injection (SFI). Energizing each injector one at a time allows faster changes in the air/fuel mixture and better fuel control to reduce emissions and improve fuel economy and performance.

Fuel injectors do not spray fuel continuously, but are "pulsed" on and off by the powertrain control module (PCM). On most engines, the injectors receive power (battery voltage) when the ignition is on. But the injectors do not open until the PCM driver circuit completes the ground connection to the injector solenoid.

When the PCM commands the injector on, the driver circuit closes the ground connection allowing current to flow through the solenoid coil in the top of the injector. This creates a powerful magnetic field in the coil that pulls the injector pintle or ball valve open. When the injector valve opens, pressure in the fuel supply to the injector forces fuel out of the injector nozzle. Fuel sprays into the intake port, mixes with air and is sucked into the cylinder when the intake valve opens. When the injector has delivered its dose of fuel, the PCM opens the driver circuit and stops the flow of current through the injector. The magnetic field collapses and spring pressure forces the injector valve to snap shut. This stops the flow of fuel and the injector ceases to spray fuel until the next pulse from the PCM.

One way to tell if the injectors are working, therefore, is to listen for a buzzing noise when the engine is running. Another is to hold the tip of a steel screwdriver next to each injector to see if it is attracted by a magnetic field.

Injectors can sometimes fail if the solenoid coil inside develops an open or a short. One way to check the electrical condition of the injectors is to measure the resistance across the injector terminals. This is done by unplugging the injector's wiring connector (key off) and measuring the resistance between the injector terminals with an ohmmeter. Injectors typically fall into one of two categories: high resistance (12 to 17 ohms) or low resistance (2 to 7 ohms). If resistance is out of specifications (refer to a service manual for the exact specifications), the injector is defective and should be replaced.

If you don't have the exact specifications, another method is to measure and compare the resistance readings of all the injectors. If one is more than a few ohms higher or lower than the rest, it is probably defective and should be replaced.

NOTE: It doesn't take much of a change in resistance to affect injector performance. A shorted injector may pull so much current through the driver circuit that it may prevent other injectors from opening or cause the engine to stall or not start.

The amount of air entering a fuel injected engine is controlled by the throttle body. The throttle is connected by a cable to the gas pedal, but on some newer vehicles there is no direct connection. The throttle is controlled by the PCM using a small electric motor and inputs from a gas pedal position sensor.

To maintain the correct air/fuel ratio under changing operating conditions, the amount of air entering the engine must be measured or estimated by the PCM. In "Mass Air Flow" EFI systems, a "Vane" (VAF) or "Hot Wire" (MAF) airflow sensor is located ahead of the throttle body to measure incoming air. In "Speed-density" EFI systems, no airflow sensor is used. The PCM estimates airflow using inputs from the "throttle position sensor" (TPS), "manifold absolute pressure "(MAP) sensor and "manifold air temperature" (MAT) sensor.

The air/fuel ratio is controlled by increasing or decreasing the amount of fuel delivered by the injectors as engine speed and load change. The PCM does this by increasing or decreasing the on-time of the injectors. Increasing each injector pulse allows more fuel to spray into the engine while decreasing each injector pulse reduces fuel delivery. When the engine warms up, the PCM goes into a special fuel control mode called "Closed Loop" operation. The PCM starts to use the rich/lean signal from the "oxygen" (O2) sensor in the exhaust manifold to fine tune the fuel mixture. If the O2 sensor reads lean (too much air, not enough fuel), the PCM responds by increasing the duration of the injector pulses. You can see this change on a scope that displays waveforms. When the O2 sensor then reads rich (too much fuel, not enough air), the PCM decreases the duration of the injector pulses. This constant flip-flopping back and forth from lean to rich and back again results in an average fuel mixture that is properly balanced for best fuel economy and emissions.

When a cold engine is first started, the PCM is in "Open Loop" fuel control mode and delivers a relatively rich fuel mixture. If the PCM remains in open loop once the engine is warm, it will hurt fuel economy and emissions. A defective coolant sensor or oxygen sensor can prevent the PCM from going into closed loop. Loop status (open or closed) can be read on a scan tool.

The amount of fuel delivered by the injectors is also affected by fuel pressure. A "fuel pressure regulator" maintains system pressure at a preset level. A diaphragm inside the regulator is connected to engine vacuum so the regulator can increase or decrease fuel pressure in proportion to engine load. On some newer vehicles, the pressure regulator is not on the engine but is part of the fuel pump assembly inside the fuel tank. This is called a "returnless" system because there is no fuel return line from the engine back to the tank. Fuel pressure is generated by a high pressure electric pump usually located in or near the fuel tank. When the ignition is turned on, the PCM energizes a relay that routes voltage to the pump. A no-start condition due to no fuel can be caused by a bad fuel pump relay, a defective fuel pump or wiring problems in the fuel pump circuit.

For the engine to start and run properly, fuel pressure to the injectors and the volume of fuel delivered by the pump must both be within specifications. A difference of only a few pounds may be enough to cause starting and performance problems.

Pressure can be checked with a fuel pressure gauge connected to the test valve on the fuel supply rail, or teed into the fuel supply line. Fuel volume can be measured by disconnecting a fuel line, putting the hose into a container and running the pump briefly (CAUTION: KEEP SPARKS AND FLAME AWAY FROM FUEL!) A good pump should usually deliver about a pint of fuel in 15 seconds. Low fuel pressure and/or low volume can result in a lean air/fuel mixture, hard starting, misfire, detonation, increased emissions and poor performance. Low fuel pressure can be caused by a defective fuel pressure regulator, a weak fuel pump, a plugged fuel fuel filter, or a restricted fuel line.

Too much fuel pressure can cause a rich air/fuel mixture, rough idle, surging, increases fuel consumption and emissions. Too much fuel pressure can be caused by a defective pressure regulator or a plugged fuel return line.

Older fuel injection systems often have an additional "cold start" injector that serves the same function as the choke on a carburetor for starting (fuel injection systems do not have a choke). The cold start valve sprays extra fuel into a cold engine when it is first cranked for easier starting. A "warm-up regulator" provides additional fuel enrichment while the engine is warming up. Newer EFI systems do not use either of these devices and rely instead on longer injector pulses from the PCM for fuel enrichment during starting and warm-up.

Idle speed in fuel injected engines is not controlled by the opening of the throttle, but rather by an air bypass circuit. On most EFI applications, idle speed is computer-controlled by an "idle air control" (IAC) valve. A small electric motor opens and closes the bypass circuit valve so air can bypass the throttle plates.

Problems here can occur if the motor fails or the intake system has an air leak downstream of the throttle body. Air leaks typically increase idle speed so the idle control system will try to compensate for the leak by closing the idle air bypass. This may overload and burn out the motor. If idle speed is not within specifications, there is an air leak or a problem with the idle speed control circuit, valve or motor.

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