Fuel pressure in a F.I. system is typically 10 times greater than in a carbureted system. Many (but not all) fuel injection systems have an injector for each cylinder. This is called Multiport Fuel Injection. A multiport system delivers fuel through the injectors just behind the intake valve of each cylinder. When the intake valve opens, the fuel flows into the cylinder and is burned. If a fuel injection system does not have an injector for each cylinder it is known as a "Throttle Body Injection" system. In a T.B.I. system fuel is delivered through 1 or 2 large injectors just above the throttle plate at the entrance to the intake manifold.

• Fuel Pump(s) - Delivers Pressurized fuel from tank up to injection valves (injectors)
• Fuel Filter(s) - Cleans fuel and removes sediment, dirt, and water
• Fuel Pressure Regulator - Sets and keeps fuel pressure at determined value to ensure even and constant delivery values
• Fuel Rail - A hollow tube for fuel to flow through and ports for injectors to attach to
• Fuel Injector - electrically triggered solenoid valve that opens and closes, turning on and off the supply of fuel to the engine
• Throttle Valve - The "butterfly" flap controlled by accelerator linkage that regulates the air flow into an engine
• Electronic Control Unit - The computer that controls the fuel injection system (ECU) The ECU receives inputs from various sensors and adjusts injector open time to allow more or less fuel to pass through the injectors. Injector open time is changed by the computer sending a longer or shorter electric pulse to the injector.
• Air Flow Meter - A spring loaded flap mounted in the air intake system before the throttle valve that measures the air moving past it and sends an electric signal to the ECU
• The air flow meter is a "load" measuring input to the ECU
• Air Mass Meter - A more sophisticated air flow meter using a hot wire or film to measure not only air volume but density. This allows more accuracy measuring load.
• M.A.P. Sensor - A sensor connected to a manifold vacuum source to measure engine load and send an electric signal to the ECU
• Throttle Switch - A sensor attached to the throttle valve linkage that sends a signal to the ECU about throttle position (ie. open, closed)
• Temperature Sensor - A Negative Temperature Coefficient sensor (N.T.C.) that measures intake air temperature or coolant temperature and sends the information to the ECU. NTC sensors have greater resistance as they get colder and less as they get warmer
• E.F.I. vehicles may have both an air temperature sensor and a coolant temperature sensor.
• Oxygen Sensor - (Exhaust gas sensor) A probe mounted in the exhaust manifold that measures the amount of oxygen in the exhaust stream. The ECU reads the voltage generated by this sensor and adjusts the mixture for best air/fuel ratio
• Engine RPM Sensor - Engine speed may be measured by a wire attached to the ignition system or by a flywheel or front pulley mounted speed sensor(s).
• Cold Start Valve - (cold start injector) A fuel injector that is controlled by electrical wiring that is not connected to the ECU. The cold start valve is triggered to spray additional fuel into the intake manifold during a cold startup only. The amount of cold start fuel sprayed is determined by a Thermal Time Switch.
• Auxiliary Air Valve - A valve that opens when the vehicle is cold to allow additional air to bypass the throttle plate and increase idle speed. This valve has a heating element in it that warms the valve and shuts off the extra air when the engine warms up.
• Idle Speed Control Valve - A more sophisticated auxiliary air valve that is controlled by a computer and can adjust idle speed at all temperatures and engine loads.
   

• Base Ignition Timing
  A vehicles base ignition timing (if adjustable) is set by rotating the distributor in relationship to the distributor shaft. This is usually checked and adjusted at idle speed. You may have to pull off or disable vacuum units and/or jump or disconnect electrical connectors to set this. Rotating the distributor body toward the direction of distributor rotation will advance base timing, rotation it away from the direction of rotation will retard base timing. Be sure to check emission sticker or shop manual for the proper procedure for checking base ignition timing on your vehicle.
   
• Ignition timing changes for engine speed (with a distributor)
  Ignition timing must occur at just the right time. At slow engine speeds the best time for spark is just before the piston gets to the top of the compression stroke. At faster engine speeds the spark has to be introduced earlier because there is less time to burn the mixture when the piston is moving faster. When the ignition spark occurs earlier, it is said to be advanced. Timing advances with increasing RPM. The centrifugal advance mechanism in the distributor controls the RPM timing advance by using springs and flyweights. The faster the RPM, the more the flyweights move outwards against spring tension and advance the distributor timing. Most automotive engines today have a total RPM advance of about 30-32 degrees at maximum engine RPM.
   
• Ignition timing changes for load (with distributor)
  The load on an engine is the work it must do. Driving up hills or pulling extra weight increases vehicle engine load. A good indication of engine load is amount of intake manifold vacuum. Under light loads intake manifold vacuum is relatively high and the amount of air/fuel mixture drawn into the cylinders is small because the throttle valve is only opened part way. On compression this "thin" mixture will produce less combustion pressure and will allow the ignition timing to be advanced.

Under heavy loads intake manifold vacuum is low and large amounts of air and fuel enter the cylinders because the throttle plate is opened more. This rich, highly charged mixture will cause higher compression pressure and tend to detonate in the cylinder causing knocking or pinging. Under heavy load conditions, ignition timing must be retarded to bring back normal combustion. A vacuum advance mechanism on the distributor adjusts the ignition timing for engine load by using a flexible diaphragm inside a metal housing. The diaphragm is attached to the distributor breaker plate and can move the plate to adjust the ignition trigger signal. The vacuum advance unit is attached to a hose the comes from a ported vacuum source located just above the throttle valve.

When the throttle plate is closed, no vacuum can reach the diaphragm and no advance occurs. When the throttle plate is opened slightly, manifold vacuum pulls on the diaphragm to advance timing. When the throttle is wide open vacuum is low and the diaphragm returns to a neutral position and timing is not advanced. Some vehicles have vacuum retard mechanisms also. A vacuum retard mechanism applies vacuum to the opposite side of the diaphragm and retards timing to help the car meet emissions requirements.

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