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OBJECTIVES. After studying Chapter 24, the reader will be able to: Prepare for ASE Engine Performance (A8) certification test content area “C” (Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair). Describe how to check an electric fuel pump for proper pressure and volume delivery.
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OBJECTIVES After studying Chapter 24, the reader will be able to: • Prepare for ASE Engine Performance (A8) certification test content area “C” (Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair). • Describe how to check an electric fuel pump for proper pressure and volume delivery. • Explain how to check a fuel-pressure regulator. • Describe how to test fuel injectors. • Explain how to diagnose electronic fuel-injection problems. • Describe how to service the fuel injection system.
PORT FUEL-INJECTION PRESSURE REGULATOR DIAGNOSIS • To test a vacuum-controlled fuel-pressure regulator, follow these steps: • Connect a fuel-pressure gauge to monitor the fuel pressure. • Locate the fuel-pressure regulator and disconnect the vacuum hose from the regulator. • With the engine running at idle speed, reconnect the vacuum hose to the fuel-pressure regulator while watching the fuel-pressure gauge. • Using a hand-operated vacuum pump, apply vacuum (20 in. Hg) to the regulator. The regulator should hold vacuum. If the vacuum drops, replace the fuel-pressure regulator.
PORT FUEL-INJECTION PRESSURE REGULATOR DIAGNOSIS FIGURE 24-1 If the vacuum hose is removed from the fuel-pressure regulator when the engine is running, the fuel pressure should increase. If it does not increase, then the fuel pump is not capable of supplying adequate pressure or the fuel-pressure regulator is defective. If gasoline is visible in the vacuum hose, the regulator is leaking and should be replaced.
PRESSURE TRANSDUCER FUEL PRESSURE TEST FIGURE 24-2 (a) A fuel-pressure graph after key on, engine off (KOEO). (b) Pressure drop after 10 minutes on a normal port fuel-injection system.
DIAGNOSING ELECTRONIC FUEL-INJECTION PROBLEMS USING VISUAL INSPECTION • All fuel-injection systems require the proper amount of clean fuel delivered to the system at the proper pressure and the correct amount of filtered air. • The following items should be carefully inspected before proceeding to more detailed tests. • Check the air filter and replace as needed. • Check the air induction system for obstructions. • Check the conditions of all vacuum hoses. Replace any hose that is split, soft (mushy), or brittle. • Check the positive crankcase ventilation (PCV) valve for proper operation or replacement as needed • Check all fuel-injection electrical connections for corrosion or damage. • Check for gasoline at the vacuum port of the fuel-pressure regulator if the vehicle is so equipped.
DIAGNOSING ELECTRONIC FUEL-INJECTION PROBLEMS USING VISUAL INSPECTION FIGURE 24-3 A clogged PCV system caused the engine oil fumes to be drawn into the air cleaner assembly. This is what the technician discovered during a visual inspection.
STETHOSCOPE FUELINJECTION TEST FIGURE 24-4 All fuel injectors should make the same sound with the engine running at idle speed. A lack of sound could indicate an electrically open injector, a break in the wiring, or a stuck closed injector. A defective computer could also be the cause of a lack of clicking (pulsing) of the injectors.
STETHOSCOPE FUELINJECTION TEST FIGURE 24-5 Fuel should be heard returning to the fuel tank at the fuel return line if the fuel-pump and fuel-pressure regulator are functioning correctly.
SCAN TOOL VACUUM LEAK DIAGNOSIS • If a vacuum (air) leak occurs on an engine equipped with a speed-density-type of fuel injection, the extra air would cause the following to occur: • The idle speed increases due to the extra air just as if the throttle pedal was depressed. • The MAP sensor reacts to the increased air from the vacuum leak as an additional load on the engine. • The computer increases the injector pulse width slightly longer due to the signal from the MAP sensor. • The air-fuel mixture remains unchanged. • The idle air control (IAC) counts will decrease, thereby attempting to reduce the engine speed to the target idle speed stored in the computer memory.
SCAN TOOL VACUUM LEAK DIAGNOSIS FIGURE 24-6 Using a scan tool to check for IAC counts or percentage as part of a diagnostic routine.
PORT FUEL-INJECTION SYSTEM DIAGNOSIS • To determine if a port fuel-injection system—including the fuel pump, injectors, and fuel-pressure regulator—are operating correctly, take the following steps. • Attach a fuel-pressure gauge to the Schrader valve on the fuel rail. • Turn the ignition key on or start the engine to build up the fuel-pump pressure (to about 35 to 45 PSI). • Wait 20 minutes and observe the fuel pressure retained in the fuel rail and note the PSI reading. • The check valve in the fuel pump • Leaking injectors, lines, or fittings • A defective (leaking) fuel-pressure regulator • Reenergize the electric fuel pump. • Clamp the fuel supply line, and wait 10 minutes (see caution box). If the pressure drop does not occur, replace the fuel pump. If the pressure drop still occurs, continue with the next step.
PORT FUEL-INJECTION SYSTEM DIAGNOSIS FIGURE 24-7 Checking the fuel pressure using a fuel-pressure gauge connected to the Schrader valve.
PORT FUEL-INJECTION SYSTEM DIAGNOSIS • To determine which unit is defective, perform the following: • Reenergize the electric fuel pump. • Clamp the fuel supply line, and wait 10 minutes (see caution box). If the pressure drop does not occur, replace the fuel pump. If the pressure drop still occurs, continue with the next step. • Repeat the pressure buildup of the electric pump and clamp the fuel return line. • If the pressure drop still occurs, one or more of the injectors is leaking. Remove the injectors with the fuel rail and hold over paper. Replace those injectors that drip one or more drops after 10 minutes with pressurized fuel.
PORT FUEL-INJECTION SYSTEM DIAGNOSIS FIGURE 24-8 Shutoff valves must be used on vehicles equipped with plastic fuel lines to isolate the cause of a pressure drop in the fuel system.
TESTING FOR AN INJECTOR PULSE • One of the first checks that should be performed when diagnosing a no-start condition is whether the fuel injectors are being pulsed by the computer. • Checking for proper pulsing of the injector is also important in diagnosing a weak or dead cylinder.
TESTING FOR AN INJECTOR PULSE FIGURE 24-9 (a) Noid lights are usually purchased as an assortment so that one is available for any type or size of injector wiring connector. (b) The connector is unplugged from the injector and a noid light is plugged into the injector connector. The noid light should flash when the engine is being cranked if the power circuit and the pulsing to ground by the computer are functioning okay.
TESTING FOR AN INJECTOR PULSE • Possible noid light problems and causes include the following: • The light is off and does not flash. • The noid light flashes dimly. • The noid light is on and does not flash. FIGURE 24-10 Use a DMM set to read DC volts to check the voltage drop of the positive circuit to the fuel injector. A reading of 0.5 volt or less is generally considered to be acceptable.
CHECKING FUEL-INJECTOR RESISTANCE • Each port fuel injector must deliver an equal amount of fuel or the engine will idle roughly or perform poorly. • The electrical balance test involves measuring the injector coil-winding resistance. • For best engine operation, all injectors should have the same electrical resistance. • To measure the resistance, carefully release the locking feature of the connector and remove the connector from the injector.
CHECKING FUEL-INJECTOR RESISTANCE FIGURE 24-11 Connections and settings necessary to measure fuel-injector resistance. (Courtesy of Fluke Corporation)
CHECKING FUEL-INJECTOR RESISTANCE FIGURE 24-12 To measure fuel-injector resistance, a technician constructed a short wiring harness with a double banana plug that fits into the V and COM terminals of the meter and an injector connector at the other end. This setup makes checking resistance of fuel injectors quick and easy.
MEASURING RESISTANCE OF GROUPED INJECTORS • To measure the resistance of these injectors, it is often easiest to measure each group of three that is wired in parallel. • The resistance of three injectors wired in parallel is one-third of the resistance of each individual injector. • For example, • Injector resistance = 12 ohms (Ω) • Three injectors in parallel = 4 ohms (Ω)
MEASURING RESISTANCE OF GROUPED INJECTORS FIGURE 24-13 (a) The meter is connected to read one group of three 12-ohm injectors. The result should be 4 ohms. This reading is a little low, indicating that at least one injector is shorted (low resistance). (b) This meter is connected to the other group of three injectors and indicates that most, if not all three, injectors are shorted. The technician replaced all six injectors and the engine ran great.
MEASURING RESISTANCE OF INDIVIDUAL INJECTORS • While there are many ways to check injectors, the first test is to measure the resistance of the coil inside and compare it to factory specifications. FIGURE 24-14 If an injector has the specified resistance, that does not mean that it is okay. This injector had the specified resistance yet did not deliver the specified amount of fuel because it was clogged.
PRESSURE-DROP BALANCE TEST • The pressure balance test involves using an electrical timing device to pulse the fuel injectors on for a given amount of time, usually 500 ms or 0.5 second, and observing the drop in pressure that accompanies the pulse. • If the fuel flow through each injector is equal, the drop in pressure in the system will be equal.
PRESSURE-DROP BALANCE TEST FIGURE 24-15 Connect a fuel-pressure gauge to the fuel rail at the Schrader valve.
INJECTOR VOLTAGE DROP TEST • Another test of injectors involves pulsing the injector and measuring the voltage drop across the windings as current is flowing. FIGURE 24-16 An injector tester being used to check the voltage drop through the injector while the tester is sending current through the injectors. This test is used to check the coil inside the injector. This same tester can be used to check for equal pressure drop of each injector by pulsing the injector on for 500 ms.
SCOPE TESTING FUEL INJECTORS • A scope (analog or digital storage) can be connected into each injector circuit. • There are three types of injector-driven drive circuits and each type of circuit has its own characteristic pattern. FIGURE 24-17 A digital storage oscilloscope can be easily connected to an injector by carefully back probing the electrical connector.
SCOPE TESTING FUEL INJECTORS • Saturated Switch Type • Peak-and-Hold Type • Pulse-Width Modulated Type FIGURE 24-18 The injector on-time is called the pulse width. (Courtesy of Fluke Corporation)
SCOPE TESTING FUEL INJECTORS FIGURE 24-19 A typical peak-and-hold fuel-injector waveform. Most fuel injectors that measure less than 6 ohms will usually display a similar waveform.
IF THREE OF SIX INJECTORS ARE DEFECTIVE, SHOULD I ALSO REPLACE THE OTHER THREE? FIGURE 24-20 A set of six reconditioned injectors.
IDLE AIR SPEED CONTROL DIAGNOSIS • On an engine equipped with fuel injection (TBI or port injection), the idle speed is controlled by increasing or decreasing the amount of air bypassing the throttle plate. • Again, an electronic stepper motor or pulse-width modulated solenoid is used to maintain the correct idle speed. • This control is often called the idle air control (IAC).
IDLE AIR SPEED CONTROL DIAGNOSIS FIGURE 24-21 An IAC controls idle speed by controlling the amount of air that passes around the throttle plate. More airflow results in a higher idle speed.
IDLE AIR SPEED CONTROL DIAGNOSIS FIGURE 24-23 Some IAC units are purchased with the housing as shown. Carbon buildup in these passages can cause a rough or unstable idling or stalling. FIGURE 24-22 A typical IAC.
THERE IS NO SUBSTITUTE FOR A THOROUGH VISUAL INSPECTION FIGURE 24-24 (a) Nothing looks unusual when the hood is first opened. (b) When the cover is removed from the top of the engine, a mouse or some other animal nest is visible. The animal had already eaten through a couple of injector wires. At least the cause of the intermittent misfire was discovered.
FUEL-INJECTION SERVICE • Fuel-injection system service should include the following operations: • Check fuel-pump operating pressure and volume. • Test the fuel-pressure regulator for operation and leakage. • Flush the entire fuel rail and upper fuel-injector screens including the fuel-pressure regulator. • Clean the fuel injectors. • Decarbon the engine assembly. • Clean the throttle plate and idle air control passages. • Relearn the onboard computer.
FUEL-INJECTION SERVICE FIGURE 24-25 Checking fuel-pump volume using a hose from the outlet of the fuel-pressure regulator into a calibrated container.
FUEL-INJECTION SERVICE FIGURE 24-26 Testing fuel-pump volume using a fuel-pressure gauge with a bleed hose inserted into a suitable container. The engine is running during this test.
FUEL-INJECTION SERVICE FIGURE 24-27 A typical two-line cleaning machine hookup, showing an extension hose that can be used to squirt a cleaning solution into the throttle body while the engine is running on the cleaning solution and gasoline mixture.
FUEL-INJECTION SERVICE FIGURE 24-28 To thoroughly clean a throttle body, it is sometimes best to remove it from the vehicle.
FUEL-SYSTEM SCAN TOOL DIAGNOSTICS • Diagnosing a faulty fuel system can be a difficult task. • However, it can be made easier by utilizing the information available via the serial data stream. • By observing the long-term fuel trim and the short-term fuel trim, we can determine how the fuel system is performing. • Short-term fuel trim and long-term fuel trim can help us to zero in on specific areas of trouble. • Readings should be taken at idle and at 3000 RPM. Use the following chart as a guide.
