FUEL-INJECTION SYSTEM DIAGNOSIS AND SERVICE

1. 81 FUEL-INJECTION SYSTEM DIAGNOSIS AND SERVICE

2. Figure 81-1 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.

3. TECH TIP: Pressure Transducer Fuel Pressure Test Using a pressure transducer and a graphing multimeter (GMM) or digital storage oscilloscope (DSO) allows the service technician to view the fuel pressure over time. - SEE FIGURE 81–2(a) . Note that the fuel pressure dropped from 15 PSI down to 6 PSI on a TBI-equipped vehicle after just one minute. A normal pressure holding capability is shown in - FIGURE 81–2(b) when the pressure dropped only about 10% after 10 minutes on a port-fuel injection system.

4. Figure 81-2 (a) A fuel-pressure graph after key on, engine off (KOEO) on a TBI system.

5. Figure 81-2 (b) Pressure drop after 10 minutes on a normal port fuel-injection system.

6. Figure 81-3 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 on this TBI system.

7. Figure 81-4 All fuel injectors should make the same sound with the engine running at idle speed. A lack of sound indicates a possible electrically open injector or a break in the wiring. A defective computer could also be the cause of a lack of clicking (pulsing) of the injectors.

8. TECH TIP: Stethoscope Fuel-Injection Test A commonly used test for injector operation is to listen to the injector using a stethoscope with the engine operating at idle speed. - SEE FIGURE 81–4 . All injectors should produce the same clicking sound. If any injector makes a clunking or rattling sound, it should be tested further or replaced. With the engine still running, place the end of the stethoscope probe to the return line from the fuel-pressure regulator. - SEE FIGURE 81–5 . Fuel should be heard flowing back to the fuel tank if the fuel-pump pressure is higher than the fuel-regulator pressure. If no sound of fuel is heard, then either the fuel pump or the fuel-pressure regulator is at fault.

9. Figure 81-5 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.

10. TECH TIP: Quick and Easy Leaking Injector Test Leaking injectors may be found by disabling the ignition, unhooking all injectors, and checking exhaust for hydrocarbons (HC) using a gas analyzer while cranking the engine (maximum HC = 300 PPM). This test does not identity which injector is leaking but it does confirm that one or more injector is leaking.

11. Figure 81-6 Using a scan tool to check for IAC counts or percentage as part of a diagnostic routine.

12. TECH TIP: No Spark, No Squirt Most electronic fuel-injection computer systems use the ignition primary (pickup coil or crank sensor) pulse as the trigger for when to inject (squirt) fuel from the injectors (nozzles). If this signal were not present, no fuel would be injected. Because this pulse is also necessary to trigger the module to create a spark from the coil, it can be said that “no spark” could also mean “no squirt.” Therefore, if the cause of a no-start condition is observed to be a lack of fuel injection, do not start testing or replacing fuel-system components until the ignition system is checked for proper operation.

13. Figure 81-7 Checking the fuel pressure using a fuel-pressure gauge connected to the Schrader valve.

14. Figure 81-8 Shutoff valves must be used on vehicles equipped with plastic fuel lines to isolate the cause of a pressure drop in the fuel system.

15. Figure 81-9 (a) Noid lights are usually purchased as an assortment so that one is available for any type or size of injector wiring connector.

16. Figure 81-9 (b) The connector is unplugged from the injector and a noid light is plugged into the harness side of the 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 normally.

17. Figure 81-10 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.

18. Figure 81-11 An ohmmeter is connected to the injector electrical terminals to read injector coil resistance.

19. Figure 81-12 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.

20. TECH TIP: Equal Resistance Test All fuel injectors should measure the specified resistance. However the specification often indicates the temperature of the injectors be at room temperature and of course will vary according to the temperature. Rather than waiting for all of the injectors to achieve room temperature, measure the resistance and check that they are all within 0.4 ohm of each other. To determine the difference, record the resistance of each injector and then subtract the lowest resistance reading from the highest resistance reading to get the difference. If more than 0.4 ohm then further testing will be needed to verify defective injector(s).

21. Figure 81-13 (a) The meter is connected to read one group of three 12-ohm injectors. The result should be 4 ohms and this reading is a little low indicating that at least one injector is shorted (low resistance).

22. Figure 81-13 (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.

23. Figure 81-14 If an injector has the specified resistance, this does not mean that it is okay. This injector had the specified resistance yet it did not deliver the correct amount of fuel because it was clogged.

24. Figure 81-15 After connecting a pressure gauge, unplug the electrical connector from an injector and attach the test lead from the pulse unit to the injector.

25. Figure 81-16 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.

26. Figure 81-17 A digital storage oscilloscope can be easily connected to an injector by carefully back probing the electrical connector.

27. Figure 81-18 The injector on-time is called the pulse width. (Courtesy of Fluke Corporation)

28. Figure 81-19 A typical peak-and-hold fuel-injector waveform. Most fuel injectors that measure less than 6 ohms will usually display a similar waveform.

29. Figure 81-20 A set of six reconditioned injectors. The sixth injector is barely visible at the far right.

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31. Figure 81-21 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.

32. Figure 81-22 A typical IAC.

33. Figure 81-23 Some IAC units are purchased with the housing as shown. Carbon buildup in these passages can cause a rough or unstable idling or stalling.

34. Figure 81-24 (a) Nothing looks unusual when the hood is first opened.

35. Figure 81-24 (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.

36. REAL WORLD FIX: There Is No Substitute for a Thorough Visual Inspection An intermittent “check engine” light and a randommisfire diagnostic trouble code (DTC) P0300 was being diagnosed. A scan tool did not provide any help because all systems seemed to be functioning normally. Finally, the technician removed the engine cover and discovered a mouse nest. SEE FIGURE 81–24 .

37. Figure 81-25 Checking fuel-pump volume using a hose from the outlet of the fuel-pressure regulator into a calibrated container.

38. Figure 81-26 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.

39. Figure 81-27 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. Typical two-line cleaning machines include Carbon Clean, Auto Care, Injector Test, DeCarbon, or Motor-Vac.

40. Figure 81-28 To thoroughly clean a throttle body, it is sometimes best to remove it from the vehicle.

41. TECH TIP: Check the Injectors at the “Bends and the Ends” Injectors that are most likely to become restricted due to clogging of the filter basket screen are the injectors at the ends of the rail especially on returnless systems where dirt can accumulate. Also the injectors that are located at the bends of the fuel rail are also subject to possible clogging due to the dirt being deposited where the fuel makes a turn in the rail.

42. TECH TIP: Be Sure to Clean the Fuel Rail Whenever you service the fuel injectors, or if you suspect that there may be a fuel-injector problem, remove the entire fuel rail assembly and check the passages for contamination. Always thoroughly clean the rail when replacing fuel injectors.

43. FUEL-PUMP RELAY CIRCUIT DIAGNOSIS 1 The tools needed to diagnose a circuit containing a relay include a digital multimeter (DMM), a fused jumper wire, and an assortment of wiring terminals.

44. FUEL-PUMP RELAY CIRCUIT DIAGNOSIS 2 Start the diagnosis by locating the relay center. It is under the hood on this General Motors vehicle, so access is easy. Not all vehicles are this easy.

45. FUEL-PUMP RELAY CIRCUIT DIAGNOSIS 3 The chart under the cover for the relay center indicates the location of the relay that controls the electric fuel pump.

46. FUEL-PUMP RELAY CIRCUIT DIAGNOSIS 4 Locate the fuel-pump relay and remove by using a puller if necessary. Try to avoid rocking or twisting the relay to prevent causing damage to the relay terminals or the relay itself.

47. FUEL-PUMP RELAY CIRCUIT DIAGNOSIS 5 Terminals 85 and 86 represent the coil inside the relay. Terminal 30 is the power terminal, 87a is the normally closed contact, and 87 is the normally open contact.

48. FUEL-PUMP RELAY CIRCUIT DIAGNOSIS 6 The terminals are also labeled on most relays.

49. FUEL-PUMP RELAY CIRCUIT DIAGNOSIS 7 To help make good electrical contact with the terminals without doing any harm, select the proper-size terminal from the terminal assortment.

50. FUEL-PUMP RELAY CIRCUIT DIAGNOSIS 8 Insert the terminals into the relay socket in 30 and 87.