Chapter 34 Advanced Diagnostics 659 Copyright by Goodheart-Willcox Co., Inc. operation to determine which part or parts could be mal- functioning and causing the symptoms. For example, if an engine misses only when cold, think of what components are involved in cold engine operation. You should think of the coolant temperature sensor, the intake air temperature sensor, and the cold start injector. Th ese components are monitored and controlled by the ECM, which enriches the air-fuel mixture when the engine is cold. If a vehicle’s service history is available, review it to determine if the vehicle has been properly maintained and if there were previous repairs made to the vehicle. Th e service history may contain valuable information that will help you when trying to pinpoint the cause of a problem. Service manuals contain information to help facilitate troubleshooting. Th e chart shown in Figure 34-3 summa- rizes the basic service procedures that can be performed to help fi nd intermittent problems in one particular vehicle. Isolating Electromagnetic Interference Electromagnetic interference (EMI), or radiation interference, occurs when an induced voltage enters another system’s wiring. Sources of EMI can include loose, misrouted, or unshielded spark plug wires police and CB radios and aft ermarket accessories. In the past, electromagnetic interference was limited to noise in the radio speakers. In late-model vehicles, EMI can cause a computer-controlled system to malfunction. For example, induced voltage from a loose spark plug wire could enter a sensor wire. Th is unwanted voltage then enters the computer as false data. Numerous computer mal- functions or false outputs can result. See Figure 34-4. To isolate the source of electromagnetic interference, try turning off or disabling circuits or devices. If, for exam- ple, removing the drive belt from the alternator corrects the problem, suspect problems inside the alternator. If the problem only occurs with the heated windshield turned on, check components within the heated windshield circuit. Figure 34-3. This diagnostic chart shows how different factors and parts can cause abnormal operating conditions. The technician can perform the service procedures listed to simulate intermittent problems. (Nissan) Variable Factor Influential Part Target Condition Service Procedure 1 Mixture ratio Pressure regulator Made lean Remove vacuum hose and apply vacuum. Made rich Remove vacuum hose and apply pressure. 2 Ignition timing Crankshaft and camshaft position sensors Advanced Rotate distributor clockwise, find problem or reprogram. Retarded Rotate distributor counterclockwise, find problem or reprogram. 3 Mixture ratio feedback control Oxygen sensor Suspended Disconnect oxygen sensor harness connector. ECM Operation check Perform on-board diagnostic system (On-board Diagnostic Test Mode II) at 2000 rpm. 4 Idle speed IAC valve-AAC valve Raised Turn idle adjusting screw counterclockwise. Lowered Turn idle adjusting screw clockwise. 5 Electrical connection (Electrical continuity) Harness connectors and wires Poor electrical connection or improper wiring Tap or wiggle. Race engine rapidly. See if the torque reaction of the engine unit causes electric breaks. 6 Temperature ECM Cooled Cool with an icing spray or similar device. Warmed Heat with a hair drier. [WARNING: Do not overheat the unit.] 7 Moisture Electric parts Damp Wet. [WARNING: Do not directly pour water on components. Use a mist sprayer.] 8 Electric loads Load switches Loaded Turn on headlamps, air conditioning, rear defogger, etc. 9 Closed throttle position switch condition ECM ON-OFF switching Rotate throttle position sensor body. 10 Ignition spark position Timing light Spark power check Try to flash timing light for each cylinder using ignition coil adapter (SST).