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Methods for diagnosing mechanical noise faults in engines include the temperature method, speed method, load method, auscultation method, exhaust color observation method, lubrication condition modification method, electrical circuit testing method, and replacement of qualified components method. These approaches can directly diagnose noises while also making subtle noises more pronounced and reducing or eliminating more obvious ones. Additionally, diagnostic priorities and methods must be tailored based on whether the vehicle is new, in service, or recently repaired. Close observation of changes in the vehicle’s technical condition is also crucial for selecting appropriate diagnostic approaches. Below is a brief overview of several commonly used methods.
1. Aural Inspection Aural inspection comprises three methods: airborne listening, direct contact listening, and internal listening.
Airborne listening involves standing in front of the vehicle and detecting abnormal sounds transmitted through the air. Direct contact listening uses a wooden rod (or metal rod, long-handled screwdriver, etc.) pressed against a specific vehicle component to detect abnormal sounds via body vibrations. This method can identify noises from engines, gearboxes, solenoid valves, relays, generators, fuel pumps, and similar components. Internal listening involves inserting a hollow tube into the crankcase (or transmission, drive axle) to directly detect airborne noises. Inserting a rubber hose (or plastic tube) into the oil filler port—without submerging it in oil—enables internal listening. This method eliminates external noise interference and proves particularly effective for diagnosing faint or externally indistinguishable abnormal noises.
Using the three auscultation methods described above allows for relatively quick identification of the malfunction location, providing the optimal approach for accurate diagnosis and troubleshooting while minimizing component disassembly or unnecessary detours.
2. Speed Method
Engine noise-generating components vary in structure, load capacity, location, lubrication conditions, and degree of looseness, resulting in different noise-producing RPM ranges. However, each type of engine noise possesses a characteristic frequency. When the operating frequency matches an integer multiple of this characteristic frequency, resonance occurs, amplifying the noise. Engine mechanical noises are directly related to rotational speed (excitation frequency). Therefore, engine speed is divided into four segments: 500–800 r/min is idle; 800–1200 r/min is slightly elevated idle; 1200–2000 r/min is medium speed; and above 2000 r/min is high speed. Since various engine noises have corresponding optimal diagnostic RPMs, some sounds are more pronounced at idle or slightly elevated idle. However, during acceleration or at medium-to-high RPMs, the increased frequency of the noise, combined with amplified background noise, can mask the specific sound, making it difficult to discern. Examples include piston “knocking” and piston pin noise. Certain noises are difficult to detect at idle or even during gradual acceleration. However, when accelerating rapidly from idle to medium speed, the sudden increase in impact load causes distinct and continuous knocking sounds, such as those from loose connecting rod bearings or crankshaft bearings. Still other noises become more pronounced during rapid engine deceleration (when the accelerator pedal is abruptly released from high engine speed), such as those caused by looseness between the piston pin and connecting rod bushing, or a fractured crankshaft.
Given this unique relationship between abnormal noises and engine speed, diagnosing engine noise faults requires conducting multiple speed tests. These should include stable speeds across various operating ranges and rapid acceleration at different rhythms. This approach fully exposes the noises, facilitating their accurate capture and clarification of their relationship with engine speed. Only by hearing the noise firsthand can it be definitively identified. Therefore, the proper utilization of engine speed is crucial for diagnosing abnormal noises.
3. Cylinder Deactivation (or Fuel Cut) Method
Whether a specific cylinder is operating significantly impacts the intensity and tone of mechanical noises, making this method highly useful for analyzing noise characteristics. By terminating or restoring operation to a particular cylinder, the source of the noise can also be pinpointed. Inspection methods vary depending on the engine type. The spark-cutting method involves disconnecting a cylinder’s high-voltage distributor cap wire or shorting it with a screwdriver to prevent spark generation at the spark plug. This technique is used for carbureted gasoline engines. The fuel-cutting method entails loosening the high-pressure fuel line fitting nut for a specific cylinder (for diesel engines) or disconnecting the control wire from the injector for that cylinder (for electronically controlled gasoline injection engines).
After fuel or spark interruption, engine abnormal noises exhibit the following three variations: a. Noise remains unchanged. This refers to situations where the primary characteristics of the abnormal noise show little or no change after fuel or spark interruption. Note that the decrease in engine speed and frequency of the noise caused by the interruption is not included here. The noise is unrelated to the cylinder deactivation (or fuel cutoff). This phenomenon indicates that the noise originates outside the crankshaft-connecting rod mechanism, typically from a loose component or a malfunction in the valve train. It can be inferred that the cylinder being deactivated (or fuel-cut) was already non-functional due to a pre-existing fault before inspection. b. Noise diminishes or disappears. If the noise associated with a specific cylinder’s misfire (or fuel cutoff) diminishes, it indicates a fault in that cylinder. If the noise only weakens but does not disappear, it suggests other faults exist or that other faults are affecting that cylinder. If the noise disappears after misfire (or fuel cutoff), it indicates only that cylinder has a fault, while the others are normal or essentially normal. c. Abnormal noise becomes pronounced or increases in frequency. If noise becomes noticeable after cutting ignition (or fuel), this means previously absent or faint noise has become prominent, or low-frequency noise has increased. Piston pin noise and loose throttle seat ring noise exhibit these characteristics.
4. Accelerator pedal shake test
The throttle-flutter method involves abruptly depressing the accelerator pedal from its free state to rapidly increase engine RPM, then safely releasing the pedal to quickly reduce engine speed, followed by repeating this sequence. This technique is primarily used for preliminary diagnosis of engine faults. Based on the pedal depression distance and duration, it can be categorized into three types:
A. Rapid Accelerator Pedal Method: Press the accelerator pedal (approximately halfway through its travel) to rapidly increase engine RPM from around 500 RPM to approximately 1000 RPM, then immediately release the pedal. Repeat this rapid pulsing action. This method diagnoses piston knocking.
B. Medium-Speed Accelerator Pedal Vibration Method: Safely depress the accelerator pedal to allow the engine to accelerate smoothly from 1000 r/min to 1900 r/min, then immediately release the pedal. Repeat this vibration cycle. This method diagnoses connecting rod bearing noise.
C. Medium-to-High Speed Accelerator Pedal Vibration Method: Safely depress the accelerator pedal to increase engine speed from 1000 r/min to 2500 r/min.
Additionally, if you have any experience diagnosing engine noises, feel free to share and discuss.
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