How to Diagnose your Vehicle if The Engine Produce Abnormal Sounds? 1 of 2


Mechanically Abnormal Sound and Noise
The engine consists of many parts, and each part operates by a sliding or rotational movement. The portion that slides or moves by rotational movement has clearance without exception and when this clearance becomes larger than the specification, an abnormal sound or noise can be heard.
The sound resulting from abrasion in the engine has the following characteristics
1. Occurrence condition
• When the engine is cold, the sound is loud.
• When the oil viscosity is insufficient, the sound is loud.
• When the hydraulic pressure is low, the sound is loud.
• When accelerating, the sound is loud.
• When the load is heavy, the sound is loud.
2. Sound types
• Sliding sound: the sound of things rubbing together.
• Hitting sound: the sound produced by hitting.
• Other sounds: Generally obscured and complicated sounds.
• Sound when idling
• Sound in the specified engine speed range
• Sound at various engine speed range
Abnormal Sound and Noise Caused by Abnormal Combustion
1. Knocking
During acceleration, a high-pitch hitting sound is produced.
The piston and the valve receive a bad influence and the engine may be damaged.
Main cause
• Poor fuel quality
Octane number of the fuel is lower than the required
• Ignition timing is advance
If the ignition timing is advanced, combustion is performed suddenly. As a result, knocking occurs.
• Spark plug failure
The overheated spark plug becomes a heat spot and causes pre-ignition. Suitable temperature for spark plug: approximately 450 to 950 degrees centigrade (self-cleaning temperature)
• Carbon accumulation in the combustion chamber
If carbon accumulates in the combustion chamber, the carbon prevents the heat from being dissipated and the engine overheats. The heated portion becomes a heat spot and causes pre-ignition.
• Air fuel mixture ratio is lean
During high speeds and with a heavy load, if the air fuel mixture ratio is lean, knocking will occur easily.
• Over load operation
When the engine load is excessive, knocking will occur easily. If the vehicle is continued to be driven while knocking occurs, the malfunction as shown below could occur.
• Overheating
• Metal damage due to engine overheating
• The amount of fuel consumption increases due to reduced heat efficiency
• Spark plug, piston and valve melt down.
• Cylinder head gasket breakage.
2. Run on (dieseling)
This is a phenomenon in which spontaneous combustion occurs. This phenomenon is caused when the fuel sucked in by the inertia of the crankshaft or fl ywheel, overheated spark plug that ignites the unburned gas and the carbon accumulated in the combustion chamber becomes a heat source even after the ignition switch is turned off. In an EFI engine, when the ignition switch is turned on, the fuel stops. Therefore, run on does not occur.
Main cause
• Inappropriate fuel
The fuel self-ignition temperature or octane number is low.
• High intake temperature
The compressed air fuel mixture temperature becomes higher than the self-ignition temperature.
• High-pressure compression (high-speed, high-load running)
Combustion temperature and the combustion chamber wall temperature are both high. When the pressure is high, the compressed air fuel mixture temperature also becomes high.
• Carbon accumulation inside the combustion chamber
When carbon accumulates inside the combustion chamber, the carbon prevents the heat from being dissipated and the engine overheats. The overheated portion becomes a heat spot.
• Spark plug failure
Spark plug is over-burned and becomes heat a spot.
• Idling speed is high
The amount of intake air when the throttle valve is fully closed is large.
• Engine overheating
Temperatures of combustion chamber parts become higher.
• Ignition timing retards
The maximum burning temperature lowers, but the combustion duration becomes longer and the exhaust temperature becomes higher. The temperature near the exhaust valve rises and becomes the heat source of the spontaneous combustion.

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