A common issue reported with an Alto mini car is a rhythmic, metallic knocking sound coming from the lower part of the engine. The noise is heavy and persistent, resembling a "clunking" sound, and can cause noticeable vibrations in the vehicle body, especially when the problem becomes severe.

During testing and analysis, it was found that the abnormal noise occurs primarily in the lower section of the engine. The sound becomes more pronounced during rapid acceleration or deceleration, and it remains consistent regardless of the engine's temperature. This suggests that the issue may be related to the crankshaft bearings.

Under normal conditions, the rear main bearing typically produces a dull and heavy sound, while the front bearing tends to make a lighter, more brittle noise. However, when the crankshaft bearing is faulty, this balance is disrupted, leading to unusual sounds.

The possible causes of the abnormal noise at the crankshaft bearing include:

• Excessive radial clearance in the main bearing
• Loose main bearing cap bolts
• Bent or deformed crankshaft
• Burned main bearing shells
• Loose or cracked main bearing bushings
• Excessive wear on the axial thrust washer
• Improperly smooth main bearing surface

If the axial thrust washer is excessively worn, it can result in excessive axial clearance, causing the crankshaft to tilt and produce a non-rhythmic, irregular noise. In contrast, if the abnormal noise is due to other issues, it tends to be more noticeable when the engine is cold after starting. During such times, the cylinder head of the affected cylinder may vibrate in sync with the noise.

When performing single-cylinder ignition cut-off tests, the abnormal noise usually remains unchanged. However, if two adjacent cylinders are disabled simultaneously, the noise may decrease or become less prominent, indicating that the main bearing between those two cylinders is likely the source of the problem.

Upon removing the crankshaft, it was discovered that the axial thrust washer was severely worn. After replacing it with a new one and reinstalling the crankshaft, the test drive showed a significant improvement, and the abnormal noise was eliminated.

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