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Engine sludge is the thick black deposit that coats an engine's interior cavities.  MolaKule at BITOG reports the following about black sludge formation:

A. Black Sludge is defined as thick to solid material with low water content, of dark color, light oil insolubles, and typically found in rocker cover, cylinder head, timing chain cover, oil sump, oil pump screen, and oil rings in variable quantities.

B. Cause of Black Sludge in more modern engines.
1. Promoted by mixed driving conditions in which there is stop and go and highway operations.
2. Starts with Nitrous oxides in Lean-Burn engines reacting with blow-by gasses.
3. High spark advance, high temperature of combustion, lean fuel/air ratios, high compression.
4. Fouled air ventilation systems (PCV), combined with low operating temperature driving conditions.
5. Longer oil change intervals.
6. Higher oil temperatures.
7. Lower sump capacity.

 

Engine Sludge