Engine sludge. It’s a black gelatinous substance that wreaks havoc in engines. And long before the engine’s demise, engine sludge can foul engine sensors and interfere with performance. Some mechanics call it the “black death.” So, how does sludge in oil develop? And, how do you prevent it?
How Engine Sludge Forms
Engine sludge is the result of a series of chemical reactions.
The lubricant degrades as it is exposed to oxygen and elevated temperatures. The higher the temperature, the more rapid the rate of degradation. In fact, every 18°F (10°C) increase in temperature doubles the rate of oxidation.
The byproducts of this reaction form highly reactive compounds that further degrade the lubricant. Their byproducts react with other contaminants, forming organic acids and high-molecular-weight polymeric products. These products further react, forming the insoluble product known more commonly as sludge.
What begins as a thin film of lacquer or varnish builds up on hot or cold metal surfaces and bakes into an expensive mess.
The Impacts of Sludge
Sludge can block the oil passages and oil-pump pick-up screen, resulting in oil starvation. Often, the negative effects are cumulative rather than sudden.
Many engines with variable valve timing (VVT) use oil-pressure-operated mechanical devices to change valve timing, duration and lift. Sludge in oil can plug the solenoid screen or oil passages and affect the operation of VVT mechanisms, eventually leading to a costly repair bill. Sludge reduces efficiency and increases the time and money spent on maintenance.
Synthetic Oil Helps Prevent Sludge
Fortunately, sludge and varnish deposits are something that can be controlled. Using thermally stable synthetic base oils reduces the rate of degradation (oxidation).
Antioxidant additives further help reduce the rate of degradation. One of the most widely used is zinc dithiophosphate. Not only is it an excellent oxidation inhibitor, but it is also an outstanding anti-wear additive.
