Engine Sludge

First appearing in Australian vehicles in the late 1980s, engine sludge is a phenomenon that can affect almost any make of later model petrol engine and can cause extensive engine damage.

The term engine sludge, also known as black sludge, describes the formation of heavy deposits in the crankcase of petrol engines. Its formation can usually be traced to inadequate servicing, such as missed or extended oil change intervals or to the use of poor quality or inappropriate oils.

Sludge can resemble heavy grease or tar and results from the thickening and oxidation of the engine oil. The resulting material blocks the engines lubricating system, causing the engine to seize.

The best method for removing heavy sludge build-up is to dismantle, clean and repair the engine components, however, lesser amounts may be treated without fully dismantling the engine.

Engine sludge deposits are most commonly found in petrol engines, although sludge can accumulate in diesel engines through a slightly different process.

Diesel oils are exposed to much higher levels of unburnt fuel and combustion soot and over time, these contaminants build up to a level that thickens the oil. This eventually causes oil starvation as with petrol engines.

The small diesel engines used in light commercial vehicles and 4WDs are most at risk of accruing black sludge. Their typically high power output, small oil capacity and increased heat load from turbochargers make them a prime candidate for such problems. Fuel system problems, such as worn injectors, that over deliver or dont atomise the fuel properly, can add considerably to the soot and fuel loads the oil has to deal with, speeding up the thickening process.

Engine sludge can be avoided with regular servicing in accordance with the manufacturers specifications and by the use of appropriate, good quality oils and filters. In the case of diesel engines, fuel system maintenance is equally critical to engine life.

Most vehicle manufacturers recommend more frequent oil and filter changes for vehicles used for city driving, categorising it as severe use. Severe use oil change intervals can be as little as 5,000km to 7,500km.