Oil sludge

Oil sludge under the valve cover of an internal combustion engine

Oil sludge or black sludge is a gel-like or semi-solid deposit inside an internal combustion engine, that can create a catastrophic buildup. It is an emulsion of engine oil soluble combustion productions, water and solids. Particles accumulating in the sludge, can cause excessive engine wear. The sludge can also block the oil pump inlet screen or oil channels, causing oil starvation.

One type of sludge is called 'hot sludge' and is formed when engine oil oxidizes at high temperatures. Cold sludge is formed when engines don't get up to operating temperature, for example when the vehicle is primarily driven short distances.

Causes

Oil sludge may occur due to a variety of different factors. Some of the most common causes are:[additional citation(s) needed]

Prior to the phase out of leaded fuel, oil sludge consisted of up to 50% lead by weight.

Precautions

Oil sludge is generally preventable through frequent oil changes at manufacturer specified intervals, however, while uncommon, some engines do have a tendency to build up more sludge than others.

Engine oils typically contain dispersant additives to inhibit sludge formation.

In response to oil sludge problems, car manufacturer started adding oil life monitors to vehicles to warn drivers to change the engine oil on time.

Examples

Sludge formation was first found in combustion engines in the 1960s. During the 1980s sludge formation became a common issue again.

In 2004, Volkswagen informed 426,000 Passat and Audi A4 owners in the US warning them of sludge problems with its 1.8-liter turbocharged engines.

In 2007, Toyota settled a lawsuit with 7.5 million vehicle owners on sludge formation, although the company insisted the issue primarily occurred for vehicles which weren't serviced frequently enough.

References

  1. ↑ Rogers, D. T.; Rice, W. W.; Jonach, F. L. (1956-01-01). "Mechanism of Engine Sludge Formation and Additive Action". SAE Technical Paper Series. Vol. 1. doi:10.4271/560067.
  2. 1 2 Haycock, Roger F.; Hillier, John E. (2005-07-01). Automotive Lubricants Reference Book. John Wiley & Sons. ISBN 978-1-86058-471-8.
  3. 1 2 Costs and Benefits of Reducing Lead in Gasoline. Office of Policy Analysis, Office of Policy, Planning, and Evaluation, U.S. Environmental Protection Agency. 1984.
  4. 1 2 "Vehicle Engine Sludge | Consumer Reports". www.consumerreports.org. Retrieved 2026-09-18.
  5. 1 2 Lillywhite, J.R.F.; Sant, P.; Saville, S.B. (1990-01-01). "SLUDGE FORMATION: INVESTIGATION OF SLUDGE FORMATION IN GASOLINE ENGINES". Industrial Lubrication and Tribology. 42 (1): 4–10. doi:10.1108/eb053400. ISSN 0036-8792.
  6. ↑ Dimitroff, Edward; Quillian, R. D. (1965-02-01). "Low Temperature Engine Sludge — What? — Where? — How?". SAE Technical Paper Series. Vol. 1. doi:10.4271/650255.
  7. 1 2 3 Brooks, Michael (2007-01-09). "TOYOTA DEAL: HERE COMES THE SLUDGE JUDGE". The Center for Auto Safety. Retrieved 2026-09-18.
  8. 1 2 Monopoly, United States Congress Senate Committee on the Judiciary Subcommittee on Antitrust and (1971). Marketing Practices in the Gasoline Industry: Hearings Before the Subcommittee on Antitrust and Monopoly of the Committee on the Judiciary, United States Senate, Ninety-first [and Ninety-second] Congress Pursuant to S. Res. 334 [and 32, Section 4]. U.S. Government Printing Office.
  9. ↑ Palafox, J. A. (April 2012). How to Keep Your Vehicle Running Like a Dream. Xlibris Corporation. ISBN 978-1-4691-9551-3.
  10. 1 2 Guilfoyle, Nicole (2004-09-07). "Monitors Could Combat Oil Sludge". The Center for Auto Safety. Retrieved 2026-09-18.
  11. ↑ "Club Evolution - The Return of the Black Death". www.clubevolution.com.au. Retrieved 2026-09-18.
  12. ↑ "Toyota settles owners' lawsuit over oil sludge". Los Angeles Times. 2007-01-12. Retrieved 2026-09-18.