Causes of sludge in engine oil

Sludge is the thick clots of oil in your engine.

In Summary

•Lubricating oil in its natural state is supposed to be clean and must be easy to flow to reach all the moving parts within the shortest time possible.

•Once the oil starts to get dirty, the ability to flow is reduced

An engine with sludge.
An engine with sludge.

Oil sludge is the thickening and breaking down of engine oil inside the engine. 

 The main purpose of engine oil is to lubricate all moving components within an engine. In other words, if your lubricating oil is contaminated by any foreign matter, the lubrication is then compromised. 

As the engine keeps working so, is the lubricating oil. Within a given period, the engine oil must be changed to keep the good health of the engine and all its moving internal components. 

Lubricating oil in its natural state is supposed to be clean and must be easy to flow to reach all the moving parts within the shortest time possible. Once the oil starts to get dirty, the ability to flow is reduced drastically and this compromises its efficiency. Sometimes due to extremely high operating temperatures, the lubricating oil may become too thin and may also affect its efficiency as well. For this reason, it is recommended to use manufacturers specified engine oil for a specified mileage or period according to whichever is achieved first. 

Sludge in simple terms is the thick clots of oil in your engine which turns to carbon like oil debris when heated up. In most cases, prolonged sludge turns into small charcoal-like residue clinging on all engine components. The easiest place of finding sludge is on the back of your yellow engine oil filler cap. 


  • Condensation: After the vehicle has been running during the day, especially in arid and semi-arid areas, during the night the temperatures drop slow. This extreme drop in temperatures causes the vapour that was initially produced in the lubricating oil to condense. The condensed vapour drops back into the oil pan. At the next start, the engine oil mixes with the vapour which in turn causes the oil to form sludge.
  • Engine overheats: Prolonged overheats cause the engine oil to heat up and evaporate. Once this happens the oil melts on the engine parts and the residue turns into sludge which may block out the oil orifice within the engine causing engine knock related problems.
  • Wrong engine oil: Some manufactures are very specific on the type of oil to use. They even indicate it on the oil filler cap, especially for the high compression engines. Going out of this range may lead to a very thin grade of oil which easily evaporates or may lead to the use of a very high grade of oil which will have no ability to flow hence sludge formation.
  • Short trips full of idling: This may be hard to believe but trust you I stop and go short distance drives may also lead to sludge formation in the engine especially during cold seasons. The Ability of oil to flow is good at optimum engine temperatures. Failing to hit working temperatures limits oil flow leading to the likelihood of sludge formation. 

The same thing also happens for vehicles that have been parked for long periods without moving. It is recommended to flush your engine oil every six months of middle sitting

  • Contaminants: worn out piston rings leads to compression gases escaping through the cylinder walls into the crankcase. Carbon content in these gases contaminates the engine oil which turns into sludge causing damage to all engine moving parts.

Signs and symptoms 

  • Engine oil pressure light stays on for longer periods after the car has been started 
  • Check engine light stays on and upon computerised diagnosis reveals  faulty vvti/avcs sensor but upon replacing it still the check engine light won't go off
  • Engine oil draining becomes slower than usual 
  • Engine oil filter has molten oil substances 
  • Oil filler cap has got charcoal-like 
  • Substances at the backside
  • Camshaft and entire top cover has lots of sticky residues


  • Always ensure you carry out periodic service with correct engine oil and oil filter on time. 
  • Avoid running your engine with a temperature light in the upper limit 
  • Carry out engine flush with recommended sludge removers.
  • Sludge can easily cause an engine knock and should not be taken lightly.  Only periodic maintenance can reduce sludge formation on an engine.


The writer is the owner of Boosted Auto, a vehicle repair shop