Oil Sludge!

EDT Automotive Jaguar Enthusiasts’ Club X350 Forum Co-ordinator, Mike Kennedy, explains the causes and results of engine oil sludging, and suggests preventative measures.

oil sludge


Fresh engine oil is a clear, free-flowing liquid blend of base stock and additives that do not contain fuel, water, coolant, dirt, or other contaminants. When regular engine oil changes are neglected, normally free flowing lubrication oil breaks down, becomes contaminated, could cease to flow and then transform into a thick cocktail of waste by-products. Internal engine damage can then become a serious possibility, as these pictures of Oil Sludging, found via a Google search, will testify.

Why then does engine oil break down to combine with contaminants and form sludge deposits? Chemistry teaches us that engine oil is unstable and will decompose in the presence of oxygen at high temperatures. The process, called oxidation, can also occur naturally after exposure to normal operating conditions for extended periods of time. It is accelerated by exposure to severe operating conditions and elevated high temperature. In addition, accelerated oxidation may be triggered by a combination of any or all of these factors.

During oxidation, the chemical bonds that define molecular oil structure become broken with some of the reaction by-products accumulating and interacting to form a highly viscous complex mixture of solids, liquids and gases. They may contain a variety of solid carbon-based dirt, metallic particles, liquid coolant, fuel, oil and water droplets. But what more can we learn about engine sludge?

Consider if you will, my ancient workshop compressor. The drive belts have been replaced a couple of times and the only other routine service this two cylinder compressor has received is to have condensation drained from the air tank and the odd infrequent crank case oil change. When the used and unfiltered lubrication oil is drained from the unit, it shows no evidence of contamination or oxidation and looks just like fresh, new oil. Should it not be blackened? The answer is in the negative because the oil has never been subjected to combustion by-product fouling as in the case of a fuel burning petrol or diesel engine.

Somewhat simplified in description, an internal combustion engine is an air compressor in which fuel is mixed with compressed air, ignited and then burned. Petrol or diesel engines can be regarded as air compressors in which the oil becomes subjected to oxidation when exposed to high temperatures and contamination by combustion by-products. The combustion process generates heat and a variety of reaction products. These enter the crankcase as blow-by gas where it escapes past the piston rings and proceeds to contaminate the oil with chemical contaminates. This might include incompletely burned fuel wash, carbon soot, water, coolant and other reactant products and by-products. Even though oil temperature may be high enough to boil off water droplets and other volatile contaminants via a positive crankcase ventilation system, this crankcase broth will inevitably morph into a deposit that does not drain easily when the oil is changed.

Common causes of sludge deposits in internal combustion engines can indicate either short journey driving, lack of proper oil change maintenance or too low or too high an oil temperature or advanced levels of oil contamination. Sludge formation is not a new phenomena. Even in the earliest days of motor transport, sludge deposits seriously limited the durability and lifespan of an internal combustion engine. Over the years though, oil base stocks improved and detergent oil additives were developed to keep micro sludge-forming solid particles in suspension within the oil. Anti-oxidation additives were developed to slow the formation of sludge and deposits. Engine oil filters were introduced to help remove suspended solid particles from the oil stream and slow the precipitation of flow restricting sludge on internal engine surfaces. Changes and improvements in engine oil formulation and additive treatment have accelerated in time with the introduction of glycol-based coolant. If leaked from an engine cooling system into the oil system, that defect also commonly exacerbates sludge formation. Sludge deposits may also be due to inferior oil or break down of built in oil additives and oil improver chemicals after an overly extended period of lubricant use.

When detergent oils first appeared, engine repair businesses prospered. Tired, worn out engines, choked full of sludge deposits became common place. This was due to being previously serviced with non-detergent oils and then being filled and run with fresh detergent oil. A detergent oil will promptly attack, soften and release sludge deposits within an engine block. Bearing surfaces could then become flooded with detergent oil containing an abrasive and high concentration of newly suspended sludge particles. At the time of the introduction of detergent engine oils, a number of chemical “wonders” for engine oil sludge control appeared and were often used with disappointing results.

Oil technology is now significantly improved. A mass of well proven engine and user friendly oil quality control products are easily available to the motorist. It is worth noting that an absence of sludge deposits within a pre-owned and operated vehicle engine may indicate the engine has recently been “flushed”. This is noticeable if fresh oil contains an excessive concentration of solids which ideally should be removed and cleared by sump removal and cleaning or by some other searching alternative irrigation process.

It always pays to be suspicious of maintenance information which may not disclose substantive documentation in regard to scheduled oil changing. This may often be further confirmed when a used vehicle offered for sale is stated to have just been serviced but displays dirty engine oil and a highly sludged cylinder head around the rocker gear. Removing an oil filler cap and viewing within the valve cover often gives the poor maintenance game away. Exposed to heat for an extended period of time, old oil eventually oxidises and transforms into a black mass resembling thick molasses. That  points to the possibility that oil starvation may be just around the next corner. Costs for remedial work will be expensive. Buyer beware!

Sludged engines may also occur because of two additional factors. One is the trend towards ever-higher engine oil temperatures, an end users complete neglect or postponement of routine maintenance or reliance on a “Service Engine” light. Many owners think a warning light was designed to indicate when he should add engine oil. I’d suggest its interpretation is better described as “Too Late”! Engine noises and low oil pressure are indications of inadequate engine bearing lubrication. When oil sludge is found, the following occurrences of overextended oil change intervals, elevated oil temperatures should be suspected as the most likely causes. But there are always exceptions.

New and re-manufactured engine durability is directly related to the recognition, control and elimination of all factors that support engine sludge formation including, but not being limited to a lack of proper maintenance, inappropriate oil temperature, use of an inferior engine oil or excessive other engine oil contamination. Over fuelling or use of higher octane fuel leads to the increased production and formation of carbon based soot. Soot will be ejected via the exhaust system or become absorbed in suspension within the engine sump oil in microscopic form. This leads to rapid blackening of the oil and a high concentrate of abrasive particles within the sump and oil filter. Obviously that is an undesirable occurrence and an improved oil change maintenance regime should be actioned.

My water/methanol injected Jaguar 4.2 XJR is now subject to more than double the number of manufacturer recommended oil changes. On this highly modified vehicle, the oil and filter is replaced every four to five thousand miles. Methanol is a high soot producing fuel that if used to excess can rapidly contaminate and blacken engine oil. That being the case, I recognise the cause of the increased contamination but more importantly the imperative need for regular oil drops. Filtration has been uprated to encompass what many people consider to be the best filtration media in the World today. Namely, the use of the much respected K&N filter range.

In conclusion then, if you wish to do the best for your engine, change the oil and filtration media on a regular basis and also make use of the Terraclean and EDT deep clean processes to remove the gunk that normal changing leaves behind.


EDT Automotive