Engine sludge is the thick black deposit that coats an engine's interior cavities.  MolaKule at BITOG reports the following about black sludge formation:

A. Black Sludge is defined as thick to solid material with low water content, of dark color, light oil insolubles, and typically found in rocker cover, cylinder head, timing chain cover, oil sump, oil pump screen, and oil rings in variable quantities.

B. Cause of Black Sludge in more modern engines.
1. Promoted by mixed driving conditions in which there is stop and go and highway operations.
2. Starts with Nitrous oxides in Lean-Burn engines reacting with blow-by gasses.
3. High spark advance, high temperature of combustion, lean fuel/air ratios, high compression.
4. Fouled air ventilation systems (PCV), combined with low operating temperature driving conditions.
5. Longer oil change intervals.
6. Higher oil temperatures.
7. Lower sump capacity.


Engine Sludge


From BITOG: How does Detergents and Dispersants work in oil?

Detergents and dispersants in an engine oil work as follows.

Detergent and dispersant additives are used to keep the engine's metal surfaces clean, prevent the formation of deposits and to neutralize the harmful effects of corrosive acids that are formed by the combustion of diesel and gasoline fuels.

All engine oils, as they deteriorate either due to oxidation or by contamination, will form insoluble sludge and varnishes and resins that can become deposited on the surfaces of the engine. Once deposited, these sludge, varnish and resins can block oil lines and oil passages causing the flow of the engine oil from reaching the parts be lubricated. This in turn can result in increased wear, heat buildup and eventual engine failure.

Further engine oils can be exposed to fuel soot due to incomplete combustion of the fuel or carbon being introduced into the engine by various emission controls, acids formed by the combustion of the fuel, and the ingestion of moisture and dirt from the engine's air intake system. If these contaminants are allowed to buildup in the engine oil they can result in:

  • Increased engine wear, especially in the valve train area

  • Increased deposits, especially in the piston rings and crownland of the piston.

  • Increased cylinder bore polishing

  • Thickening of the engine oil's viscosity

  • The formation of oxidation precursors in the engine oil

  • Poor oil pumpability especially during cold weather conditions

  • Plugging of oil filters

  • Rapid depletion of the engine oil's additive system

  • Decreased engine durability and life.

In order to prevent the formation of these deposits, sludge and resins, it is important that the engine oil contain an effective detergent/dispersant additive system.

Detergents are oil soluble bases that are derived from the organic soaps or salts of calcium, magnesium or sodium or, barium. Primarily, today they are calcium or magnesium based. These materials are often referred to as organo-metallic compounds and they are polar in nature, which allows them to cling to the surfaces of particles. Detergents serve two principal functions.

First, they lift any deposits from the surfaces from the surfaces of the engine to which they adhere to and then chemically combine to form a barrier film, which keeps the deposits from coming out of suspension and coagulating. Detergents form two kinds of barrier films. On small particles, (generally less than 0.02 microns in size), detergents form an absorbed film which slows down coagulation of the particles. On much larger particles, (ranging from 0.5 to 1.5 microns in size), detergents cause the particle surfaces to acquire an electrical charge of the same sign so they can repel each other.

The polar metallic heads of detergents have a great affinity for each other. These molecules attract each other like magnets and form clusters called "micelles".

The deposit precursors being oil-insoluble have a greater affinity for the detergent molecule than the oil molecules. They are attracted to the detergent micelles (much like iron fillings are drawn to a magnet) and trapped within them. Thus, they are kept in solution in the engine oil and cannot settle out to form deposits in the engine.

The number of particles that can be contained in a micelle is limited. When a number of particles exceed the capacity of the type of detergent chemistry being used deposits can form. Therefore it is necessary that the engine oil be drained before this happens if engine cleanliness is to be maintained.

Secondly, detergents neutralize any acids formed by the combustion of the fuel by chemically reacting with the acids in order to form harmless neutralized chemicals.

Dispersants are polar additives that are used to disperse sludge and soot particles for the purpose of preventing agglomeration, settling and deposits. Dispersants envelops particles and keep them finely divided. Dispersants are polymeric and ashless compounds. These compounds are based on long chain hydrocarbons, which are acidified and then neutralized with a compound containing basic nitrogen.

The hydrocarbon portion provides oil solubility, while the nitrogen portion provides an active site that attracts and holds potential deposit forming materials to keep them suspended in the engine oil.

This dispersant molecule combines a compact, electrically polar head and a long, oil soluble tail, which might look like this.


Dispersant Molecule


In a succinimide dispersant, (which is the most widely used type of dispersant chemistry used), the piece on the left containing nitrogen (N) is the polar head; the piece both containing nitrogen (N) and oxygen (O) is the connecting link. "R" is the long, oil soluble tail. The polar heads attach themselves to any deposits or acids that may be formed by the combustion of the fuel to form micelles which are taken into solution in the oil by the R's. These micelles can trap deposit precursors up to 0.05 microns in size by proving a thick absorbed barrier film or they can also hold larger particles up to 0.1 micron in size by electrical charge repulsion. In this state, the acids and deposits cannot see the engine's metallic surfaces.


Dispersant Action


As mentioned earlier in this discussion on detergents and dispersants, one of their functions is to help neutralize any acids that are formed by the combustion of the fuel. Each of these additives contributes to the neutralization of these acids by proving the engine oil with a Total Base Number (TBN).

TBN measures an engine oil's ability to neutralize acids that are formed by combustion. As long as an engine oils TBN stay above a certain limit during use (generally ½ of its original TBN number), during use the engine oil is still fit for service. In fact a new oil's TBN is less important than the TBN during service, which protects the engine. An engine oil must have the ability to retain its TBN reserve (alkalinity reserve) that is contributed by both the detergents and dispersants during its entire drain interval.

Of these two additives, it is detergents that offer the best alkalinity reserve. Though dispersants are a necessary additive for the formulation of engine oils, and engine oil's total TBN that is derived through the use of high dispersant chemistry does not offer adequate protection an engine needs against the corrosive attack of acidic combustion by-products. Dispersants are more rapidly depleted than detergents because of the way they chemically react with acids that are formed and by the way they react with other particulate contaminants.

Detergents, on the other hand, because they chemically react with the acids and any other particulate contaminants that are present in the engine oil, have the ability to retain their TBN reserve over longer periods of time, thus providing a more protective form of TBN over the entire drain period of the engine oil.


For sludged engines, Widman (see Selection of the Right Motor Oil for the Corvair and other Engines) recommends the use of a 10W-30 Heavy Duty Engine Oil (HDEO) followed by 15W-40 HDEO because HDEOs have strong detergent/dispersant additive packages already built-in.  (See Widman's Bottom Line Recommendations: 10, 11, 12).  Keep an eye on the oil and, when it looks like it's thickening up, change it.  Detergents are polar molecules and work by displacing deposits from the surface of engine. Once removed from the engine's surface, sludge is kept from agglomerating by dispersants.  The net effect is that any sludge cleaned from the engine will remain suspended in the oil, without any danger of clogging the oil pickup screen.  For more severe cleaning, it is best to use a slow-acting cleaner (like Auto-Rx) rather than solvent-based fast engine flushes.

Widman's Bottom Line Recommendation #10 is:

Forget the myth that you can’t put high detergent oils in older engines or engines that have been using poor quality oil. I do it every day! 50% of this market is API SF or lower, frequently without thermostats. They are full of sludge. Some drain plugs come out looking like a cork, with an inch or so of thick sludge on the end. No matter what the engine, I put in a 10W-30 high detergent CI-4 oil and instruct the customer to come back when it thickens up, or the following week if he doesn’t want to check it himself. Once it no longer thickens up quickly we move on to 15W-40 and add a 1200 mile engine cleaner. At the end of that cycle we move to whatever oil the engine should have.

If you've just removed the valve cover(s) or oil pan and discovered sludge, you can use mineral spirits to wash the sludge off those parts.  Don't try to remove sludge inside the engine as hard deposits can be dislodged, which can later clog your oil pickup screen.  Don't be concerned with trying to remove insoluble varnish deposits as these are harmless and can eventually be removed by the detergents in your oil.

Widman recommends the use of pre-CJ-4 Heavy Duty Engine Oils in older engines (see Bottom Line Recommendations: 6, 7, 37).  That is, oils that are rated for both gasoline and diesel engines and that the diesel performance level is CI-4+, CI-4, or CH-4.  The gasoline performance level listed for these oils will typically be SM, SL, or SH. The upper part of the API donut would typically show a combined performance level like API Service CJ-4/SM or API Service CI-4/SL.  Since CJ-4 oils are much more common now than the earlier CI-4 and CH-4 performance levels and since these HDEOs are far superior to the old engine oils meeting the obsolete performance levels (ML, MM, MS, SA, SB, SC, SD, SE, SF, etc), we also recommend its use in any flat-tappet engine newer than 1930.


Obviously, it best to prevent sludge from forming in the first place:

  1. Use a good quality engine oil.
  2. Change the engine oil at regular intervals.  Use the manufacturer's recommendations or use Used Oil Analysis to determine the optimum extended oil change interval.

  3. Plan your trips so that the engine reaches its normal operating temperature as much as possible.

  4. Keep the engine in good tune (ignition timing, carburetor jetting, etc)

  5. Keep the PCV (Positive Crankcase Ventilation) and EGR (Exhaust Gas Recirculation) systems in good working order.

  6. Keep the cooling system in good working order.  Use the recommended thermostat or even consider using a winter thermostat.


Often, people acquire cars with engines that have not been well maintained and poorly maintained engines can be severely sludged.  Sludge is not a problem if just coats the interior surfaces of the engine but, if it ends up clogging the oil pump's pickup screen, it can cause severe engine damage.  I recommend using modern, fully-formulated (ie, with additive packages meeting non-obsolete API service categories) engine oils in all engines newer than 1930 because modern oils minimize engine wear and prevent sludge formation.  See Impact of Low Quality Oils on Engine Wear and Sludge Deposits.   The use of non-detergent oil (API SA, formerly ML - Motor Light) is only recommended in vehicles earlier than 1930.