Oil Tech: Sludge

Posted in Blog by admin with 3 Comments

Apr2012 30



One of the most dangerous substances to an engine’s lubrication system is oil sludge. Anyone who is familiar with Volkswagens and Audis knows that the early 1.8t engines (AEB) that came in Passats and A4s have a notorious engine oil sludge problem. VW’s fix is to use a larger filter, the one for earlier diesel engines, which adds approximately a half-liter of oil capacity.

Oil sludge can wreak havoc on an engine. In the case of the aforementioned Passats and A4s, an engine with a sludge problem is a fairly expensive fix if caught soon enough, and has the potential to be very expensive if the problem is overlooked for any significant amount of time. Initially, the sludge will clog up oil passages in the engine block and cylinder head, the oil pickup tube attached to the oil pump, and the oil lines providing lubrication to the turbocharger. If the issue is not addressed in a timely manner, the sludge will severely restrict oil flow to the engine and turbocharger, which could potentially ruin the turbocharger bearings, the engine crankshaft and connecting rod bearings, and the cam journals in the cylinder head. It can also accumulate on the piston ring lands (the notch where the piston ring sits) and restrict movement of the rings, which can result in accelerated wear of the rings and cylinder wall, loss of compression, broken rings, etc. In short, an unchecked oil sludge problem can result in serious damage requiring engine and/or turbocharger replacement.

On a side note, this is very common on the AEB engines, and anyone looking to upgrade to the better-flowing “big port” AEB cylinder head should pay special attention to cam journal wear. The repair manual states that the cam journal clearance wear limit is 0.1mm, measured using plastigauge with the cam followers not installed.

Oil sludge refers to a nasty substance that forms in the oil, and can range in color from light brown to black and in texture from gooey to almost solid. It can form for a number of reasons, and typically the color and texture will help to diagnose the issue. We will divide the reasons into two basic groups: water/coolant intrusion and oil breakdown.

Water or coolant in the oil will typically cause a gooey light brown sludge resembling a chocolate milkshake in appearance. Coolant intrusion is exactly what it sounds like, coolant in the oil, and is typically the result of a leaking gasket, such as the cylinder head gasket, but can also result from a faulty oil cooler. Water can get into the oil, as well. Typically this would be the result of condensation, and becomes prevalent on vehicles that do not properly get up to operating temperature, thus, not producing enough heat to properly evaporate the condensation from the crankcase. This occurs most often on vehicles that are only taken on short trips or due to a faulty (stuck open) thermostat.

Sludge resulting from breaking down the oil is usually thicker and darker. It can be tar-like or almost rock hard, and is usually black. Oil breakdown can also occur for several reasons. As discussed in the Acids & Bases oil tech article, heat and combustion gasses, usually resulting from blow-by, can cause the oil to break down. The blow-by can also contain a number of contaminants, such as unburnt fuel, dirt, or soot. While modern engine oils contain detergents and are chemically basic (pH over 7), over time, contaminants and acids that enter the oil, along with heat, can neutralize the oil and consume the detergents.

This is why it is important to maintain a regular, reasonable oil change interval with a high quality oil that meets the manufacturer’s specifications. Removing the contaminants and acids from your engine will prolong the life of the engine, not only by reducing the potential for corrosion or wear due to dirty oil, but also by reducing the potential for engine sludge. In addition to regular oil changes, keeping the PCV system functioning properly will help alleviate corrosive blow-by gasses and moisture from remaining in the crankcase.

It should be noted that, for vehicles that are used for a lot of stop-and-go driving, idle for long periods, or are subjected to heavy duty use, such as high engine temps, heavy loads from hard driving or towing, or are operated in dusty areas, a shorter oil change interval can help keep oil sludge at bay.

In short, the best way to keep your engine’s lubrication healthy and strong is to maintain it properly with manufacturer approved oils. If you are looking to extend your oil change interval or check that it is proper for a vehicle that sees heavy use, an oil analysis can provide insight into how much life is left in your oil.

For the complete series of Oil Tech posts, check out this link

Post dicussion

3 Responses to “Oil Tech: Sludge”

  • Allen May 14, 2012

    A killer named sludge may live in your engine and can choke the life from your car, regardless of maintenance or mileage. And the automakers whose engines are susceptible to sludge still aren’t always eager to help. Cleaning engine oil sludge (from any cause) can prevent engine problems and improve the performance of your vehicle.

  • user July 16, 2012

    Numerous late-model piston engines from many manufacturers have suffered from failures due to oil sludge contamination. These problems happen when fine engine oil passages become clogged with sludge, and often result in catastrophic failure of the engine.

  • Frank October 25, 2012

    I made the mistake of buying a 2000 1.8t Passat engine code ATW with 103,000 miles the turbo needed to be changed I did it my self no problem but after I installed the turbo I tested it and it ran good but almost right away the oil pressure light came on that’s when I new I had problems,after checking I found out about the slugging problem and man this one was sludge up vary bad I have never seen any thing this bad to fix this the right way it will need many new parts,be ware of SLUDGE!!!!!!!!!!

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