Perhaps one of the most damaging problems a vehicle's engine can encounter is engine sludge. An engine can build up sludge when moisture gets into the oil, and in lower temperatures, solids in the oil can turn into gel. This problem can be exacerbated by higher temperatures, so cooling system problems can also contribute to engine build-up. These deposits can build up and cause several types of problems, including blocking the passage of oil to key components in the engine, causing excessive wear of engine components, and various other damaging effects that can threaten the life of an engine.
Any leaks that can cause oil contamination will contribute greatly to engine sludge build-up. Once the sludge has built up in the cylinder heads or small passages in the engine block called galleries. When these galleries get clogged, they starve the engine of oil necessary for proper function. With smaller engines and engine compartments--combined with aluminum components that had a higher heat transfer rate and emissions standards that required higher engine temperatures--oil has become more susceptible to overheating and therefore the formation of sludge.
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To combat this problem, synthetic oils have been developed to eliminate contaminants and excessive build-up. While this certainly helps keep build-up lower, it will not eliminate the problem. Regular maintenance above and beyond the manufacturer's recommendations may be necessary, as some engines are prone to engine sludge. A coolant system that is not working properly can be a major contributor to engine sludge, so regular maintenance and coolant flushing is a good way to prevent engine sludge build-up. Regular oil changes can also help prevent build-up, though again, this will not eliminate the problem entirely.
If engine sludge build-up does occur in your engine, there is really no cost-effective way to fix the problem. Many engines can seize up if engine sludge becomes a problem, and engine replacement may be the only remedy. Sludge can sometimes be cleaned be taking the engine apart and using kerosene to break up sludge, but this is certainly not a cost-effective option and it can lead to its own set of problems as well.
When purchasing a used vehicle, remember that vehicles formerly used in rental fleets or as lease vehicles may be more susceptible to sludge build-up because of spottier maintenance schedules. Cars that were driven primarily for short distance commutes or around-town driving may also be more susceptible to sludge problems, and to combat this problem, a vehicle should be taken for a longer drive periodically to properly heat the engine oil.