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Causes of engine oil mixing with the coolant

It is common sense that the oil of any engine should never mix with the coolant or water.

This is so because whereas oil is meant to lubricate all moving parts in the engine, the coolant is supposed to cool the engine which normally generates heat at high temperatures when it is running. A vehicle engine is designed in such a way that it has a top cover which is supposed to cover the whole engine.

The cylinder head, which is also part of the cooling system, sits on the engine block, the structure that houses the cylinders or combustion chambers. The cylinder head also forms the ceiling of the combustion chambers. Between the cylinder head and engine block is the cylinder head gasket which will be the focus of our discussion.

The cylinder head gasket seals both the cylinder head which houses the engine valves at the topmost part of the engine and the engine block which also houses the pistons and crank shafts at the lower part.

The engine block has two different channel networks bored into it, one directing the engine oil to where it is needed and the other one doing the same for the coolant/water in the cooling system of the engine.

During the early development of the internal combustion engine, a thin sheet called the cylinder head gasket was introduced into it. Its purpose was to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in any engine and, as part of the combustion chamber, it shares the same strength requirements as other combustion chamber components.

Whenever oil is discovered in the engine coolant on inspecting the radiator top-up tank, one will notice that the coolant has formed into a milky colour. It is quite a bad occurrence when the two mix and usually means that the gasket has failed to function as required, creating an internal leakage which could easily damage the engine.

Some of the reasons why engine oil mixes with the coolant are:

  • A broken/damaged head gasket. Remember this sits between the block and cylinder head. In case it gets damaged, it will allow oil and coolant to eventually mix.
  • A cracked cylinder head due to overheating or an accident. If this is not replaced, it sometimes leads to the mix of oil and coolant. This happens because a cracked head will misdirect the coolant and in the long run it will mix with the oil.
  • A damaged engine block since this is the lower part of the engine which houses most of the moving parts that use the oil and if this gets damaged, would lead to the mixing of the fluids.
  • Extreme engine overheating, most especially in diesel engines. When engines overheat, they tend to burn and destroy the gasket which in the long run loses its functionality.
  • When a vehicle passes through water that rises to top-of-the-engine level. Sometimes, this water will enter the engine, either through the top cover, or any other open part of the engine. It could also enter through the exhaust pipe, especially whenever one revs the engine while driving through the water.

Note that whenever there is an oil/coolant mixture, you need to immediately contact a knowledgeable mechanic. If the fault is extreme, your vehicle may need an engine overhaul, but if it is minor, then the engine is simply flushed and the oil and filter changed to rectify the problem.        

Robert Kato is a mechanic who can be reached on:
robertkato999@gmail.com or 071-2-212507/0704137708.

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