Learn to interpret oil texture in your engine

Just like a pathologist studying a blood sample, a trained technician can learn a lot about the inside of an engine by studying a sample of used oil from that engine. This requires the facilities of a specialised laboratory with sophisticated equipment, and, in fact, there are firms in South Africa that offer precisely such a service. They are used mainly by big transport operators and companies in the heavy construction and manufacturing industries.

Just like a pathologist studying a blood sample, a trained technician can learn a lot about the inside of an engine by studying a sample of used oil from that engine. This requires the facilities of a specialised laboratory with sophisticated equipment, and, in fact, there are firms in South Africa that offer precisely such a service. They are used mainly by big transport operators and companies in the heavy construction and manufacturing industries.

But for us, mere shade tree mechanics, we have to rely on the art of interpreting the information that the dipstick gives us. The dipstick, of course, is just a metal blade, the bottom end of which dips into the oil in the sump, while the top end has a handle which sticks out from a tube. To "read" the dipstick, you pull it out by the handle, wipe it clean, push it back into the tube as far as it will go, then pull it out immediately. It will bring some oil from the sump with it, which you study closely for the following things:

l The oil level. This was the original purpose of the dipstick. The normal pattern is that immediately after an oil change the level will be at the "Full" mark, and it will slowly drop because some oil will unavoidably get past the oil control rings, especially at higher revs, and end up in the combustion chambers where it will be burnt together with the fuel.

When a fuel injector is leaking, or the petrol which you last put in at the pump had a small amount of diesel in it, the downward migration will increase. The oil level might then remain constant or even rise slightly. A rising oil level should always be investigated.

Returning to the normal pattern of a slowly dropping oil level, try to maintain the level in the upper half of the indicated range, but be careful not to overfill when topping up .

Overfilling can easily happen when a garage attendant decides to put in half a litre of oil when the level on the dipstick is only just below the full mark. It's best to check the level yourself before starting the engine in the morning. Excessive oil consumption indicates a worn engine and a visit to the bank manager.

l A coolant leak into the oil, perhaps past a cylinder head gasket, is another possible cause for a rising oil level. It will show up in brown bubbles or a crusty, brown residue above the oil level line on the dipstick.

If in doubt, put a drop of oil from the dipstick on a hot exhaust manifold. If it crackles, like frying bacon, it's an indication of water contamination. In severe cases the oil will look like chocolate milkshake. Always treat coolant contamination as an emergency.

l The oil should be smooth and glossy and somewhat transparent. It's natural for oil to darken slowly due to oxidation. But if it rapidly turns black and opaque, it points to excessive blow-by and a looming engine overhaul. Expect oil in a diesel engine to become black sooner due to soot formation, even if the engine is in prime condition.

l Oxidised and contaminated oil will lose interfacial tension, and a simple test for this is to put a drop of oil from the dipstick on a water surface. If the oil spreads out across the water surface, instead of forming beads, it may be time for an oil change.

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