Why does engine oil turn black after a few hundred kilometers?

Engine oil is often called the blood of the engine, and it is the most critical factor in ensuring the normal operation of the engine. When it comes to truck maintenance, the most important thing is to change the engine oil. The main function of engine oil is lubrication, which reduces friction between moving parts and minimizes wear. Of course, it also has another important function, which is cleaning. It will absorb all impurities in the oil channel, just like an omnipotent cleaner. Most of the impurities are black, which causes the color of the engine oil to turn black. 

So the oil will turn black after the engine has been running for a while. 

The main source of black is the carbon particles formed after the incomplete combustion of gasoline, because the color of carbon is black.

When gasoline burns in the cylinder, there are many conditions where it cannot be completely burned. At this time, carbon deposits will be generated and attached to the cylinder wall. When the piston moves downward, these carbon particles will be scraped off and blended into the engine oil, causing the engine oil to turn black.

In addition, the iron filings from engine wear and tear also become black powder after high temperature oxidation and blend into the engine oil. Another reason is the oxidation of the engine oil itself. If the engine runs for a long time, the additives in the engine oil will also oxidize and deteriorate, and the color will change to black.

This is like the oil we usually use for cooking. It is clear at first, but if it is used to fry a lot of food, food residues and seasonings will be mixed into the oil, and the color will become darker due to its own oxidation. The blackening of engine oil is actually a normal phenomenon and does not mean that the engine oil has failed. For example, if a drop of ink is dropped into a basin of water, the whole basin of water will be black, but there are not many impurities and dirt in the water, only one drop. The blackening is just a visual effect and may not have a big impact on performance. 

Some people find that the engine oil turns black after a few hundred kilometers of driving. This is definitely not related to the new engine oil. It is because there is old engine oil has not been drained out completely. After the engine has run for a while, it will be mixed and look a little black. Is this a big deal?

In fact, it is normal that the oil cannot be drained completely. There are two most commonly used ways to drain the oil. One is gravity draining, which is to open the drain screw of the oil pan and let the oil flow out naturally; the other is to use a device to insert a pipe into the oil pan and pump it out.

No matter what method is used to drain the oil, it is impossible to drain all the oil. At least some oil will adhere to the surface of each component.

Unless after draining the oil, add a bucket of new oil and let the engine run for a while before draining it, which is equivalent to rinsing it with new oil, and then adding new oil, it can greatly reduce the residue of old oil.

Of course, it is not necessary to do so. Because the old oil residue has little effect on the engine. The new and old oils are mixed, 99% is new oil and 1% is old oil. In this system, the new oil will dominate and play the main lubricating role. In other words, if the oil is replaced in the normal process, it will not affect the performance of the new oil, and of course it will not damage the engine. Don't think there is a problem when you see the new oil turn black in a short time. 

However, when changing the engine oil, you must at least make sure that the oil in the oil pan is completely drained. When the oil stops flowing out in streams and starts to drip out drop by drop, it is completely drained. In terms of time, it takes about three minutes. As long as the oil draining time is more than three minutes, there is no need to worry about the problem of oil retention.