Will leaving some engine oil behind damage the engine?

发布时间：2022年03月14日 15:26

When car owners change their engine oil, they all hope that the old oil can be completely drained out. Often, they find that after adding a fresh batch of oil, there’s still some left over—some cars have more leftover oil than others. Does this mean the old oil wasn’t fully drained? Could incomplete oil drainage damage the engine? And why is it impossible to drain the oil completely? First, draining engine oil relies on gravity for it to flow out naturally. Due to the angle of the oil drain plug and the irregular structure of the oil pan, there will always be some oil that simply cannot flow out by gravity. Moreover, as engine oil is used over time, it accumulates many impurities, making it impossible to drain it completely. To illustrate this point, imagine filling a large basin with water and then adding sand into it.

　　When changing their oil, car owners all hope that the old oil can be completely drained out. Often, they find that after adding a new batch of oil, there’s still some left over—some cars have more leftover oil than others. So, does this mean the old oil wasn’t fully drained?

　　Will leaving some engine oil behind damage the engine? Why is it impossible to drain all the oil completely?

　　First, oil draining relies on gravity to allow the oil to flow out naturally. Due to the angle of the oil drain port and the irregular structure of the oil pan, there will always be some oil that simply cannot drain out by itself. Over time, as the oil is used, it accumulates many impurities, making it impossible to completely drain it all out. To illustrate this point, imagine filling a large basin with water and then poking a hole in the bottom of the basin—no matter how hard you try, you’ll find that some of the water and sand will always remain trapped inside. The same principle applies to engine oil; thus, it’s perfectly normal for some oil to remain behind.

　　Second, as we all know, engine oil has a certain viscosity, and some oil will inevitably remain on components such as the crankshaft and connecting rods. This oil cannot be completely drained at once by gravity or suction alone.

Will leaving some engine oil behind damage the engine?

　　Will leaving some oil residue damage the engine?

　　As mentioned earlier, oil cannot be completely drained by gravity alone. What really concerns car owners is whether the remaining old oil might damage the engine. Actually, there’s no need to worry. Engine oil serves several crucial functions: cleaning, lubricating, cooling, preventing corrosion, and sealing. Metal particles generated from internal engine wear are carried along with the circulating oil and eventually settle in the oil pan. This is because the oil pump in the oil pan filters the oil as it pumps it upward into the engine’s upper components before the oil enters the lubrication passages. Unless a large amount of old oil has been mixed in, the engine will not be damaged.

　　It’s important to note that engine oil, during engine operation, is affected by the high temperatures generated from fuel combustion as well as by the catalytic effects of various media and metals. As a result, it gradually deteriorates and ages, producing harmful substances such as carbon deposits, varnish films, and sludge. Therefore, it’s essential to regularly clean the lubrication system and remove residual oil to ensure the engine remains “healthy.”

Will leaving some engine oil behind damage the engine?

　　How should leftover motor oil be stored? During maintenance, some vehicles always end up with a little extra motor oil. You might want to save it for use in the next maintenance service, but you’re also worried that the oil could go bad and harm your car. So, how should you store this excess motor oil?

　　First, let’s understand the causes of机油 degradation: 1. Moisture: Under normal circumstances, the water content in engine oil should be below 0.03%. Once the water content exceeds 0.1%, the additives in the oil will lose their effectiveness, accelerating the oxidation process of the oil. Moreover, the organic acids produced by oil oxidation, along with acidic oxides from engine exhaust gases, react with water to form inorganic acids. These acidic substances intensify corrosion within the engine. 2. Oxygen: Oxygen also affects various additives in engine oil. Don’t assume that only high-quality oils contain many additives—every type of engine oil has additives. Especially under sunlight exposure or high-temperature conditions, oxygen in the air reacts more rapidly with the oil. 3. Impurities: Impurities primarily affect the cleanliness of the oil. When impurities mix into the oil, they can impair its lubricating performance, even leading to abnormal wear of engine components. This, in turn, can compromise the engine’s sealing performance and may even result in oil burning.

　　If the vehicle owner adds this degraded oil to the vehicle, the engine components that require lubrication and cooling will not receive adequate protection, leading to increased engine load and wear, ultimately resulting in engine damage and necessitating an early major overhaul.

　　Storage Method 1: Cover the bottle mouth of the engine oil with a layer of plastic film, then screw the oil bottle cap back on. Be sure to maintain airtightness at the bottle mouth; use a thicker plastic film and then seal it with wax. Otherwise, it will be difficult to effectively prevent air from entering.

　　2. Store the sealed motor oil in a cool, dry place to prevent oxidation and deterioration—for example, in a corner of the wall. Of course, many car owners also like to keep it in the trunk; that’s perfectly fine too, just in case you need it unexpectedly.

　　3. Oil that has been sealed for a long time will inevitably contain some moisture and impurities. Before use, never shake it. When adding oil, don't pour it all in at once—there will always be a small amount of moisture or impurities remaining at the bottom of the bottle. If you add this residue into the engine, it could impair engine performance or even cause adverse effects. Therefore, when adding oil, leave a small amount at the bottom of the bottle so that any moisture and impurities remain there.

Will leaving some engine oil behind damage the engine?