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Different engines have specific normal oil consumption values, and the normal oil consumption for different engine models varies. For example, BMW’s limit for oil consumption is 0.7 liters per thousand kilometers, while for diesel engines used in trucks, many manufacturers consider levels of 2.24 liters per 10,000 kilometers or less to be normal. Below are the reasons why an engine burns oil, provided as a reference for automotive repair when encountering issues such as high oil consumption or oil burning. http://img.mp.sohu.com/upload/20170718/78e07c408b9743e09f9d9a18ee59197b_th.png1. External leakage of engine oil: There are many reasons for engine oil leakage, including the oil pipelines, oil drain ports, oil pan gaskets, valve cover gaskets, oil pump gaskets, fuel pump gaskets, as well as the seals around the timing chain cover and the camshaft seals. All of the above potential leakage factors cannot be ignored, as even minor leaks can lead to significant oil consumption. For example, if one drop leaks every 6 seconds, it means 0.56 liters of engine oil are consumed per 100 kilometers. The best way to detect leaks is to place a light-colored cloth under the engine and check it after starting the engine. The location of the oil droplets on the cloth can be used to determine the leakage site. 2. Failure of the front and rear oil seals – Damage to the oil seals of the front and rear main bearings will definitely lead to oil leakage. This condition can only be detected when the engine is operating under load. The main bearing oil seal must be replaced once it is worn out, as this will lead to high levels of leakage, just like in the case of oil leakage. 3. Wear or failure of the main bearing: A worn or faulty main bearing will cause excess oil to be thrown out, which then ends up on the cylinder walls. As the bearing wear increases, more oil will be thrown off. For example, if a bearing design gap of 0.04 millimeters provides proper lubrication and cooling, and if this gap is maintained, the amount of oil discharged will be normal; neither the lubricant manufacturer nor the bearing will be damaged. When the gap increases to 0.08 millimeters, the amount of oil discharged will be five times the normal amount. If the gap is increased to 0.16 millimeters, the amount of oil discharged will be 25 times that of the normal level. If the main bearing spews out too much oil, more of it will splash onto the cylinders, preventing the piston and piston rings from effectively controlling the oil. This can lead to oil burning or carbon buildup on the pistons and piston rings. Typically, if too much oil is lost from the main bearings, the rod bearings will run out of oil; as a result, at certain low speeds, there isn’t enough oil splashed onto the cylinder walls, which leads to wear of the piston rings and piston, and an inability to control the oil flow when the engine is running at high speeds. Therefore, the consequence of worn main bearings is high oil consumption. 4. Wear or damage of the connecting rod bearings: The effect of the clearance in the connecting rod bearings on the engine oil is similar to that of the main bearings. Furthermore, the oil is thrown more directly against the cylinder wall. Worn or damaged rod bearings result in an excessive amount of oil being flung against the cylinder walls; the pistons and piston rings, which are designed to control the normal amount of oil, are unable to manage this excess oil effectively. As a result, the extra oil enters the combustion chamber and is burned, leading to high oil consumption. Note: Insufficient bearing clearance not only leads to wear of the bearings themselves, but also to wear of the piston, piston rings, and cylinder walls. 5. Wear or damage to the camshaft bearings: Camshaft bearings are usually lubricated under pressure; if the clearance is too large, excess oil will leak out. The leaked engine oil will soak the valve guides and valve stems, resulting in increased oil consumption. 6. Wear of the crankshaft journals: Worn crankshaft journals have the same effect on engine oil as bearing wear. When they wear out and become oval, the clearance between them and the circular bearings becomes uneven. The gap between the out-of-round crank journal and the bearing changes during rotation, causing more engine oil to be thrown out. Out-of-round bearings need to be re-ground and paired with bearings of a smaller size. 7. If the cylinder liner is tapered or out of round, in the case of a slight taper and out-of-roundness (decrease in cylindricity and concentricity), oil consumption can be controlled by the piston and piston rings. However, as the cylinder liner taper and out-of-roundness increase, it becomes increasingly difficult to control oil consumption. This is the result of a combination of many factors. As the gap between the piston and the cylinder liner increases, it will cause the piston to swing during operation ； This instantaneous tilting motion will cause excess oil to accumulate on one side of the piston, and the same phenomenon occurs with the piston rings. In this way, as the piston continues to move back and forth, some engine oil will leak into the combustion chamber. For each rotation of the crankshaft, the piston completes two strokes, one up and one down. When the engine is running at 3000 rpm (about 60 miles per hour), the piston rings operating in the deformed cylinder liner undergo 6000 changes in size and shape per minute. As a result, at high operating speeds, the piston rings may not be able to adjust their clearance with the cylinder liner in a timely manner (especially when the cylinder liner is worn, which leads to an excessive clearance). Therefore, as long as the above conditions occur, it will lead to excessive oil consumption in the engine. 8. Cylinder liner deformation is different from the out-of-roundness of the cylinder liner caused by wear as mentioned in point 7; there are other factors as well, such as uneven heating or inconsistent tightening of the cylinder head bolts. All these can lead to twisting deformation of the cylinder liner, preventing the piston rings from making proper contact with the surface of the cylinder liner and thereby reducing its oil-scraping function ； As a result, an excessive amount of oil remains in the area in question, eventually entering the combustion chamber where it is burned, leading to increased oil consumption. 9. The “PCV” crankcase positive pressure ventilation valve or tube serves to recirculate the mixture that has entered the crankcase from the engine’s combustion chamber, thereby reducing the amount of unburned hydrocarbons in it. The intruding mixture is a combination of air, fuel, and combustion exhaust gases; during the power stroke, due to high pressure, it seeps into the crankcase through the gaps between the piston/piston rings and the cylinder liner. A PVC system usually has a pipe running from the crankcase to the carburetor or intake manifold. The vacuum created as air enters the engine’s intake manifold draws mixed exhaust gases out of the crankcase and into the combustion chamber, where they can be reused. The PVC (crankcase positive ventilation) valve can become clogged by sludge, paint film, or other impurities present in the mixed exhaust gases. This will cause the oil to deteriorate and excessive deposits to form, resulting in blockage of the piston rings (oil rings), increased oil consumption, and premature wear of the piston rings ； Increased crankcase pressure leads to the failure of the crankshaft seals, allowing oil to leak out and worsening the engine’s performance. 10. Abrasive wear during honing: If the cylinder liner has been honed or polished, it must be cleaned strictly in accordance with the requirements, to prevent residual metal debris or abrasives from damaging the surface of the piston ring grooves. The cleaning method is as follows: After honing, the cylinder liner must be thoroughly cleaned with a brush dipped in soapy water, and then oiled immediately ； Or clean the cylinder wall with 10# lubricating oil and wipe it thoroughly clean. Repeat the above process until all foreign objects are removed. Regardless of the method used, an inspection is required in the end: wipe the surface of the cylinder liner with a piece of white cloth; if the cloth remains clean after wiping, it indicates that the cylinder liner has been cleaned properly. Note: Gasoline or kerosene should not be used to clean the honed cylinder walls. Because they are unable to remove the abrasives attached to the cylinder wall, and instead carry them into the micro-pores of the honed surface. Therefore, cylinder liners that have not been properly cleaned may cause premature wear and piston ring failure, ultimately leading to increased oil consumption. 11. Wear of piston ring grooves: The flatness of the end surfaces of the piston ring grooves, as well as whether the gap between the piston rings and these grooves is appropriate, are important factors determining whether the piston rings can provide an effective seal. Generally, the clearance next to the piston ring grooves in an automobile engine should not exceed 0.002”-0.004”. As the piston moves up and down, the piston rings must be properly seated in the piston ring grooves. If the piston ring groove is deformed, it will prevent the piston rings from functioning properly, allowing engine oil to leak into the combustion chamber. Worn piston ring grooves will cause an increase in the clearance, allowing excess engine oil to enter the combustion chamber. Conversely, an excessive clearance can cause the piston rings to strike the piston ring grooves, leading to further wear of those grooves; if the situation is not addressed, it may even result in the breakage of the piston ring edges. 12. Damage or fragmentation of the piston ring land: Damage or fragmentation of the piston ring land prevents the piston ring from fitting properly in its groove, allowing excess engine oil to enter the combustion chamber. Furthermore, it will also cause complete damage to the cylinder liner, piston, and piston rings. Therefore, close attention is required; once such signs appear, it must be replaced immediately.