2. Front and rear bearings seal failure Front and rear main bearing oil seals damage will lead to engine oil leakage. This situation can only be discovered when the engine is running under loading conditions. Leaking main bearing oil seals must be replaced immediately as it will cause a high amount of oil leakage. Your regular maintenance/service centre will be able to provide advice on how often to check main bearing seals. 3. Main bearing failure or fault Worn or faulty main bearings will result in excessive engine oil leaking to the cylinder wall. This phenomenon will aggravate when bearing wear increase. For example, the bearing design clearance of 0.04 mm provide normal lubrication and cooling functions. When the gap increase to 0.08 mm, 5 times more engine oil will leak into the cylinder wall and 25 times when the gap increase to 0.16 mm. If too much oil is splashed out of the main bearing, more oil will be splashed on the cylinder, resulting in the piston and piston rings unable to effectively control the oil. This can cause oil burning or carbon deposits on the piston and piston rings. In general, if main bearing oil loss is high, the connecting rod bearing will be short of oil, resulting in insufficient oil splashing on the cylinder wall under certain low speed conditions, causing wear of the piston rings and pistons. Therefore, the result of main bearing wear is high engine oil consumption. 4. Connecting rod bearing failure or wear The effect of connecting rod bearing clearance is similar to the main bearing. Worn or damaged connecting rod bearings cause too much oil to be thrown onto the cylinder wall, causing the pistons and piston rings designed to control the normal amount of oil to be unable to effectively control the excessive oil. The excessive engine oil is thus burned in the combustion chamber leading to high engine oil consumption. Note: Insufficient bearing clearance will cause wear on itself and also on the pistons, piston rings and cylinder wall. 5. Camshaft bearings damaged or wear Camshaft bearings are usually pressure lubricated. Engine oil will leak when there is a big clearance. The leaked engine oil will soak in the valve guide and valve stem and cause oil consumption to increase. 6. Crankshaft journal wear Worn crankshaft journals have the same effect on oil as bearing wear. When it wears and loses roundness, the gap between them and the round bearing will be uneven. The gap between the out-of-round crankshaft journal and the bearing changes during the rotating motion, which will throw out more oil. The out-of-round bearings need to be reground and matched with smaller bearings. 7. Conical and out-of-round cylinder liner For cylinder liners that are ground into a slight taper and out of round (decreased cylindricity and concentricity), the oil consumption can be controlled by the piston and piston ring. However, with the continuous increase of cylinder liner taper and out-of-roundness, the control of oil consumption become increasing difficult. 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 tilt and swing will result in excessive oil retention on one side of the piston, and the same situation also occurs on the piston ring. In this way, as the piston continues to swing back and forth, some oil will escape into the combustion chamber. For each revolution of the crankshaft, the piston completes two strokes, one up and down. When the engine is running at 3000 rpm (approximately 90 kph), the piston ring running in the deformed cylinder liner will undergo 6,000 changes in size and shape per minute. Therefore, as long as the above situation occurs, it will cause high engine oil consumption. 8. Cylinder liner deformation Different from the cylinder liner out-of-round due to wear mentioned in Point 7, there are other reasons, such as uneven heating or uneven tightening of cylinder head bolts, which may cause the cylinder liner to be twisted and deformed, causing the piston ring to fail. Proper contact is formed with the surface of the cylinder liner, and the oil scraping function is reduced; as a result, excessive oil remains locally, which eventually escapes into the combustion chamber and is burned, resulting in increased oil consumption. 9. Positive Crankcase Ventilation Valve (PCV) blockage The main function of PVC (Crankcase Positive Pressure Ventilation) is to recycle the mixed gas from the engine combustion chamber into the crankcase to reduce the content of unburned hydrocarbons. The blow-in mixed gas is a mixture of air, fuel oil and combustion exhaust gas. It flows into the crankcase through the gap between the piston or piston ring and the cylinder liner due to the high pressure during operation. PVC systems usually have a pipe that routes the crankcase to the carburetor or intake manifold. The vacuum generated during the intake of the engine's intake manifold sucks the mixed blow-by gas out of the crankcase, enters the combustion chamber, and is recycled again. The PVC valve may be blocked by sludge, paint film or other impurities in the mixed blow-by gas. This will cause the engine oil to deteriorate and generate excessive deposits. As a result, the piston ring (oil ring) will be blocked, the oil consumption will increase, and the piston ring will wear prematurely; the crankcase pressure will increase, resulting in the failure of the crankshaft sealing ring, the oil leakage, and deterioration of engine working situation. 10. Piston ring groove wear The surface flatness of the piston ring groove and the gap between the piston ring and the piston ring groove are important factors of the seal ability of piston ring. Generally, the side clearance of the piston ring groove of an automobile engine cannot exceed 5-10 mm. During operation, the piston ring must fit properly in the piston ring groove. If the piston ring groove is deformed, the piston ring will not operate properly and the engine oil will leak into the combustion chamber. In addition, worn piston ring groove will increase the side clearance, causing excessive oil to escape into the combustion chamber. 11. Piston land damaged or chipped The damage or fragmentation of the piston land will result in the piston ring inability to be embedded properly in the piston ring groove, this will also cause excessive engine oil flow into the combustion chamber. 12. Valve stem or guide wear If the valve stem and the duct are worn out, the vacuum suction generated during the intake will draw the oil and oil vapor between the valve stem and the duct into the intake manifold, and finally enter the combustion chamber and burn it. If this situation cannot be improved, when the engine is replaced with a new piston ring, the engine oil consumption will increase due to the increase in intake vacuum suction. When the engine is overhauled, sludge and other sediments originally attached to the valve stem and duct surface are removed, thus increasing the gap, resulting in higher engine oil leakage loss. The problem of high engine oil consumption caused by excessive valve guide clearance can be improved by continuously trimming the valve stem. Sometimes new valves also need to be trimmed in this way. Adopting advanced integrally fastened type valve oil seal (Bonded Valve Seal) can effectively prevent oil leakage and loss.