Case Analysis And Explanation

1. External engine oil leakage
There are many causes of oil leakage, including: engine oil pipeline, oil drain, engine oil pan gaskets, valve chamber cover gasket, engine oil pump gasket, fuel pump gasket, timing chain cover seals and camshaft seals.
The above possible leakage factors cannot be ignored, because even small leakage may cause a large amount of engine oil consumption. For example, a drop every 6 seconds means that 0.56 liters of engine oil is consumed per 100 kilometers. The best way to check leakage is to put a light-colored cloth at the bottom of the engine and check it after starting the engine. The location of the leakage can be determined based on the location of the oil droplets on the cloth.

2. Front and back oil seals failure
Damage to the front and rear main bearing oil seals will definitely lead to oil leakage. This condition can only be found when the engine is running with load. The main bearing oil seal must be replaced after it has worn out, as it can cause high leakage as the external engine oil leakage.

3. Main bearing wear or failure
A worn or faulty main bearing expels excessive oil and it will be expelled to the cylinder wall. As the bearing wear increases, more engine oil will be expelled. For example, if the bearing designed clearance of 0.04 mm provides normal lubrication and cooling functions, and the bearing clearance can be maintained, the amount of expelled oil is normal and the bearing will not be damaged.
When the clearance is increased to 0.08 mm, the amount of expelled oil will be five times of the normal amount. If the clearance is increased to 0.16 mm, the amount of expelled oil will be 25 times of the normal amount. If the main bearing expels excessive oil, more oil will be splashed on the cylinder, so that the piston and piston ring cannot effectively control the oil. This can cause high engine oil consumption or carbon deposits on the piston and piston ring. Usually, if the engine oil is excessively lost on the main bearing, the connecting rod bearing will be short of oil, resulting in insufficient oil splashed on the cylinder wall in some low-speed circumstances. Consequently, the piston ring and piston will be worn, and the oil cannot be controlled when the engine runs at a high speed. Thus, the consequence of main bearing wear is high engine oil consumption.

4. Connecting rod bearing wear or damage
The impact of the connecting rod bearing clearance on the engine oil is similar to that of the main bearing. In addition, the engine oil is more directly expelled on the cylinder wall. A worn or damaged connecting rod bearing causes excessive oil expelled on the cylinder wall, so the piston and piston ring designed to control the normal engine oil volume cannot effectively control excessive engine oil, thus allowing excessive oil to be burned in the combustion chamber, i.e., the engine oil consumption is high.
Note: Insufficient bearing clearance will cause the self-wear and wear of piston, piston ring and cylinder wall.

5. Camshaft bearing wear or damage
Camshaft bearings are generally lubricated by pressure. If the clearance is too large, excessive engine oil will be lost. The leaking oil will immerse the air valve guide pipe and air valve stem, resulting in increased engine oil consumption.

6. Crankshaft journal wear
The worn crankshaft journal has the same affect on the engine oil as the bearing wear. When it is worn to be out of round, the clearance between it and the circular bearing may be uneven. The clearance between the out-of-round crankshaft journal and the bearing is too small and will change in the rotational motion, which will expel more engine oil. The out-of-round bearing needs to be re-ground, and a bearing with smaller size shall be used to match it.
For the cylinder liner worn to be slightly conical or out of round (reduced cylindricity and concentricity), the consumption of engine oil can be controlled by the piston and piston ring. However, as the cylinder liner conicity and out-of-roundness are continuously increased, it becomes increasingly difficult to control the engine oil consumption. This is the result caused by many factors comprehensively.
As the clearance between the piston and the cylinder liner increases, the piston will oscillate during operation; this momentary tilting oscillation will result in excessive engine oil retention on one side of the piston, the same circumstance will also occur on the piston ring. In this case, as the piston continues to reciprocate, some engine oil will plunge into the combustion chamber.
The piston completes two strokes (upper and lower strokes) whenever the crankshaft rotates a circle. When the engine runs at 3000 rpm (approximately 60 miles per hour), the piston ring operating in the deformed cylinder liner will be subject to the change of 6000 cycles per minute in size and shape. As a result, in the case of fast operation, the piston ring may not timely adjust its fitting clearance with the cylinder liner in time (causing excessive fitting clearance particularly when running to the cylinder liner wear position). Therefore, as long as the above circumstance occurs, the engine oil consumption will be too high.

8. Cylinder liner deformation
Different from the cylinder liner out-of-round circumstance caused by the wear as mentioned in 7, there are some other causes such as uneven heating or uneven tightness of cylinder lid bolt. All these factors may cause distortion and deformation of cylinder liner, failure to form the appropriate fitness contact between the piston ring and cylinder liner, and reduced oil scraping function; consequently, excessive engine oil is retained locally, and finally plunged to and combusted in the combustion chamber, resulting in increased engine oil consumption.

9. Positive Crankcase Ventilation (“PVC”) valve or pipe blockage
The Positive Crankcase Ventilation (“PVC”) is mainly to recycle the mixed gas expelled to the crank case from the engine combustion chamber to reduce the content of non-combusted hydrocarbon substances. The expelled mixed gas means the mixture of air, fuel and combusted exhaust gas. It is expelled to the crank case through the clearance between the piston/piston ring and cylinder liner during the power stroke.
PVC systems typically include a pipe that routes from the crank case to the carburetor or air intake manifold. The vacuum generated during air intake of the engine intake manifold sucks the mixed gas from the crank case to the combustion chamber for recycling.
PVC (Crankcase Positive Pressure Ventilation) valve may be blocked by sludge, paint film or other impurities in the expelled mixed gas. This will cause the 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, causing the crankcase seal to fail and the oil to seep out, deteriorating the operating conditions of the engine.

10. Grinding abrasive materials wear
If the cylinder liner is ground or polished, it must be strictly cleaned as required to prevent residual metal debris or abrasive damage to the piston ring groove surface. The cleaning method is as follows: After grinding, the cylinder liner must be thoroughly cleaned with a brush and soapy water, and then immediately coated with oil; or the cylinder wall shall be cleaned with 10# lubricating oil and carefully cleaned. Repeat the above process until all foreign matters have been removed. No matter which method is used, the final inspection is required: wipe the surface of the cylinder liner with a white cloth. If the white cloth is still clean after wiping, it indicates that the cylinder liner has been cleaned.
Note: The ground cylinder wall cannot be cleaned with petrol or kerosene. Because they cannot remove the abrasives attached to the cylinder wall and bring the abrasives into the grinding grain micropores. Therefore, a cylinder liner that has not been properly cleaned may cause premature wear and failure of the piston ring, eventually resulting in an increase in engine oil consumption.

11. Piston ring groove wear
The end face flatness of piston ring groove and clearance between the piston ring and piston ring groove have a great impact on the sealing of piston ring. Generally, the side clearance of engine piston ring groove shall not exceed 0.002”-0.004”.
When the piston moves up and down, the piston ring must be properly embedded in the piston ring groove. If the piston ring groove is deformed, the piston ring will not work properly and the oil will plunge into the combustion chamber. A worn piston ring groove will cause an increase in the side clearance, causing excessive oil to plunge into the combustion chamber. Conversely, excessive side clearance will cause the piston ring to hit the piston ring groove, causing the piston ring groove to wear further. If the situation is not improved, it may even cause the piston ring land to break.

12. Piston ring land damage or breakage
The damage or breakage of the piston ring causes the piston ring to not fit properly in the piston ring groove, resulting in excessive oil expelled to the combustion chamber. In addition, it will cause the thorough damage to the cylinder liner, piston and piston ring. So pay close attention to it. Once there is such a sign, it must be replaced immediately.

13. Valve stem or guide pipe wear
If the valve stem and the guide pipe are worn out, the vacuum suction generated during the air intake will suck the oil and oil vapor between the valve stem and the guide pipe into the air intake manifold and will be eventually combusted in the combustion chamber. If this circumstance is not improved, the oil consumption will increase as the intake vacuum suction increases after the engine is replaced with a new piston ring; when the engine is overhauled, sediments such as sludge originally attached to the surface of valve stem or guide pipe will be cleaned and the clearance will be further increased. As a result, the engine oil leakage and loss will become more apparent.
For the engine with top-mounted air valve, both the exhaust and intake valves may have oil loss. The problem of high engine oil consumption caused by excessive clearance of air valve guide pipe can be improved by continuously trimming the valve stem. Sometimes, new valves need to be trimmed as well. The advanced Bonded Valve Seal can be used to effectively prevent engine oil leakage loss.

14. Connecting rod bending deformation
The bent and deformed connecting rod will cause the failure of the piston to run straightly along the cylinder liner, affecting the normal sealing function of the piston ring and resulting in increased engine oil consumption. In addition, the curved deformation of the connecting rod will also cause the fit clearance between the connecting rod bearing and the piston pin to change, resulting in the premature wear of connecting rod bearing. Consequently, more engine oil will be expelled on the cylinder wall.

15. Wear or inappropriate position of piston pin
If the piston pin is worn or improperly assembled, the engine oil that flows under pressure to the piston pin will be expelled on the cylinder wall, and the piston ring will not scrape the excessive engine oil. This not only leads to excessive engine oil loss, but also the formed carbon deposits block the oil circuit, causing the piston ring to get jammed.

16. Too tight assembly of piston pin
If both ends of the piston pin are assembled too tightly, the piston cannot perform the corresponding normal expansion and contraction under the repeated alternate cooling and heating working environment of the engine, and the piston will be deformed consequently, which will cause the cylinder wall to be scratched and inevitably lead to oil expelling and excessive oil loss.

17. Oil circuit blockage
After the engine runs for a long time under the severe working conditions, the carbon deposits and foreign matters generated can easily block the oil circuit in the piston and the piston ring. At this point, the engine oil cannot return to the crank case through the normal channel, but stays in some parts such as the air valve guide pipe, resulting in increased oil consumption.
If the oil circuit in the connecting rod or other parts is blocked, the engine will be inappropriately lubricated, the wear will be intensified, and the engine oil consumption will increase. In order to avoid the circumstance above, prevention shall be carried out as described in item 28. Of course, it is not necessary to reserve the side clearance for this purpose.

18. Unbalanced torque of main bearing cover bolt or connecting rod bolt
If the torque of main bearing cover bolt or connecting rod bolt is unbalanced, the bearing to be out of round and deformed, reducing the service life of bearing, and excessive engine oil will be expelled from the bearing (its impact on engine oil consumption is as described in items 3 and 4). The bearing cover bolt must be installed with the torque wrench and shall be tightened strictly according to the torque as required by the manufacturer. If the torque of the connecting rod bolt is unbalanced, the connecting rod will be deformed, and the consequences are as described in item 14.

19. Unbalanced torque of cylinder cover bolt
The stress generated by the unbalanced torque of the cylinder cover bolt will cause severe deformation of the cylinder and lead to the oil expelling condition as described in items 7, 8. The cylinder cover bolt must be installed with the torque wrench and shall be tightened strictly according to the torque and sequence as required by the manufacturer.

20. Dust-polluted cooling system
Corrosion particles, scale, deposits or other products in the water jacket and radiator, as well as corrosion of the water pipe, can adversely affect the cooling efficiency of the cooling system. The resulting cylinder deformation will directly cause the engine oil loss. The causes are described in items #7 and #8.
Defects of the cooling system cause the engine to overheat, and some cylinders may have localized overheating area, which may cause scratches and sticking of the cylinder, piston and piston ring, resulting in increased fuel consumption. The overheated engine and overall high oil temperature of the oil pan also cause an increase in fuel consumption.

21. Dirty oil
Failure to change oil according to the specified oil change period and improper maintenance of the engine oil filter will make the engine oil dirty, cause the engine oil block the oil clearance between the piston and piston ring and lead to the increased oil consumption as described in #17.
Dirty oil can also cause increased wear on bearing, cylinder, piston, and piston ring. These worn parts, as specifically described in the respective corresponding sections, result in an increase in fuel consumption. Special note: dirty oil itself causes higher consumption than the clean oil.

22. Excessive oil volume in the oil pan
Due to the wrong insertion of the dipstick, it fails to reach the bottom, resulting in the measured oil level being lower than the actual oil level. Therefore, the new oil needs to be added to cause too high oil level. If the bottom end of the connecting rod of the pressure-lubricated engine touches the oil level, or the oil ring of the splash-lubricated engine is immersed in the oil pool too deep, excessive oil may be expelled on the cylinder wall and enter the combustion chamber.

23. The piston ring equipped is not suitable to the engine type or work type
If an improperly sized piston ring (e.g., 0.020" enlarged piston ring is used in the 0.040" enlarged cylinder) is equipped, the oil at the upper part of the cylinder cannot be scraped back to immediately cause oil expelling due to their mismatching.
The clearance between the bottom of the piston ring and the ring groove is also increased, further increasing the oil consumption, as described in #26. A variety of specially designed and manufactured piston ring set shall be provided for different types of engines under different working conditions. Each type of piston ring set is made for a specific purpose. If it is not used appropriately, it is impossible to control the engine oil consumption. It is very important to use the correct piston ring set.

24. Engine high vacuum
The increased speed of the modern engine, increased overlap angle of the air valve, and the improved compression characteristics increase the vacuum degree of the engine. When some new engines decelerates, the air suction vacuum degree will reach 25 InHg (635 mmHg) height (old engine design = 508 mm Hg height).
High vacuum requires the development of a new oil ring that effectively seals the both sides (upper and lower) of the piston ring groove to prevent oil from leaking from both sides and back of the oil ring during high vacuum and deceleration. This is often a major cause of blue smoke or high fuel consumption, so it is important to use an oil ring with side end face sealing capacity when needed.

25. Timing gear or chain wear
The wear of the timing gear or chain causes the timing out-sync of air valve and crankshaft. The adjustment of the engine cannot be achieved due to the excessive side clearance generated due to the wear of gear and chain: the timing of the previous circle may be different from the one of the next circle. When the movement of the gas valve and the piston is not synchronized, excessive oil consumption will be caused, because the excessive vacuum in the combustion chamber will suck a large amount of engine oil for combustion.

26. Too small circumferential end clearance during piston ring mounting
When installing a new piston ring, it must be noted that the piston ring still shall have the sufficient circumferential end clearance at the minimum diameter of cylinder to compensate the thermal expansion. Typically, the cast iron ring of engine requires a clearance of 0.003-0.005 inch/ inch aperture.
Due to directly subjecting to the combustion gas coming from the combustion chamber, the temperature rise temperature and operating temperature of piston ring are higher than the one of cylinder. The temperature of cylinder wall is low due to the function of water jacket, which means that the piston ring is expanded greatly. Therefore, there must be a clearance, i.e., the circumferential end clearance, for compensation. Otherwise, the end face of piston ring will interfere with and impact the cylinder wall during the operation of engine to further cause scratch and adhesive wear, resulting in the increased oil consumption accordingly.
If the engine continues to run, especially when the load is heavy, the adhesive wear will become more severe. The end face of the piston ring is pressed inwards toward the piston ring groove, and the clearance between the ring and the cylinder wall will be increased. The high-temperature and high-pressure combustion gas of the combustion chamber directly burns the lubricating oil on the cylinder wall along the passage, and the expelled gas enters the oil bottom, which greatly increases oil consumption. Severe interference can even cause breakage of the piston ring, with the consequences as described in #27.
The excessive circumferential clearance of piston ring will also increase the engine oil consumption.
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27. Worn or broken piston ring
If the piston ring is broken or excessively worn, the compressive stress and the clearance cannot be maintained, and excessive engine oil will be sucked into the combustion chamber during the induction stroke, and the combustion gas will be expelled downwards along the piston during the power stroke. Both of them cause combustion and carbonization of the engine oil at the piston, cylinder wall and piston ring.
The broken piston ring is more destructive, and the broken piece with the sharp cut is likely to cut into the side of the piston ring groove, causing damage to the ring land and thorough damage to the piston. When the engine is overhauled, the worn piston ring shall be replaced immediately instead of being reused. The new piston ring includes a quick positioning surface for immediate control of oil consumption. The used piston ring, even if it is only slightly worn, can result in excessive oil consumption due to the failure of appropriate positioning caused by polished surface.

28. Piston ring sticking
Obviously, the sticky piston ring cannot control the engine oil. Therefore, this should be avoided as much as possible. Firstly, the piston ring should be installed to ensure the correct piston ring side clearance, so that the piston ring is still movable in the ring groove at operating temperature when the engine is operating.
In addition, the components of the engine shall be clean and free of dust particles when the piston ring is installed. Otherwise, the piston ring may be stuck. Thirdly, oils with excellent performance shall be selected to reduce the formation of carbon deposits, sludge and paint film. Fourthly, oil should be changed regularly and the oil filter shall be cleaned. Fifth, the engine overheating shall be avoided.

29. Air valve timing lag
The lagged valve timing extends the intake valve closing time after the start of the induction stroke, increases the vacuum degree in the cylinder and enhances the probability of combustion of engine oil in the combustion chamber above the cylinder after being sucked through the clearance between the piston, ring and cylinder liner.

30. Excessive engine oil pressure
Incorrect oil pressure setting and failure of the safety relief valve will cause the excessive oil pressure. The result is that the engine is wetted by excessive oil, causing the same results as bearing wear.

31. Engine oil viscosity
The too low viscosity of the used engine oil may cause high oil consumption. Refer to the vehicle maintenance manual to select the appropriate oil viscosity based on driving conditions and ambient temperature.

32. Piston design
Some of the latest engines have adopted the new piston ring design to meet emission requirements. Sometimes this design will have a slight "tap" during startup. Sometimes it will increase the oil consumption.

33. Internal gasket/ air intake port damage
In the new engine design, various composite materials composed of metal and other materials are often used. Due to the difference in thermal expansion and cold contraction of different materials, thermal stress fatigue or damage may occur in the filler and sealing after long-term operation, which also leads to the increased fuel consumption level.

34. Premature spark knock
Most new engines are equipped with knock sensors to adjust the timing system to reduce emissions and improve engine power and performance. The premature spark knock is caused by the advance ignition of fuel during combustion.
The advance ignition causes a sharp rise in the pressure built up on the piston, damaging the normal movement of the piston ring, causing the failure of seals on the top and bottom sides of the piston ring and ultimately resulting in the air expelling through the piston ring and fuel consumption. The same problem can also be caused by faults of intake flow sensor and throttle position sensor.

35. User’s modification and parts selection for engine performance improvement
Modified parts for improvement of engine performance/power added on the stock or serviced engine may cause the high engine oil consumption.

36. Engine lugging
Lugging means that the engine is made to run at low speed when required to run at high speed (higher power/ torque), which will cause greater pressure on the piston and increased engine oil consumption.

37. Misoperation of overspeed operation
The engine runs at a high speed under unsuitable conditions. Various relevant causes such as crawling and frequent start/stop in the urban traffic will result in the increased engine oil consumption. You may also refer to item 36 above.

38. Sealing leakage of turbocharger
The sealing leakage of the turbocharger causes the engine oil to enter into the combustion chamber where it burns and forms carbon deposits, hindering the normal operation of the engine and further causing higher oil consumption.

39. High air intake resistance
Excessive air intake system resistance increases the vacuum in the engine and increases the oil consumption, as described in item 24. An example is the severe blockage of the air filter.

40. Fuel dilution
If the fuel not fully burnt enters the lubrication system, the engine oil will become thinner and more volatile, which will result in higher engine oil consumption. Excessive fuel may enter the lubrication system and be mixed with the engine oil due to fuel nozzle leakage, fuel pump problem, high intake resistance or excessive idle operation.