Reasons for Oil Consumption
What is excess oil consumption
All engine manufacturers have specific acceptable levels of oil consumption - what the expected oil consumption should be as part of normal operating conditions. These levels of oil consumption help define what is excessive in a given engine.
Engine, Age, and Oil Consumption
Oil consumption changes as the engine ages. Typically, a new engine will have a "break-in" period which might experience a higher level of oil consumption.
The increased oil consumption falls off after the engine breaks in. As the engine ages and gets toward the end of its life cycle oil consumption tends to rise again.
Heavy Loads, Extreme Conditions, Increased Demand
There are many other factors effecting oil consumption. Pulling heavy loads and operating in mountainous terrain will tend to cause an increase in oil consumption.
The harder a diesel engine works the more oil it will consume. Idle time also must be taken into consideration. If the engine is constantly idling you will consume more oil.
Viscosity Effects Oil Consumption
Changing the viscosity of your engine oil can also make a difference in oil consumption. Changing from a 15W-40 to a 10W-30 will have a slight increase in oil consumption due to having a slightly lighter viscosity oil in the engine.
Proper Oil Change Intervals
Extending oil change intervals beyond reasonable limits can also lead to excessive oil consumption.
Impact of Low Oil on The Engine
When an engine is consuming an excessive amount of oil and constantly running a gallon or more below the recommended level, the engine is putting excessive stress on the oil left in the engine.
Additives in the oil are going to be consumed at a faster rate if the engine is run with a low oil level thus reducing the life of the oil.
Deposits and Lost Performance
Additional problems can occur if the engine is burning oil. Carbon deposits will form on the valves and in the combustion chamber.
If oil is getting into the combustion chamber it can cause deposits to form on the top of pistons. Burning oil also will cause deposits on the exhaust valves. With these deposits the engine will not have the proper airflow which will result in a loss of power and overall engine performance.
What causes excess oil consumption
Under normal operating conditions, excess oil consumption is generally a mechanical problem.
Oil is consumed in one of two ways: It is either burned or is leaked
In a significant number of instances where oil consumption is a problem it turns out to be a leak issue. Either the valve cover gasket is leaking or one of the main seals is leaking. The pan gasket can also cause the leak.
The quality of the oil being used can have an impact.
If the engine is using a higher quality oil, the piston ring and ring belt areas will tend to stay clean. If these areas remain clean and free of carbon deposits less oil will be consumed. When the ring areas get dirty and covered with carbon the rings cannot move freely. The engine will start to have a higher oil consumption rate.
Oil Consumption - Troubleshooting
Failure to change the oil at proper intervals or to take proper care of the oil filter may cause the oil to be so dirty it will promote clogging of the oil passages in the piston rings and pistons. This will increase the oil consumption as described in "Clogged Oil Passages". Dirty oil will also increase the rate of wear on bearings, cylinders, pistons and piston rings. All of these worn parts will contribute to a further waste of oil.
Too Much Oil in Crankcase
Due to an error in inserting the oil dip stick so it does not come to a seat on its shoulders, a low reading may be obtained. Additional oil may be added to make the reading appear normal with the stick in this incorrect position which will actually make the oil level too high. If it gets so high the lower ends of the connecting rods touch the oil in a pressure lubricated engine or the dippers go too deep into the oil in a splash lubricated engine, excessive quantities of oil will be thrown on the cylinder walls and some of it will work its way up into the combustion chamber.
High Engine Vacuum
Engine vacuum has increased in modern engines because engine rpm, valve overlap and compression habits have also increased with these models. Some of the late model engines will draw as high as twenty five inches of vacuum on deceleration, as compared to twenty inches in older engines. This high vacuum characteristic has made it necessary for the development of an oil ring to seal both (top & bottom) sides of the ring grooves and eliminate oil from passing around the back and sides under high vacuum or deceleration. Such vacuum could be the main cause of smoking and oil consumption so it is important to use a side sealing piston ring when required by the application.
Worn Timing Gears
Worn timing gears can cause the valves to be out of time with the crankshaft. The large amount of backlash, which is caused by this wear, will prevent proper engine adjustment because timing may vary from one revolution of the crankshaft to another. When the valve and piston motions are not synchronized, extremely high oil consumption may result. This will be caused by excessive vacuum which draws large quantities of oil into the combustion chamber where it will be burned.
Piston Rings Fit with Too Little End Clearance
When fitting new rings sufficient end clearance is required to allow for expansion due to heat. Normal gap clearance in automotive engines with cast iron rings usually runs .003-.005 per inch of bore diameter. The rings will heat more rapidly and will operate at a higher temperature than the cylinder because they are exposed to the direct heat of the burning gases from the combustion chamber. The cylinder walls are kept at a lower temperature by the water in the water jacket. This means the rings expand more than the cylinder and this expansion must be allowed for by use of a gap - known as end clearance - between the two ends of each ring. If sufficient end clearance is not provided, the ends of the rings will butt while the engine is in operation.
Butting will cause scuffing and scoring of rings and cylinders which leads to oil consumption. If the engine is allowed to be used for continued operation, especially under heavy load, scoring will become more severe. The ends of the rings will be forced inward, away from the cylinder wall, and a space opens up between the rings and the cylinder. This provides a direct path for hot gases from the combustion chamber to burn the oil on the cylinder and greatly increases the oil consumption of the engine. Severe cases of butting may also cause ring breakage, with the same results as described in "Worn or Broken Piston Rings". Excessive ring end clearance leads to increase oil consumption as well.
Worn or Broken Piston Rings
When piston rings are broken or are worn to such an extent the correct tension and clearances are not maintained, they will allow oil to be drawn into the combustion chamber on the intake stroke and hot gases of combustion to be blown down the cylinder past the piston on the power stroke. Both of these actions will result in burning and carboning of the oil on the cylinders, pistons and rings.
Broken rings are especially damaging because their loose pieces with jagged ends are likely to cut into the sides of the piston grooves. This causes land breakage which results in the complete destruction of the piston assembly. Instead of reinstalling worn rings during engine overhaul, it is always advisable to replace them. New rings have quick-seating surfaces which enable the rings to control oil instantly, unlike rings which have been used in the past. Used rings, even those only slightly worn, will still have polished surfaces which will not seat-in properly and will lead to excessive oil consumption.
Piston Rings Stuck in Grooves
Oil cannot be controlled by piston rings which are stuck in their grooves. Every effort should be made to prevent rings from becoming stuck.
First, rings should be installed with sufficient side clearance to enable them to remain free while the engine is working under load at normal operating temperatures.
Second, every precaution should be taken at the time of assembly to ensure all parts of the engine are clean of any dirt particles which might cause the rings to stick.
Third, a good grade of oil should be used to lessen the possibility of carbon or varnish.
Fourth, the oil should be kept clean by regularly scheduled oil changes and proper care of the oil filter.
Fifth, every precaution should be taken to keep the engine from becoming overheated from any cause.
Late Valve Timing
Late valve timing will keep the intake valve closed too long after the intake stroke has started, and will increase the vacuum in the cylinder. The high vacuum will have a tendency to suck oil up past the piston and rings into the upper part of the cylinder where it will be burned.
Oil Pressure Too High
An incorrect oil pressure setting or a faulty relief valve may cause the oil pressure to be too high. The result will be the engine will be flooded with an abnormally large amount of oil in a manner similar to worn bearings.
Refer to the engine owner's manual for the proper oil viscosity to be used under specific driving conditions or ambient temperatures. Use of oil with lighter than prescribed viscosity can lead to higher than normal oil consumption.
Lugging is running the engine at a lower RPM in a condition where a high RPM (more power/torque) should be implemented. This causes more stress loading on the piston and can lead to increases in engine oil consumption.
Leaking Turbocharger Seal
A leaking turbocharger seal will draw oil into the combustion chamber where it will burn and form carbon deposits which contribute to further oil consumption as they interfere with proper engine function.
Restricted Air Intake
Excessive restriction in the air intake system will increase engine vacuum and can increase oil consumption as noted in "High Engine Vacuum". A heavily plugged air filter would be one example of this situation.
According to IPD, before an engine would be disassembled, a study should be done to track the actual amount of oil usage and determine if the engine truly is consuming questionably high amounts of oil. There are different methods to calculate excessive oil consumption. Some deal with calculations involving load factors and BSCO (brake specific oil consumption) and grams per brake horsepower (g/bkW-h). Combining the O.E. information, we've found the cimplest guideline we've developed is shown in the chart below. This chart compares the engines fuel usage in comparison to its oil consumption.Oil usages should be recorded over at least two consecutive regular service oil change periods to determine a reliable base line.
There are conditions and circumstances where engines falling in this questionable range could still be within acceptable limits.
Obviously operation conditions can play major roles and consideration must be given to things like:
- Load factors
- Oil density and additives
- Operating practices
- Operating temperatures
- Maintenance programs and practices
- Equipments applications
Reader Comments (0)
You must login to post comments.