After spending more than 20 years in Off Road racing and with links to an engine building facility, one the most frequently asked questions is “why does my engine use oil” so I thought I would share with OBT some common and not so common causes of higher than normal oil consumption in no particular order. Most of you will already be aware of the reasons as to why, but this is for those who may want a little more information.
Cheers
Glenn
1. Worn Valve Stems and Guides
When wear has taken place on valve stems and valve guides, the vacuum in the intake manifold will draw oil and oil vapor between the intake valve stems and guides, into the intake manifold and then into the cylinder where it will be burned. If this condition is not corrected when new piston rings are installed, an engine is likely to use more oil than it did before because the new piston rings will increase the vacuum in the intake manifold. When deposits on the valve stems are removed the seal previously formed will be removed and leakage will be more pronounced. This is particularly true on overhead valve engines where loss of oil may occur on the exhaust valves as well as on the intake valves.
2. Dirty Cooling Systems
Rust, scale, sediment or other formations in the water jacket and radiator, or corrosion of the water distributing tube, will prevent a cooling system from performing its duties efficiently. A defective cooling system causes overheating of the engine with the possibility of developing localised hot spots in some of the cylinders. This may also lead to scuffing and scoring of cylinders, pistons and rings which results in high oil consumption.
3. Dirty Oil
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 that it will promote clogging of the oil passages in the piston rings and pistons. This will increase the oil consumption. Dirty oil will also increase the rate of wear on bearings, cylinders, pistons, and piston rings. All of these worn parts, as explained in individual items on each part, will contribute to a further waste of oil.
4. Piston Rings Fit with Too Little End Clearance
When fitting new rings, care must be taken to see that, with the rings in the smallest part of the cylinder, sufficient end clearance is allowed for expansion due to heat. 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 that 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 or 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 – so that 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. Excessive ring end clearance leads to increased oil consumption as well.
5. Worn or Broken Piston rings
When piston rings are broken or are worn to such an extent that 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 cylinder, piston 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. Used rings, even those that have been only slightly worn will still have polished surfaces that will not seat-in properly and will lead to excessive oil consumption.
6. Honing Abrasive
If cylinder honing is performed on an engine, cleaning instructions should be carefully followed to prevent metal fragmentation or abrasive damage to the rings’ seating surfaces. After honing thoroughly wash cylinder walls with soapy water and a scrub brush and oil immediately thereafter and carefully wipe clean. Repeat until all evidence of foreign matter is removed
Note: Do not use petrol or kero to clean the cylinder walls after honing. Solvents of this nature will not remove the grit from the cylinder wall and often carry particles of abrasives into the pores of the metal. Failure to properly clean the cylinder walls will leave abrasives that will cause rapid wear and ring failure and will result in elevated oil consumption.
7. Worn Ring Grooves
For piston rings to form a good seal, the sides of the ring grooves must be true and flat – not flared or shouldered – and the rings must have the correct side clearance in the grooves. As the pistons move up and down, the rings must seat on the sides of the grooves in very much the same way that valves must seat to prevent leakage. New rings in tapered or irregular grooves will not seal properly and, consequently, oil will pass around behind the rings into the combustion chamber. Worn grooves are usually flared or tapered causing increased side clearances which permit more than the normal amount of oil to pass the rings into the combustion chamber. Excessive side clearances also create a pounding effect by the rings on the sides of the piston grooves. This promotes piston groove wear and, if the condition is not corrected, breakage of rings lands may occur.
8. Cracked or Broken Ring Lands
Cracked or broken ring lands prevent the rings from seating completely. In addition to this, they also lead to serious damage to the cylinders as well as complete destruction of the pistons and rings. Cracked or broken ring lands cannot be corrected by any means other than piston replacement and this should be done as soon as there is the slightest indication of a crack.
9. 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.
10. Oil Viscosity
The use of oil with a viscosity that is too light may result in high oil consumption. Refer to the bikes owner’s manual for the proper oil viscosity to be used under specific riding conditions or ambient temperatures.
11. Internal Gasket/Intake Breach
Engine designs sometimes implement a combination of composite materials and metals. Gaskets and seals can sometimes breach or become stressed over time due to differences in heat expansion and contraction differences causing oil consumption levels to increase.
12. Restricted Air Intake
Excessive restriction in the air intake system will increase engine vacuum and can increase oil consumption. A heavily plugged air filter would be one example of this situation.
