Engine oil consumption is now increasing; as the oil change intervals become higher.
Modern engines consume so little oil; that many vehicle owners forget to regularly check their levels.
As a result, oil level warning systems are becoming standard equipment on many vehicles. Most new engines today; use less than half a quart of oil in 3,000 to 5,000 miles.
Furthermore, some have almost no engine oil consumption. But, as the miles accumulate; wear and oil consumption naturally go up. An engine that burns oil will usually foul the spark plugs.
That, in turn will cause ignition misfires, higher emissions; and likely damage the catalytic converter.
Excessive internal oil consumption; will also shorten the service life of oxygen sensors and catalytic converters. So, as engines wear; the combined loss; from external and internal oil leaks; will just add to the increase of oil consumption.
External Oil Consumption Sources;
- Crankshaft seals
- Oil pan gasket
- Timing cover gasket
- Cylinder head gasket
- Valve cover gaskets
- Intake manifold gasket
Internal Oil Consumption Sources;
A large puff of blue oil smoke from the exhaust; after an extended idling period; usually indicates internal engine oil consumption caused by;
- Worn piston rings
- Worn valve seals
- Leaking intake manifold gaskets
- Clogged oil drain back holes
So, How Can Engine Oil Enter The Combustion Chamber
Consequently, worn or cracked intake valve stem seals; can leak oil through the valve guides. Also, the spark plugs might show some oil ash accumulation; on the side of the electrode facing the intake valves.
Oil leakage through the exhaust valve guides isn’t as common; since normal exhaust flow generates positive pressure. On the other hand; most oil consumption is through the pistons and piston rings.
Fuel Washed Cylinders
So, There is a condition that contributes to engine oil consumption; generally referred to as “fuel wash“. It is caused by engine flooding; at initial startup; or in early operation of a newly rebuilt engine.
This phenomenon can cause very serious damage to;
- Cylinder bores
- Piston rings
When flooding occurs; from a fuel system malfunction; the excess fuel washes the oil film from the cylinder walls. At this point; metal to metal contact occurs and scuffing takes place. This condition is similar to and also sometimes referred to as dry start.
Piston Ring Sealing
Oil washing, is an indication of engine oil passing through the piston rings. So, to better understand ring related oil consumption; let’s look at piston and piston ring design.
For example, many top rings are flat with a convex or barrel shaped outer edge; that contains a molybdenum inlay.
Crankshaft Bearing Clearance And Oil Consumption
Excessive connecting rod bearing clearance affects oil consumption. The piston and cylinder are splash lubricated; by oil passing through the connecting rod bearing and onto the cylinder wall.
Even more on high mileage or incorrectly rebuilt engines. Doubling the connecting rod bearing clearance; will quadruple the oil flow to the piston rings. As a result, dramatically increasing oil consumption.
Issues With High-Viscosity Engine Oils
So, Using high-viscosity oil might prevent low tension piston rings; from contacting the cylinder wall; which can increase oil consumption. Also, engine oil slinging off the crankshaft not only lubricates the rings; but cools them as well. And, since high viscosity oil reduces oil flow through the connecting rod bearing; cylinder lubrication and cooling will be negatively affected.
While trying to reduce oil flow to the piston rings; the oil film still has to reach the very top of the cylinder wall. High viscosity generic oils; might not adequately lubricate the top and second piston rings; especially during cold startups.
The flash point of the oil must also be high enough to resist vaporizing under high cylinder wall temperatures. Using non-synthetic base oils in synthetic applications; allows this oil film to be burned away during combustion.
In practically all cases; synthetic oils not only protect the upper cylinder; but also protect the top and second piston rings from; momentarily micro welding to the cylinder wall during high load driving conditions. So, as miles accumulate; synthetic oils; also keep pistons free of varnish deposits that can cause low-tension piston rings to stick in their grooves.
Basic Tips To Follow;
- All engines consume oil; so check the oil level before the oil is drained. Compare the mileage on the odometer with the mileage on the lube sticker; to estimate the engine’s oil consumption rate.
- Oil flows downhill. Use a bright flashlight to examine the engine for leaks; beginning with the camshaft or rocker arm covers.
- If there’s oil dripping from the bellhousing area; remember that automatic transmission oil is usually red while engine oil is black or brown. Check the level of each to help determine the source of the leak.
Oil consumption with no apparent oil smoke; often indicates collapsed oil control ring expanders or worn oil control rings.
- Poor lubrication can cause modern piston rings to overheat and lose their tension. When combined with excessive varnish; the piston rings can stick in a collapsed position.
- Excessive compression ring blowby will; force engine oil into the intake air ducting or intake manifold.
- Also, a combination of low speed driving and neglected oil changes on variable displacement engines; can cause the piston rings to stick in their grooves on the deactivation cylinders.
- Crusted oil ash deposits on spark plugs and upstream oxygen sensors; are the best indicators of excessive internal oil consumption.
Preventing engine oil consumption is even more important; now that many modern vehicle models come with extended oil change intervals. So monitor your engine oil level and condition more frequently; at least once or twice a month. And change the oil as necessary; don’t wait for the manufacturer’s suggested interval.
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