Oil consumption has become an issue because oil change intervals now extend to 10,000 or more miles and because modern engines consume so little oil that many vehicle owners forget to regularly check their engine’s oil level. Worse still, many owners will often run their engines out of oil because they don’t know how to check the oil level. For that reason, oil level warning systems are becoming standard equipment for many vehicles.
That said, I don’t know of any specific number that would indicate excessive oil consumption for any specific vehicle. A ballpark number for oil consumption on a new engine might be a quart of oil during initial break-in. After break-in, oil consumption should then stabilize at perhaps one quart per 2,000 or 3,000 miles. For engines with 150,000 or more miles on the odometer, consuming one quart of oil every 2,000 miles should not be an issue. As engines wear, the combined loss from external and internal oil leaks might increase oil consumption to one quart per 1,000 miles, which should not be a problem if the spark plugs don’t become fouled with oil ash or the exhaust doesn’t emit visible oil smoke.
Internal Oil Consumption
Assuming that the engine has no obvious external leakage at the crankshaft seals, oil pan, timing cover or cylinder head and camshaft cover gaskets, let’s consider how engine oil might enter the combustion chamber through internal leaks. An example of internal leakage is turbocharger shaft seals leaking oil into the engine intake, as indicated by the coating of engine oil inside the ducting between the turbocharger and engine. If the intake manifold on some V-block engines seals the upper crankcase, oil can enter through one or more intake port gaskets. Similarly, worn or cracked intake valve stem seals can leak oil through the valve guides, especially during deceleration and extended idle-speed operation.
In either case, 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, which is where our story goes next.
Oil washing is an indication of engine oil passing through the piston rings (see Photo 1). 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. The moly inlay retains oil and is resistant to high combustion temperatures.
The second compression ring not only helps seal combustion pressures, but scrapes excess oil into the engine crankcase (see Photo 2). In contrast to the top ring, the second ring is saucer-shaped, with only the bottom edge of the ring contacting the cylinder wall. When combustion pressure increases, the second ring flattens against the piston ring land, which forces the full outer width of the ring against the cylinder to seal combustion gases inside the cylinder. When not under load, the ring returns to its saucer-shaped configuration, which causes the lower edge of the ring to scrape excess back oil into the crankcase.
The bottom or third piston ring’s sole duty is to scrape excess engine oil into the crankcase. In most cases, the third ring is a three-piece design consisting of a vented ring expander and two steel rails that fit over the expander. The vented expander and piston ring groove allow excess oil to flow to the inside of the piston and into the crankcase (see Photo 3).
To help meet emissions standards, manufacturers have reduced piston-to-cylinder clearances. Using a 2013 Mazda 2.5-liter, 16-valve Skyactiv engine as an example, 0.0010” minimum and 0.0017” maximum is the standard specified clearance between pistons and cylinders for new engines.
To compare, clearances were nearly double that in older engine designs to allow for thermal expansion. Since modern high-silicon content aluminum pistons experience much less thermal expansion, 0.001” provides sufficient oil clearance between the piston and a precision-machined cylinder. These tight piston skirt clearances and precision-machined cylinders also hold the piston rings square with the cylinder wall for a much better compression and oil ring seal (see Photo 4).
In the meantime, most light-duty engines reduce rotating friction by using narrow, low-tension piston rings. Low-tension piston rings also tend to last longer due to less circumferential pressure against the cylinder. Last, improved cylinder boring and “plateaued” cylinder honing techniques allow the piston rings to quickly seat into the cylinder wall. After break-in, a coarser, underlying cross-hatch pattern remains in the cylinder to keep the piston rings and upper cylinder areas well lubricated.
Connecting rod bearing clearance affects oil consumption because the piston and cylinder are splash-lubricated by oil passing through the connecting rod bearing and onto the cylinder wall. With our Mazda SkyActiv engine, oil must pass through an 0.0011” to 0.0020” connecting rod bearing clearance before it can reach the cylinder wall. Remember that doubling the connecting rod bearing clearance will quadruple the oil flow to the piston rings, which can dramatically increase oil consumption.
The engine oil must then pass through 0.0001” of an inch oil clearance between the piston skirt and cylinder before it reaches the piston rings. Using high-viscosity oil in a new engine reduces the lubrication and cooling of low-tension piston rings, which can be a serious problem on today’s turbocharged, high-performance engines.
Another issue with using high-viscosity oil is that it might prevent low-tension piston rings from contacting the cylinder wall, which can increase oil consumption.
As mentioned above, oil slinging off the crankshaft not only lubricates the rings, but cools them as well. Since high-viscosity oil reduces oil flow through the connecting rod bearing, cylinder lubrication and cooling will be negatively affected.
While on the one hand we’re trying to reduce oil flow to the piston rings, on the other hand, the oil film must 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, whereas synthetic oils tend to remain in place in the upper cylinder.
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. As miles accumulate, synthetic oils also keep pistons free of varnish deposits that can cause low-tension piston rings to stick in their grooves.
In summary, following the recommended maintenance intervals and using specified engine oils goes a long way toward preventing excessive oil consumption on modern engines.
Diagnostic Solutions: We Should Know It When We See It
- 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, which should be noted on the vehicle owner’s lube and inspection report.
- Free underhood engine oil and fluid level checks for your customers will generate a positive image for your shop.
- Oil flows downhill. When the vehicle is on the lift, use a bright flashlight to examine the engine, 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.
- 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, valve seals, intake manifold gaskets, or clogged oil drains in the cylinder head.
- 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.
- 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.