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Reasons For Motor Oil
Consumption
AMSOIL
Technical Service Bulletin
REASONS FOR MOTOR OIL
CONSUMPTION provides explanations for using “too much” oil. It serves
as a handy reference when the question of “abnormal” oil usage arises.
It is interesting to note that the only two
oil related problems contained are explained by “dirty oil” and by overfilling
the crankcase. The balance of the problems are all mechanical in nature.
Before we review the reasons why oil
consumption occurs, it should be noted that a degree of consumption should be
anticipated in all engines. What is considered normal or acceptable, however,
will vary from one engine or application to the next. For example, Ford Motor
Company considers consumption as high as one quart of oil per 1,000 miles to be
acceptable in a gasoline engine. For large diesel engines used in over the road
trucking applications, many manufacturers are not concerned until consumption
reaches one gallon of oil per 10,000 miles of operation.
1)External Oil Leaks
Some of the many points where external oil
leaks may occur include: oil lines, crankcase drain plug, oil pan gasket, valve
cover gaskets, oil pump gasket, fuel pump gasket, timing case cover and camshaft
bearing seal. No possible source of leakage should be neglected because even a
very small leak will cause extremely high oil consumption. For example, it has
been estimated that a leak of one drop of oil every twenty feet is approximately
equal to a loss of one quart of oil every 100 miles. The best way to check for
external leaks is to road test the vehicle with a large piece of light-colored
cloth tied under the engine. Oil on the cloth will indicate a leak which should
be traced to its source.
2)Front or Rear Main
Bearing Seals
Worn front or rear main bearing seals almost always result in oil
leakage. This can only be determined when the engine is operated under load
conditions. Bearing seals should be renewed when worn because a slight leak will
result in extremely high oil consumption just as it would with an external oil
leak.
3)Worn or Damaged Main
Bearings
Worn or damaged main bearings throw off an excessive amount of oil
which flows along the crankshaft and is thrown up into the cylinders. The amount
of oil throw off increases rapidly when bearing wear increases. For instance, if
the bearing is designed to have .0015” clearance for proper lubrication and
cooling, the throw off of oil will be normal as long as this clearance is
maintained and the bearing is not damaged in any way. However, when the bearing
clearance increases to .003”, the throw off will be five times normal. If the
clearance is increased to .006”, the throw off will be twenty-five times normal.
When the main bearings throw off too much oil, the cylinders are usually flooded
with more oil than can be controlled by the pistons and rings. This causes
burning of the oil in the combustion chamber and carboning of pistons and rings.
In a conventional, full-pressure lubricated engine a large loss of oil at the
main bearings may starve the downstream connecting rod bearings of lubrication
to such an extent that sometimes, especially at low speeds, insufficient oil may
be thrown on the cylinder walls. This will cause the pistons and rings to wear
to such anextent that they will not be able to control the oil at high speeds.
The effect of main bearing wear will be high oil consumption.
4)Worn or Damaged
Connecting Rod Bearings
Clearances on connecting rod bearings affect the throw off of oil in
the same proportions as mentioned for main bearings. In addition to this, the
oil is thrown more directly into the cylinders. Worn or damaged connecting rod
bearings flood the cylinders with such a large volume of oil that the pistons
and rings, which are designed to control a normal amount of oil or a reasonable
increase in the normal amount, are overloaded to such an extent that some oil
escapes past them to the combustion chamber and causes high oil consumption.
CAUTION - Insufficient bearing clearance can also produce piston, ring and
cylinder damage as well as damage to the bearing itself.
5)Worn or Damaged
Camshaft Bearings
Camshaft bearings are generally lubricated under pressure and, if the
clearances are too large, excess oil will be thrown off. Large quantities of
this oil may flood valve guide and stem areas resulting in increased oil
consumption.
6)Worn Crankshaft
Journals
Worn crankshaft journals will have the same effect on oil consumption
as worn bearings. When they are worn out-of-round, they cannot be set up with
round bearings to give uniform oil clearance. A bearing fit to the larger
dimension of a worn journal will be loose at the smaller dimension and throw off
many times the proper amount of oil. Journals which are out-of-round, rough or
scuffed should be reground and fitted with undersize bearings of the correct
size.
7)Tapered and
Out-of-Round Cylinders
In slightly tapered and out-of-round cylinders, the oil can be
controlled by the pistons and rings. However, with increased taper and
out-of-roundness, satisfactory oil control becomes more difficult to maintain.
This is due to a combination of many factors. The increased piston clearances
permit the pistons to rock in the worn cylinders. While tilted momentarily, an
abnormally large volume of oil is permitted to enter on one side of the piston.
The rings, also tilted in the cylinder, permit oil to enter on one side. Upon
reversal of the piston on each stroke, some of this oil is passed into the
combustion chamber. For each revolution of the crankshaft, the pistons make two
strokes - one up and one down. When an engine is running at 3000 R.P.M.
(approximately 60 miles per hour) the rings in tapered and out-of-round
cylinders are changing their size and shape 6000 times per minute. Consequently,
at high speeds, the rings may not have time to conform perfectly to all worn
parts of the cylinders on every stroke. Whenever this occurs, the engine
consumes higher amounts of oil due to what is commonly referred to as oil
pumping.
8)istorted Cylinders
Cylinders which are distorted so that they are out of shape - not
from wear, as in #7, but from other causes, such as unequal heat distribution or
unequal tightening of cylinder head bolts - present a surface which the rings
may not be able to follow completely. In this case, there may be areas where the
rings will not remove all of the excess oil. When combustion takes place, this
oil will be burned and cause high oil consumption.
9)Clogged “PCV” Valve
The main purpose of the PCV (positive crankcase ventilation) valve is
to re-circulate blow-by gases back from the crankcase area through the engine to
consume unburned hydrocarbons. Blow-by is a mixture of air, gasoline and
combustion gases forced past the rings on the combustion stroke. The PCV system
usually has a tube leading from the crankcase to the carburetor or intake
manifold. Vacuum within the engine intake manifold pulls blow-by gases out of
the crankcase into the combustion chamber along with the regular intake of air
and fuel. A valve can become clogged with sludge and varnish deposits and trap
blow-by gases in the crankcase. This degrades the oil, promoting additional
formation of deposit material. If left uncorrected, the result is plugged oil
rings, oil consumption, rapid ring wear due to sludge buildup, ruptured gaskets
and seals due to crankcase pressurization, oil thrown out around the filler cap
and consequent rough engine operation.
10)Honing Abrasive
If cylinder honing or glaze breaking is performed on an engine,
cleaning instructions should be carefully followed to prevent metal
fragmentation or abrasive damage to the rings’ seating surfaces. Cleaning
instructions for reconditioned cylinders: After honing thoroughly wash cylinder
walls with soapy water and a scrub brush and oil immediately thereafter, or swab
cylinders with No. 10 oil and carefully wipe clean. Repeat until all evidence of
foreign matter is removed. In either method that is used, a white cloth wiped on
the surface should remain clean. Note: Do not use gasoline or kerosene 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.
11)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. Normally, automotive ring groove side
clearance should not exceed .002-.004. 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 seat 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.
12)Cracked or Broken
Ring Lands
Cracked or broken ring lands prevent the rings from seating
completely on their sides and cause oil pumping by a process similar to that
described in #7. In addition to this, they also lead to serious damage of 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.
13)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
gum or deposits on the valve stems are removed - a procedure recommended when
overhauling an engine - 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. High oil consumption caused by too much valve guide clearance can
frequently be cured by reaming or nerraling the valve stem. In some cases new
valves may also be required. Use of a permanently bonded valve stem seal will
give added insurance against oil leakage on complete engine overhauls or on
valve jobs.
14)Bent or Misaligned
Connecting Rods
Bent or misaligned connecting rods will not allow the pistons to ride
straight in the cylinders. This will prevent the pistons and rings from forming
a proper seal with the cylinder walls and promote oil consumption. In addition
to this, it is possible that a bearing in a bent rod will not have uniform
clearance on the crankpin. Under these conditions, the bearing will wear rapidly
and throw off an excessive amount of oil into the cylinder.
15)Worn or Improperly
Fit Wrist Pins or the Wrong Pins
The use of worn or improperly fitted wrist pins or the installation
of the wrong pins, as in the case of rifle drilled rods where oil is forced to
the wrist pins under pressure, can cause such an excessive throw off of oil onto
the cylinder walls that the piston rings may not be able to control it. This
will not only result in the direct loss of the excess oil but also in the
formulation of carbon which will clog the oil passages and cause the rings to
become stuck in the grooves.
16)Wrist Pins Fit Too
Tightly
Wrist pins that are fitted too tightly at both ends prevent the
pistons from expanding and contracting freely under the repeated heating and
cooling encountered in engine operation. The piston distortion results in
scuffing and scoring, which inevitably leads to blow-by and high oil
consumption.
17)Clogged Oil Passages
After an engine has had long, hard service the oil passages in piston
rings and pistons will likely become clogged from carbon or an accumulation of
foreign matter in the oil. The passages are designed for carrying oil - in
excess of the amount needed for lubricating the cylinders - back to the
crankcase. When the passages become clogged, oil may be trapped in areas
reducing the indicated level of oil within the engine. It may also pool in areas
such as above the valve guides, which can further promote consumption.
Clogged passages in rifle drilled rods or any clogged oil line will starve the
engine of lubrication, promote wear and lead to high oil consumption. To avoid
clogging of oil passages, the same precaution should be taken as recommended in
#28. Initial side clearance is not applicable in this case.
18)Unequal Tightening of
Main Bearing Bolts or Connecting Rod Bolts
Unequal tightening of main bearing bolts or connecting rod bolts will throw the
bearing bores out-of-round enough to shorten bearing life and to cause an
abnormally large throw off of oil from the bearings. The effect on oil
consumption is described in numbers 3 and 4. When bearing bores are originally
machined, at the time of engine manufacture, the bolts are tightened to the
manufacturer’s torque. A torque wrench must be used to insure roundness of the
bearing bores whenever the bolts are tightened after having been removed and
reinstalled. Unequal tightening of connecting rod bolts may also cause
connecting rod distortion, with results similar to those described in number 14.
19)Unequal Tightening of
Cylinder Head Bolts
The strains developed by unequal tightening of cylinder head bolts
may cause serious cylinder distortion and result in oil pumping as mentioned in
#7 and #8. When re-installing a cylinder head, a torque wrench should always be
used on the head bolts. The engine manufacturer’s instructions should be
followed for the torque readings and the sequence in which the bolts are
tightened.
20)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. This is likely to cause cylinder
distortion with a direct loss of oil as mentioned in #7 and #8. A defective
cooling system causes overheating of the engine with the possibility of
developing localized 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.
21)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 as described in #17. 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. Note: as a rule, dirty oil by nature is also consumed at a higher rate
than cleaner oil.
22)Too Much Oil in
Crankcase
Due to an error in inserting the oil dip stick so that it does not
come to a seat on its shoulder, 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
that 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.
23)Incorrect Piston
Rings for Type of Engine or Type of Service
If rings of an incorrect size are installed (for instance, .020”
oversize rings in .040” oversize cylinders) they can readily cause oil pumping
because they will not fit the cylinders and will be unable to keep the oil down
from the upper cylinder walls. In this example, ring end gap will also be
greater, resulting in additional oil loss, as described in #26. Different types
of engines and their use in different types of service require individually
engineered ring sets which vary in many ways. Each set has been designed for a
particular purpose, but if one is used in an engine for which it is not
intended, it may be incapable of controlling the oil in that engine. It is
extremely important to always make sure that the correct set is used.
24)High Engine Vacuum
Engine vacuum has increased in modern engines due to the fact that
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
that you use a side sealing piston ring when called for.
25)Worn Timing Gears or
Chain
Worn timing gears or chain can cause the valves (and sometimes the
distributor) 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.
26)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. 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 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 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 - 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, with the same results as described in number 27. Excessive
ring end clearance leads to increased oil consumption as well.
27)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
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 that have been only slightly worn will
still have polished surfaces that will not seat-in properly and will lead to
excessive oil consumption.
28)Pistons Rings Stuck
in Grooves
Obviously, oil cannot be controlled by piston rings which are stuck
in their grooves, so every effort should be made to prevent rings from becoming
stuck. First, they 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 see that 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.
29)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.
30)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 that the engine will be
flooded with an abnormally large amount of oil in a manner similar to that which
occurs with worn bearings.
31)Oil Viscosity
The use of an oil with a viscosity that is too light may result in
high oil consumption. Refer to the vehicle owner’s manual for the proper oil
viscosity to be used under specific driving conditions or ambient temperatures.
32)Piston Slap
Some late model engines meeting the latest emission requirements have
changed their piston design. This can sometimes lead to a light “knock” at
startup. In some cases this can increase oil consumption levels.
33)Internal
Gasket/Intake Breach
Newer 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.
34)Spark Knock
Most new automobiles have knock sensors to adjust timing to reduce
emissions as well as increase engine power and performance. Spark knock is due
to premature ignition of the fuel during the combustion process. Pre-ignition
results in surges of pressure being forced upon the piston. This disrupts the
movement of the piston ring, resulting in a loss of ring seal on both the top
and bottom of the ring, and ultimately allowing for increased blow-by and oil
consumption past the rings. This may also occur due to a faulty mass air flow
sensor or throttle positioning switch.
35)Aftermarket
Performance Chips and Modifications
Increasing performance through the use of performance/power
enhancement products to a stock or factory engine can increase the chance of
excessive oil consumption.
36)Lugging Engine
Lugging is running the engine at a lower RPM in a condition where a
higher RPM (more power/torque) should be implemented. This causes more stress
loading on the piston and can lead to increases in engine oil consumption.
37)Inappropriate
Operation of Overdrive
Operating the overdrive mode in conditions where it is not
recommended will cause the engine to consume oil for a variety of reasons. Such
conditions include towing or stop-and-go driving in city traffic. See also
reason #36.
38)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.
39)Restricted Air Intake
Excessive restriction in the air intake system will increase engine
vacuum and can increase oil consumption as noted in #24. A heavily plugged air
filter would be one example of this situation.
40)Fuel Dilution
If un-burnt fuel is allowed to enter the lubrication system, the oil
will become thinner and more volatile. Both will result in higher oil
consumption . Excess fuel can enter and mix with the oil via a leaking fuel
injector, fuel pump problem, restricted air intake or through excessive idling.
T's Advanced Synthetics
Greg and Marcy Thurman
Your Nationwide
AMSOIL
Authorized Independent
Dealer
Nationwide US Warehouses,
Canada and Palm Harbor, FL
727-798-8552
E-Mail:
greg@tsadvancedsynthetics.com
If you
experience problems or have questions or comments about
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