Top 40 REASONS FOR MOTOR OIL CONSUMPTION
Description: 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 only two oil related problems contained are
explained by “dirty oil” and by overfilling the crankcase. The
balance of the problems is 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 gasket, 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
link 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 throws 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.
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 an extent that they will not be able to control
the oil at high speeds. The effect of main bearing wear will be high
4. Worn or Damaged Connecting Rod Bearings
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
5. Worn or Damaged 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 undersized
bearings of the correct size.
7. Tapered and Out-of-Round Cylinders
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. Distorted 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 recirculate 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
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 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
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 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
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
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
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
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
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, dirt 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
23. Incorrect Piston Rings for Type of Engine or Type of Service
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
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 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, with the same results as described
in number 27. Excessive ring end clearance leads to increased oil consumption
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
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 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
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. Preignition
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 the 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 Modification
Increasing performance through the use of performance/power enhancement
products to a stock or factory engine can increase the chance of excessive
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
the overdrive mode in conditions where it is not recommended will cause the
engine to consume oil for a variety of reasons. Such engines
include towing or stop-and-go driving in city traffic. See also reason
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
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the lubrication system, the oil will become thinner and more volatile. Both will result in higher oil consumption. Excess
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problem, and restricted air intake or through excessive idling.
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