The heart of an engine's lubrication system is the oil pump. It sucks oil in from the crankcase and pushes it through the filter and oil galleries to the bearings, camshaft and upper valvetrain. A steady supply of lubricant is absolutely essential to prevent metal-to-metal contact, wear and friction between moving parts, to help cool the bearings and pistons, and to maintain proper valvelash in engines that have hydraulic lifters or cam followers. If for any reason the pump cannot keep the oil circulating, it's bad news for the engine. The first symptom of low oil pressure is typically valve noise and/or a warning light or low oil pressure gauge reading usually at idle. From here, things can quickly take a turn for the worse. Loss of oil pressure may starve the bearings of their much-needed lubrication. With no
oil to keep the surfaces apart, bearings can wipe and fail. A seized rod
bearing will often break the connecting rod and put it through the side of the engine block. A seized main bearing will often spin and wipe out the main bore.
An oil pump failure is like cardiac arrest because the results are often fatal.
Nothing Lasts Forever
With regular oil and filter changes, an original equipment oil pump should last the life of the engine. Today, that means upwards of 150,000 miles or more. But some oil pumps may not go the distance because the pump is the only internal engine component that runs on unfiltered oil all the time. If the vehicle owner neglects regular oil changes and/or drives his vehicle in a dusty or dirty environment, the pump will suffer the consequences.
Think about it. The filter protects the bearings and other internal engine parts by trapping wear particles and debris that end up in the crankcase.
But the filter provides no protection whatsoever for the pump because the filter is located downstream of the pump. The oil pump just sucks up
whatever junk is in the crankcase and pushes it along to the filter. The
only protection for the pump is the inlet screen over the pickup tube. The screen can prevent big chunks of debris from entering the pump but allows smaller particles to pass right through. Some pickups even have slits that allow cold oil to bypass the screen when the engine is first started.
Consequently, if there is any junk in the crankcase it will be sucked right into the pump.
Pump failure can occur if anything large enough to jam the gears or rotors
enters the pump. This includes metallic debris from bearings or castings,
gasket or seal debris, shot peening remnants, glass beads from bead
blasting, or anything else that doesn't belong in the crankcase. With
twin-gear pumps, a foreign object that enters the pump can lodge between the
close-fitting gears, or the gears and housing, causing the pump to lock up.
Once the gears stop turning, something has to give. Usually the pump shaft
twists or shears off. Sometimes a pump seizure tears up the teeth on the
camshaft or distributor drive gears depending on how the pump is driven.
With front-mounted rotor-style pumps, debris usually won't lock up the pump
because it is driven directly off the crankshaft, but it can damage or
destroy the rotors.
Even if a pump doesn't fail, it loses efficiency as it wears. Over time, the
effects of pumping unfiltered oil produces scratches and wear in the gears
and pump housing that increase clearances and reduce pumping efficiency. The
result is a gradual loss of oil flow and oil pressure as the miles add up.
An oil pump, by the way, does not create oil pressure. It pushes oil from
one place to another. It is a positive displacement pump that moves oil as
it turns. Oil is incompressible so once it leaves the pump it continues to
flow until it encounters resistance in the filter, oil galleries and
bearings. It's the resistance to flow that builds pressure in the oil
system. Trying to force oil through a small opening creates more resistance
and pressure than allowing it to pass freely through a large opening.
A worn pump can't deliver the same volume of oil as a new pump, so with less
flow there's a drop in oil pressure.
As pressure builds in the oil system, it exerts pressure. A spring-loaded
"pressure relief valve" built into the oil pump (or near the pump) opens
when pressure exceeds a certain limit (typically 50 to 60 psi) and either
reroutes oil back into the pump's inlet or the oil pan. This prevents a
dangerous buildup of pressure that could rupture the oil filter or blow out
press-fit oil plugs.
At idle, most oil pumps do not produce enough flow to force open the relief
valve. Oil pumps that are camshaft driven turn only at half engine speed so
output isn't great at idle and low rpm. Even pumps that are crankshaft
driven and turn at engine speed (or double engine speed in a few instances)
don't pump enough oil to overcome the relief valve spring. The relief valve
generally only comes into play at higher rpm when the pump's output pushes
more oil into the system than it can handle. Then the relief valve opens to
vent oil and limit maximum oil pressure until the engine returns to idle or
a lower rpm.
How Much Pressure?
Vehicle manufacturers have traditionally recommend a minimum of 10 psi of
oil pressure for every 1,000 rpm of engine speed. Using these numbers, most
stock engines don't need any more than 50 to 60 psi of oil pressure. With
tighter bearing clearances, pressure goes up requiring less flow from the
pump and less parasitic horsepower loss to drive the oil pump.
In racing applications, the old school of thought was more oil pressure was
needed to keep the engine lubed. That's true if bearing clearances are
loosened up. But most engine builders today tighten clearances so less oil
flow is needed to maintain adequate oil pressure. This approach increases
the horsepower output because less power is needed to drive the pump at high
A stock oil pump is usually more than adequate for most street performance
engines, and even many racing engines. NASCAR engines typically get by just
fine with no more than 50 psi of oil pressure at 9,000 rpm! Some top fuel
dragster and funny car engines are set up so the oil pump will dump
excessive oil pressure at high rpm so more power will be routed to the rear
Some racing engines use a "dry sump" oiling system. With this type of
lubrication system, an external oil pump is used to suck all the oil out of
the crankcase to reduce "windage" (oil drag) on the crankshaft. The oil is
routed to an external tank and an oil cooler before it is recirculated back
into the engine.
High-Volume & High-Pressure Pumps
In applications where more oil flow is desired either to increase oil flow
or pressure for better bearing lubrication and cooling, an oil pump with
longer or larger gears may be installed. The physically larger surface area
of the gears pushes more oil through the pump at the same rpm than a stock
pump. A high-volume oil pump typically flows 20 to 25% more oil than a stock
pump. The increase in oil flow produces an increase in oil pressure at idle,
which helps compensate for increased bearing clearances. Consequently, some
people may install a high-volume pump in a high-mileage engine in an attempt
to restore normal oil pressure. But oil isn't metal, and the only real cure
for low oil pressure is to replace worn bearings and restore normal
High-pressure oil pumps are another option. A high-pressure pump contains a
stiffer relief valve spring that does not open until a higher pressure is
reached (75 psi or higher). The actual flow rate of a high-pressure pump may
be no different than a stock pump, or it may be higher if longer gears are
used. Either way, the pump will increase the system oil pressure reading at
high rpm when the pump is working hard, but it won't have any affect on idle
pressure when the pump is turning slowly.
A high-volume or high-pressure oil pump may be recommended in engines where
bearing clearances are looser than normal, in engines where an auxiliary
external oil cooler has been added to improve oil cooling and in racing
engines where a oil accumulator has been installed.
Rebuild or Replace?
If you're overhauling a high-mileage engine or replacing a set of worn
crankshaft bearings, it's always a good idea to replace the oil pump, too.
Replacing only the gears in a twin-gear pump can restore gear-to-gear
clearances but not gear-to-housing clearances. The end plate that covers the
pump often develops a heavy wear pattern that is most noticeable on the
outlet gear side. Regrinding the face of the plate smooth can restore end
play tolerances between the plate and gears but it can't compensate for wear
inside the housing. Deep scratches or grooves worn into the sides of the
housing will leak oil and reduce the pump's ability to move oil.
In the case of front cover oil pumps on overhead cam engines, the pump turns
at engine rpm and generates more flow at idle than crankcase-mounted pumps.
Consequently, when the pump becomes worn it isn't always necessary to
replace the entire cover assembly ‹ provided the pump housing inside the
cover isn't worn or damaged. A new drive gear can be mounted on the
crankshaft and a new rotor installed in the cover to restore normal oil
pressure. This approach eliminates the need to replace the entire cover
In cases where an engine has experienced a bearing failure or any other kind
of internal failure that puts debris into the crankcase, the oil pump should
always be replaced.
You should also replace the pump's pickup tube and screen. Pickups are
difficult to clean and can hide debris that may damage a new pump or the
If you want to rework a stock oil pump to maximize flow, use a die grinder
to smooth and blend the sharp edges of the pump inlet and outlet ports. This
will reduce turbulence and enhance flow through the pump. The clearance
between the end of the gears and the pump housing cover also should be
minimized to reduce pumping losses around the ends of the gears.
If you're working on a small block Chevy V8, another trick that can boost
oil flow is to install a big block Chevy oil pump instead of a small block
oil pump. A stock big block Chevy oil pump has 12 teeth per gear versus
seven for the small block version, and flows about 10% more oil at the same
Something else to watch out for when installing a high-volume oil pump in a
small block Chevy V8 is the nylon retainer on the pump shaft. A better
choice is a pinned steel retainer to provide extra support between the
intermediate shaft and pump shaft.
Care must also be used when tightening down the pump mounting bolts on small
block and big block Chevy V8s because the pumps do not use a mounting
gasket. The bolts should be torqued to 60 to 70 ft.-lbs. so there are no
leaks or sloppiness that would eventually cause the shaft to break.
Preventing Oiling Problems
The greatest oil pump in the world won't keep an engine properly lubed if it
is dry when the engine is first started, or if it sucks air because the oil
level in the crankcase is low or the pickup screen is mounted too far above
the floor of the oil pan.
The pickup tube should be installed so it is located no less than 3/8" above
the floor of the oil pan (to allow good intake flow), and no more than 1/2"
above the floor so it doesn't run out of oil in a sharp turn.
The pump should also be filled with oil when it is mounted on the block to
prime it and reduce the risk of a dry start. Do not use grease or assembly
lube here. In the case of front-mounted oil pumps inside the timing cover,
the pump rotors can be coated with heavy oil such as 50W or even gear oil to
keep the pump primed.
Before starting the engine, prime the oil system with a pressurized oiler.
Oil tends to drain off bearing surfaces when an engine sits for more than a
week or so without running.
On older engines with distributor-driven oil pumps, the engine can be primed
by using a drill to spin the oil pump shaft through the distributor hole.
But on engines with no distributor or those with oil pumps inside the front
cover, this isn't possible. Feeding pressurized oil into the main oil
gallery through the oil pressure sending unit fitting will route oil to all
the critical areas inside the engine and eliminate the risk of scuffing the
bearings when it is first started.
Another option to consider is installing "coated" main and rod bearings if
you're replacing crankshaft bearings. Several major bearing suppliers have
recently introduced performance engine bearings coated with a low-friction,
moly-based anti-scuff surface treatment. The special coating increases the
price of the bearings but provides added protection in the event of oil
Causes of Low Oil Pressure
Possible causes include:
Excessive main and rod bearing clearances (someone may have installed the
wrong-sized bearings or standard-sized bearings on a reground crankshaft). Only 0.001" of extra clearance in the main bearings can reduce oil pressure
by up to 20%!
Excessive camshaft bearing clearances (a bearing may have slipped out of
place when the cam was installed).
Excessive clearances inside the oil pump.
Leaks between the oil pump and engine.
Oil pump relief valve stuck open or installed backward.
A loose or mispositioned oil pickup tube (too high in the oil pan).
A defective oil pressure sending unit or oil pressure gauge.
Low oil level in the oil pan.
Cracks or leaks in the oil galleries or gallery plugs.
Cracked oil pump housing (improper installation).
Oil viscosity too thin for hot weather.
Oil viscosity too thick for cold weather.
|Presentation Available on Engine Lubricant Quality
As vehicle maintenance professionals, you all are well aware of the
importance of basic auto maintenance, especially the need to change lube oil
and filters regularly. Over the past few years, we have seen not only major
changes in automotive technology, but some say there also have been changes
in the quality of engine lubricants. The net effect of these changes is that
some engine oils that remain available in the marketplace are obsolete for
modern engines, according to the Association of International Automobile
To receive a PDF version of a Powerpoint presentation developed by the AIAM
that describes some of the excessive engine wear and sludge buildup problems
that can occur when improper motor oils are used in modern engines, email us
at firstname.lastname@example.org. Please include your name, shop, location and e-mail
|Oil Change Association Holds Convention
The Automotive Oil Change Association (AOCA) will be holding its "AOCA & All
That Jazz Annual Convention & Fast Lube Expo" April 23-26 at the Ernest N.
Morial Convention Center in New Orleans.
The sixth-annual AOCA Boot Camp will be held on Saturday, April 23rd. This
full-day program is designed to help new lube owners improve their
operations and their bottom line.
Seminars include Getting and Keeping Customers; Add-On Services: Tire
Rotation, Power Steering and A/C Service; Dude, Where's My Car?; Warranty
Claims Self Defense and Customer Satisfaction; Working With the Media; and
To Go of Grow.
On the exhibit floor, AOCA will host a new "demo theater" during which
featured exhibitors will share their insights and techniques on cabin air
filter service, water and deposit removal, training, car wash management,
on-board diagnostics and the effects of thermal expansion on lube equipment.
For more information on AOCA and the convention, visit www.aoca.org or call