Underhood Service Advertise | Subscribe | Contact Us | About Us

Oil Pumps & the Engine's Lubrication System

February 1, 2005

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 rpm.

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 wheels.

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 clearances.

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 assembly.

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 engine.

Pump Modifications
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 rpm.

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 starvation.

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 Manufacturers (AIAM).

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 esunkin@babcox.com. Please include your name, shop, location and e-mail address.

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 800-331-0329.


More Articles in Engine

Performance Gaskets And Surface Finishes
The most critical engine gaskets are the head gaskets since they have to withstand the heat and pres...
Head Gasket Service On Subaru Outbacks
Head gasket failure has been something Subaru has struggled with to some extent since the 1980s. The...
© Copyright 2014 Babcox Media, Inc. All Rights Reserved
3550 Embassy Parkway, Akron, OH 44333
(FAX) 330-670-0874