Melling Aftermarket

Common Misconceptions on Oil Systems

Oil pumps produce pressure: Oil pumps produce flow and the resistance to that flow produces pressure. The pressure relief valve limits the maximum pressure but does not do anything until the pressure has reached that point.

The inlet screen will keep debris out of the pump: Some of the new ones will but the majority have a ½ ” ID valve in the center or they have eight 3/16” x ½ slots around the edge of the screen. The valve or slots are there to allow cold thick oil into the pump. If you use thick oil with new screens, you a good chance of starving the engine of oil and ruining it.

I have low oil pressure, so I’ll put in a high pressure pump: The low oil pressure is usually caused by a leak or excessive wear in the engine. If the original pump has a 50 PSI bypass and the engine will not get over 30 PSI, then putting in a pump with a 75 PSI bypass will not change a thing. You will still have 30 PSI. You have to fix the leak or rebuild the engine.

I have low oil pressure, so I’ll put in a high volume pump: The high volume pump does not increase the flow so you will have some increase in pressure. However, you still have the original problem of a leak or worn condition. The high volume pump just delays fixing the real problem. High volume pumps are for increased demands on the oil system such as higher RPM usage, racing, remote filters and or coolers, etc.

Pressure versus Flow

Let’s consider Pressure versus Flow inside an engine. The size of any oil pump is designed to supply the correct amount of oil to meet the engine requirements. It produces a specific amount of flow at a given RPM. The resistance to the oil flow is from the bearing and lifter bore clearances.

When an engine is new, the clearances are tight and the pressure is good. As the bearings and lifter bores wear, clearances increase, resistance to flow decreases and oil pressure starts to drop.

When the pressure drops, we get our first signal that something is wrong in the engine. With the increased clearances/decreased resistance, the flow from the oil pump is at its maximum.

A few comments about the relief valve in an oil pump. The spring pressure behind the valve determines when it will open. If it is designed to open at 60-PSI, it does not have an effect on anything below 60-PSI, unless it sticks open. If it does stick open, the pressure is low at idle, but builds up to 60-PSI at the point the valve.

If an engine has excessive clearances in anything but rod bearings, the oil pressure will be uniformly low throughout the RPM range. If the rod bearings have excessive clearances, the oil pressure will be low at idle and will get worse as the RPM increases. Rod bearings turn in a circle rather than on axis, so they are subject to centrifugal force trying to pull the oil out of the bearing. The amount of loss will vary depending on the actual clearances in every bearing and lifter bore.

This gives you an idea of how much flow increases with a small increase in clearances and how pressure can drop when we exceed the flow available from the pump.

Professional Advice for the Automotive Oil Pump

I. Science of the pump

The oil pump is the heart of the engine, supplying lubricating oils to various moving parts of the engine. Most pumps consist of four parts

  1. A main body
  2. A cover assembly;
  3. A gear-type assembly ( one driven, one idling); or rotor- type assembly ( one inner , one outer)
  4. An oil pressure relief valve ( usually in the pump, but sometimes in the engine block)

Although every oil pump looks different, their function is the same: to supply lubricating oil to various moving parts in the engine. To do this, an oil pump regulates oil flow (volume) and oil pressure.

A lot of people think that the pump produces the engine’s oil pressure by itself. Actually, the pump creates only the flow of oil. The restrictions in the oil passages of the engine produce the pressure. The faster the pumps turns, the greater the flow of oil to the engine. The more oil flowing through a restricted area of the engine, the greater the pressure. Oil pressure is directly related to the clearances in various parts of the engine; as the clearances increase, so does the capacity for more oil flow. But when the engine’s need for more oil increases beyond the pump’s ability, pressure drops/

Because most pumps can produce more than 150 PSI, a pressure- regulating valve is installed in the oil pump or the engine block. The valve is loaded with a closely calibrating spring, letting oil bleed at a given pressure (oil pressure pushing against the relief valve unseats it and allows excess oil to bypass it).

The valve assembly is designed with very tight clearances, and must work freely in order to keep the engine’s oil pressure constant. Foreign material entering this area can easily jam or hinder the operation of the pressure relief valve. Also affecting engine oil pressure:

  1. Operating speed of the engine
  2. Viscosity of the oil.

Crankshaft-Driven and Chain- Driven Pumps

Most automotive oil pumps are driven by a gear on the camshaft and turned at half engine speed. However, many new engines are using crankshaft-driven pumps. These pumps are gerotor style and the inner rotor fits over the crankshaft. With this setup, the inner rotor is loose in the pump before installation. You’ll also notice that the rotors are thinner; they don’t have to be as thick because the pump is turning at engine speed.

Another pump type that’s unique is found on the 2.6L four-cylinder Mitsubishi engine. The pump and the counterbalance shafts are driven by a chain at twice engine speed. In most oil pumps, the shafts get their oil directly from the pump. In this case, the oil for the shaft goes from the pump to the engine’s main oil gallery, then back to the pump to oil the shaft. If there’s an oil leak or loose bearing, the shaft is starved for oil and the pump and engine self-destruct quickly.

Crankshaft and chain-driven pumps require great care in installation and priming. The gasket surfaces must been clean, and the gaskets and the pump need to be precisely aligned. Be sure to prime these pumps according to their manufacturer’s instructions. We strongly recommend that you prime the whole engine with an external priming device before start-up.

Oil pressure problems

The first reaction to an oil pressure problem is often to replace the oil pump- sometimes unnecessarily. Since existing problems within the engine can’t always be fixed by installing a new oil pump, you should carefully evaluate the situation before doing so. An engine could have oil pressure problems for many reasons:

  1. An air leak (usually where the screen is mounted to the pump). Check for loose connections, cracks or holes. Since air is compressible, any air inside the oiling system can cause a fluctuation of pressure.
  2. Too much oil in the crankcase (leading to air in the oil).
  3. Excessive clearances within the engine (rod and main bearings, cam bearings, seals and oil plugs).
  4. Oil leaking past the mounting pad area (usually caused by tightening the mounting bolts wrong).
  5. Malfunction of the relief valve assembly. As we mentioned before, the main function of the relief valve is to regulate oil pressure within the engine by keeping a constant flow of oil to the engine. There are three different relief valve problems ( usually caused by foreign material that is drawn into the pump from the engine) that can cause oil pressure problems:
    • The relief valve is stuck in the closed position. It won’t let extra oil pressure release itself inside the pump, so the pressure builds and ruptures the oil filter.
    • The relief valve is stuck in the open position. The pump will release too much of the pressure, creating low oil pressure and possible bearing failure.
    • The relief valve is sticking, so movement of the valve within the assembly bore of the oil pump may be restricted. In this case, the valve isn’t necessarily stuck in a fully open or closed position, and the result is erratic pressure.

Since oil pumps are manufactured with such close tolerances, it doesn’t take too foreign material to cause problems with relief valve operation. And it’s easy for dirt, bits of wire, etc. To get into the oiling, even with the screen assembly in place. So, be sure to find the root of your oil pressure problem before you decide to replace the pump.

II. Problem Solving and Prevention.

Since knowing common problems is the first step in prevention, we’ve included tips that should help you find problems and keep your oil pump running smoothly.

Oil Pressure

If you run into oil pressure, remember the following:

  1. Check the crankcase
    • Is the oil at the engine manufacturer’s recommended level?
    • Is the engine equipped with the proper dipstick?
    • Is the right oil being used?
  2. Make sure the oil pressure sending unit and gauges are working right. You can check them easily with a manual pressure gauge on the engine block
  3. If the pan is removed, examine the pump and mounting closely.
    • Is the right oil pump being used in the engine
    • Is the oil pump driveshaft or the intermediate shaft from the pump to the distributor broken? (This is usually caused by foreign debris inside the pump).
    • Is the oil pump housing cracked?
    • Are the mounting bolts tight?
    • Is the oil pump properly mounted?
    • Are the mounting surfaces leaking oil?
  4. With the oil pan off, the oiling system should be pressurized and all bearings checked for abnormal leakage. Also, check the oil pan for meal debris.

Foreign Materials

As we said before, it is easy for debris to find its way into the oiling system of your engine. It’s then washed into the pan, where the screen picks it up and draws it into the oil pump.

There are several sources of foreign material. Knowing what there are can help you avoid them.

  1. Deteriorated valve stem seals. These seals can become dry and brittle, making pieces break off and fall into the oiling system. If you check these seals and replace the, periodically when the engine is down, you can prevent this.
  2. Deteriorating nylon timing gears. These gears should be completely checked and, if necessary replaced, when then engine is taken apart.
  3. Bits of wire. These often come from a mechanic’s wire brush or from lifter retainer clips. The brushes break down with use and bristles break off and stay in the engine. Lifter clip wire usually becomes dislodged from the lifters at one of two times:
    • When the engine is running. Extreme valve train clearances let the plunger inside the lifer contact the retaining clip. If this happens, the clip can easily snap out of the lifter or a small piece could break off, as could misaligned push rods.
    • When the lifters are being removed during repairs. The most common way to remove lifters is to hook onto the retaining clip and pull out the lifters. During the removal process, the spring – type retaining clips can snap out of the lifter and fly into many holes and crevices in the block and/or cylinder head. Unless the loose wire clip is removed from the crevices of the block, heat and vibration may eventually loosen it, and the oil will wash it into the pan.
  4. Gasket cement. Some oil pumps are designed to be used with a mounting gasket, and some are designed to be mated against the engine block without a gasket. No gasket cement should be used on the mounting pad of an oil pump. Even ‘spray on’ gasket material can get inside the pump, where it will harden and cause problems.
  5. Sludge, pieces or carbon, grit, abrasive material, metal particles and dirt. Fuel and water in the oil can easily turn into sludge, varnish and carbon deposits. So oil must be filtered before it is circulated throughout the engine. The oil pump has a disadvantage, because it picks up the dirty oil from the oil pan and then pumps it through the filter. Operating with unfiltered oil can make any precision built part in the engine.

Abrasive material in the oil isn’t always caused by neglect. Many abrasives come from normal wear on various engine components like bearings, the camshaft, aluminum heads, and valve spring retainers. For this reason, most companies recommend that the oil and filter be changed every 2,000 to 3,000 miles. Even synthetic oil, designed to run 5,000 to 10,000 miles before a change, should be changed sooner, because the oil cannot magically dissolve foreign particles and keep itself clean.

Because foreign material can fall into an oil pump that’s left lying on a bench or car, a new oil pump should be left in is wrapper until it’s needed.

Oil Pump Screens

The oil pump screen is needed to keep the pump and engine running well. Many people think that oil pump screens filter out all foreign material, but this isn’t true. The main function of the screen is to pick up oil from the bottom of the oil pan without drawing any air into the oiling system. There is a mesh inside the screen assembly that helps filter out larger pieces of foreign material, but there is also a bypass hole in most screens (about ½ ” in diameter) that can open up and let even larger pieces get into the pump. (The bypass hole is there to make sure the engine gets enough oil when the screen is plugged with debris or when the oil is too thick to flow through the screen).

If we could take a screen apart, it would be easy to clean. Unfortunately, we can’t. So it’s impossible to clean completely. Any debris left inside could lock up the pump.

You can see how this is so by looking at a used screen assembly after you try to clean it. You might see a dark brown stain, which is usually a varnish type coating. The most common screen mesh has a 0.40 inch square hole between the wires. Oil flow is directly related to the size of the hole. If the varnish coating is .005 inch thick, then the square hole is down .030 inch on a side, obviously slowing or clogging the flow of oil.

There are two kinds of screens hat let debris enter an oil pump. One has a valve in the center that opens if the oil is too thick or if the screen is blocked. The second type has eight gaps to let oil flow if the mesh won’t let in.

You can see why oil pump screens must always be replaced. This is the best way to reduce oil pump and engine failure caused by trapped foreign material.

Priming the Pump

Priming the pump properly before it is installed is very important. Although oil pumps are made with close tolerances, often they aren’t self-priming. Unprimed, an oil pump could run for several seconds before picking up oil. Or worse, it might not pick up oil at all. This could lead early wear of the oil pump and the engine bearings.

Follow these suggestions when priming an oil pump:

  1. Always dip the pump in a container of clean oil, turning the rotors or gears by hand until enough oil is released from the pump.
  2. Use an electric drill in the engine to turn the pump in the direction of the distributor rotation, until the oil reaches the rocker arms.
  3. If he pump is part of the front cover housing, it helps to coat the internal parts of the pump with a small amount of thick oil.
  4. NEVER pack the oil pump full of grease or any other type of priming substance.

In addition to priming the pump, pressurising the entire engine with oil before initial startup will help lubricate the bearings and prevent a dry startup.

III. When to Replace an Oil Pump.

Generally, you can count on replacing the oil pump:

  1. At each major overhaul of the engine. Most manufactures recommend that both the pump and screen be replaced at this time.
  2. When other engine problems point to a low oil supply. Thoroughly inspect the pump and screen. Warning signals include:
    • Oil pressure that runs too low, too high or fluctuates. Be sure to follow the advice discussed earlier when deciding whether the oil pump is causing the oil pressure problems.
    • A bearing failure.
    • Failure of any internal engine parts.

IV. Installation Tips

Following are a few things to keep in mind when you replace an oil pump.

Caution is the Watchword

You should take special care when you install the oil pump. Read all instructions carefully. It will help in the long run.

Improper installation obviously leads to problems. For example, some oil pumps have a mounting neck that must be inserted into a bushing hole of the engine block before the bolts are installed. Allowing the mounting neck to slide out of the engine block even a little while you tighten the mounting bolts can break the mounting neck, break the driveshaft and/or crack the mounting pad.

Plus, be sure to check that the intermediate shaft from the distributor to the oil pump is installed correctly before you try to start the engine.

In oil pumps driven with an external spiral gear, extra wear on the cam/distributor gear will immediately transfer to the spiral gear on the pump. Replacing the worn gear before you install the pump will help solve this problem.

In oil pumps that are part of the front timing case cover, the gears and relief valve parts can be changed. Check for excessive war in the gear pocket area and see the manufacturer’s service manual for acceptable wear limits before rebuilding. Since this type of pump can’t be primed easily, gears should be coated with think oil (possibly 50W or 90W oil) for a quick and positive prime. Don’t pack the gear pocket with grease or anything else.


Unless you take the right precautions with some pumps, you could damage the driveshaft. For example, a nylon retainer is supplied with the standard small-block Chevrolet oil pump. Although this retainer is strong enough to be used with a standard pump, it may not always be strong enough to support the driveshaft of the high-volume pump. An intermediate shaft with a pinned steel retainer is recommended for use with the high volume pump. It gives extra support at the connection between the intermediate shaft and the driveshaft of the oil pump.

Driveshaft’s can also break if mounting bolts of the pump are tightened wrong. For instance, small and big block Chevrolet oil pumps should be torqued down to approximately 60-70 ft. lbs. These oil pumps don’t use a mounting gasket, so improper torqueing of the mounting bolt can either make the mounting pad leak oil ( creating an oil pressure problem ) or cause sloppiness in the driveshaft ( which can eventually cause the shaft to break).

Installing Screens

There are three types of screen assemblies that are used with oil pumps; screw-in, bolt-in or press-in. The press-in assemblies cause many problems. Many mechanics will clamp the screen in a vise and hammer on the housing of the oil pump or they’ll clamp the oil pump in a vise and hammer on the screen. Both of these methods can easily damage the housing of the oil pumps by distorting the dimensions of the gear pocket or by distorting the dimensions of the relief valve area, which in turn, causes the gears to bind or the valve to stick.

V. Keep the Heart Pumping.

The oil pump rarely fails by itself. Most failures can be traced to a cause. So, before installing a new pump, pinpoint and eliminate the true cause of each oil pump failure. And, when you do install the pump, be sure to be careful. Like the human heart, if you don’t take care of the pump, you increase the chance that problems will occur. But if you treat the oil pump well, it will give you years of service.

Recommended Engine Priming and Break-In Procedures for Camshafts

In order to give your new camshaft and lifters the best possible chance to make it through the first critical moments of operation upon initial start-up and provide a long, trouble-free service life, the following is recommended.

Pre lube by one of the following methods:

Best, Pressure tank system: The pressure tank system is by far the best method of making sure all the air is purged from the oil galleys. Pre lube until oil is seen at the rocker arms.

Next best, spinning the oil pump: Using a priming tool, turn the pump with a drill until oil is seen at the rocker arms.

Least desirable, free spinning the engine: Remove the spark plugs and turn the engine with the starter until oil pressure is shown on the gauge.

Before starting: Set the timing and be sure fuel is in the carburetor or injection system before cranking. This will assure the engine starts with the least amount of cranking.

Start the engine: After the engine starts, check for oil pressure and bring RPM to 1,500 -2,000. If the pressure is good, run the engine at this RPM range for 20 minutes. After his run-in period, final adjustments can be made and the engine can be put in service.

Why Replace Intermediate Shafts?

By the time an engine needs a new oil pump it has considerable wear. The cylinder wall, rings, cam and lifters etc. all wear. The material worn off goes to the oil pan where some is washed out at oil change time, but a portion goes through the oil pump to the filter. Pushing grit through the pump puts an additional load on the oil pump intermediate shaft. In time the deformation of the intermediate shaft can become severe enough to cause slippage and engine failure. Replacement is inexpensive insurance.

Why Replace Oil Pump Screens?

The oil pump pickup screen smooth’s the flow of oil into the pump; it usually keeps out debris that can lock up the pump. The pickup screen is the only part in an engine that assists the pump in its function. All other engine parts depend on the oil pump to assist them.

If we could take a screen apart, it would be easy to clean. Unfortunately, we can’t. Therefore, it is impossible to clean it completely. Any debris left inside has the potential of locking up the pump.

Close examination of a used screen assembly after attempts to clean it may reveal a dark brown stain, which is usually a varnish type coating. The most common screen mesh has a .040” square hole between the wires. Oil flow is directly proportional to the area of the hole. If the varnish coating is .0051 thick, our square hole is down to .0301 on a side. This is a 25% reduction on a side, and a 44% reduction in the total area and flow (.040”) 2 – (.030”) 2 / (.040”) 2 = 43.75%

The second statement references, ” usually keeps out debris”. There are two styles of screens that allow debris to enter an oil pump. The first has a valve in the center that opens if the oil is too thick or if the screen is restricted. The second type has eight gaps approximately 3/161 x ½ ” which allows oil to flow if the mesh will not.

Oil pump screens should always be replaced. There is not a more economical way to reduce oil pump and engine failure from ingested foreign material.