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Oil pump basics

Courtesy of Melling Engine Parts

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Oil pumps, pressure, and the relationship between the two!

Science of the Pump

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

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

This picture shows the most common pump designs. Although they look 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 pump 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 calibrated spring, letting oil bleed off 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:

  • Operating speed of the engine;
  • Viscosity of the oil.

Crankshaft-Driven and Chain-Driven Pumps

Most pushrod engine 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 galley, 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 be 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

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The first reaction to on 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:

  • An air leak (usually where the screen assembly 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.
  • Too much oil in the crankcase (leading to air in the oil)
  • Excessive clearances within the engine (rod and main hearings, seals and oil plugs).
  • Oil leaking past the mounting pad area (usually caused by tightening the mounting bolts wrongly).
  • 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 much foreign material to cause problems with relief valve operation. And its easy for bits of wire, etc., to get into the oiling system, 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.

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