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Lube System Basics

Those important valves inside the oil filter - and why sometimes filters bulge and even burst.

Courtesy of Fram

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Three Vital Oil System Valves

In any automotive engine lubrication system there must be a constant flow of oil under pressure to the engine. If that flow fails, the bearings and other engine components may be damaged.

Three separate valves in engine lubrication systems keep the engine supplied with the oil it needs. These valves are often mistaken for one another when questions regarding oil systems are raised. This section defines each valve and its specific function.

1. Oil Pump Pressure Regulating Valve

This valve is usually built into the oil pump and is necessary in both full flow and by-pass oil systems. Its function is to control the operating pressure of the lubrication system. The regulating valve is set by the manufacturer to maintain the correct pressure (usually between 40 and 60 psi).

The valve utilizes a ball (or plunger) and spring mechanism. When the operating pressure is below the preset pressure level, the spring holds the ball in the closed position so that oil flows to the bearings under pressure. When the desired amount of pressure is reached, the valve opens enough to maintain this pressure. Once the valve is open, the pressure remains fairly constant, with only small changes as the engine speed varies.

If the oil pressure regulating valve becomes stuck in the closed position or is slow to move to the open position after the engine has started, the pres­sure in the system will exceed the regulating valve setting. This may cause an over-pressurized oil filter. If a deformed oil filter is observed, the oil pressure-regulating valve must be serviced immediately. (Also see section on over-pressurized lube oil filters below.)

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This is the pressure regulating valve in the open position.

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This is the pressure regulating valve stuck in the closed position.

2. Relief (By-pass) Valve

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In a full oil flow system, all the oil passes through the filter to reach the engine. If the filter clogs, an alternative route to the engine must be provided for the oil, or the bearings and other internal parts may fail, due to oil starvation.

A relief - or by-pass - valve is used to allow unfiltered oil to lubricate the engine. Unfiltered oil is far better than no oil at all.

This relief valve is built into the engine block in some cars. Otherwise, the relief valve is a component of the oil filter itself. Under normal conditions, the valve remains closed. When there is sufficient contaminant in the oil filter to reach a preset level of restric­tion to oil flow (around 8 psi in most passenger cars), pressure on the relief valve causes it to open. This condition can occur when the oil filter has become clogged or when the weather is cold and the oil is thick and flows only slowly.

3. Anti-Drain Valve

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Some oil filter mountings may allow oil to drain out of the filter when the engine is stopped. When the engine is next started, oil must refill the filter before full oil pressure reaches the engine. The anti-drain valve, included in the filter when required, prevents oil from draining out of the filter. Some filter designs incorporate a combination anti-drain and relief valve with an integrated unit construction.

Over-Pressurized Filters

The oil pressure drops, the red "CHECK ENGINE" light flicks on or the oil gauge drops to zero. You stop your car and check under the bonnet: your oil filter looks like it has burst. Understandably, you think the filter has failed. You send it back to the manufacturer. They deny any responsibility. You're upset, of course.

But if the filter didn't fail, what did happen?

Lube oil system problems are usually invisible. After they occur, a burst or deformed oil filter is often the only "evidence" you have. Yet it's misleading. Just as a blown fuse is not the cause of an electrical failure, a burst oil filter isn't the cause of excessive system pressure. Rather, it is the result of a faulty regulating valve located in the engine.

How your lube system works

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This is a simplified diagram of the lube oil system showing the oil pump, regulating valve, and bearings. The pump supplies oil at volumes and pressures greater than what the system requires to lubricate the bearings and other moving parts. The regulating valve opens to allow excessive vol­ume and pressure to be diverted back to the oil pan without causing damage to the filter or engine. On most cars, the regulating valve main­tains an oil system pressure of 40 to 60 psi.

What causes over-pressurization?

Excessive engine pressure is the result of a faulty oil pressure regulating valve. There are two ways that the valve fails to operate correctly: either it sticks in the closed position, or it is slow to move to the open position after the engine has started. Unfortunately, a stuck valve can free itself after filter failure, leaving no evidence of any malfunction.

Click for larger image

This diagram shows the system operating with the regulating valve stuck in the closed position. Pressure builds up equally on all components. If the regulating valve remains stuck, the pressure will increase dramatically. Normal pressure plus 100 psi causes filter deformation. If the regulating valve still remains stuck, the gasket between the filter and the base can blow out, or the filter seam will open. The system will then lose all its oil and pose significant risk to the engine.

How to minimize the risk of an over-pressurized system

1. Change the oil and filter often, according to the engine manufacturer's instructions for frequency, viscosity, and type.

2. Warm the engine on cold mornings before starting to drive, to allow oil to flow properly.

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