Oil Pressure Test (Automobile)


Oil Pressure Test

Another mechanical test, commonly used to diagnose engine problem is the oil pressure test. This test measures the pressure at which the engine oil circulates throughout inside the engine. Figure 19.17 shows the typical path of oil flow within an engine. The oil circulation in the engine is important not only to limit the amount of wear on the moving parts of the engine but also to perform many other important functions. The oil must lubricate the moving parts to limit the amount of power loss due to friction. Oil also serves to cool the engine, removing heat from the engine parts as it flows past them. It absorbs the shocks between bearings and other engine parts to minimize engine noise and extend the life of the parts. Engine oil also forms a seal between the piston rings and the cylinder walls by filling the irregular surfaces of the rings and the walls. A certain amount of oil clings to the cylinder walls to stop or limit the amount of
leakage past the piston rings. Oil also serves as a clean­ing agent, removing varnish and carbon from the engine parts. To perform all these functions, oil must be of high quality and must be supplied in adequate amounts to all the moving engine parts under the correct pressure.
The engine oil pump provides a continuous supply of oil to the appropriate engine parts and delivers the oil at sufficient pressure to lubricate the parts properly. It picks up the oil from the oil pan through a pick up tube, which has a filtering screen on the end of it. The oil is pushed out of the oil pump into a pressure regulator, which limits the maximum amount of oil pressure. To provide enough oil to lubricate, clean, cool, and seal the parts of an engine efficiently, the oil must be delivered under pressure. Most shop manuals give specifications for the minimum and maximum allowable oil pressure of an engine. The results of an oil pressure test should always be compared to those specifications.
An oil pressure test should be performed when there is reason to believe that the oil pressure is not correct or
An engine's lubrication system (lubrication of the valve train outlined with A).
Fig. 19.17. An engine’s lubrication system (lubrication of the valve train outlined with A).

when it is required to verify that an engine is worn. Loss of performance, excessive engine noise, and poor starting can all be the result of abnormal oil pressure. Also excessive oil pressure can cause the hydraulic lifters to pump up too much and not allow the valves to close completely. Insufficient oil pressure may not be reducing the effects of friction enough, thus causing the crankshaft to be difficult to turn. This can cause poor starting and poor performance. If an insufficient amount of pressurized oil is delivered to the moving parts, these parts wear quickly and, while they are wearing, cause the engine to be noisier.
An oil pressure tester uses a gauge to read the pressure, and a high pressure hose to connect to the appropriate spot on the engine. The scale of the gauge ranges from 0 to 100 psi (690 kPa). The hose is connected to the engine block in a place where oil pressure can be monitored, such as a main oil gallery or the oil filter base plate. The preferred location for a particular engine is given in the shop manual. The engine should be run until the desired test temperature is reached. This temperature is also given in the shop manual. Once the temperature is reached, the engine should be brought to the recommended speed and the gauge should be read. The reading on the gauge should be compared to the specifications.
If the oil pressure is too low, the cause can be any of the following: a worn oil pump; excessive wear on the camshaft or crankshaft, causing excessive bearing clearances; a plugged oil pump pick up screen; or a weak or broken pressure relief valve spring. All of these require services that go beyond the scope of tune-up, and tend to be quite expensive. If the engine’s problem is due to any of these conditions, one should not attempt to perform a tune-up in an effort to hide or disguise the problem. Inform the customer of what is needed to correct the problem.
If the oil pressure is too high, the probable cause is stuck oil pressure relief valve. Typically, all that is needed is to remove the valve and clean it. After cleaning the valve with a crocus cloth, the pressure relief spring tension should be checked and replaced if it is not within specifications. Crocus cloth is a type of sandpaper that polishes metal surfaces by removing varnish or dirt but does not remove any metal from the surfaces. Normal sandpaper should not be used in place of crocus cloth as removing some metal from the valve affects the operation of the valve.

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