Why is my oil pressure low

Often drivers will look at their oil pressure gauge and become concerned with readings that are below the halfway mark.

Keep in mind that the oil pressure gauge is getting its information from an electronic sending unit and there is room for error.

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oil pressure gauge

I have often connected my trusted mechanical gauge and found oil pressure readings that were quite different than what the gauge was saying.

If you are concerned with low oil pressure readings the first place to start is to find out exactly what the specifications are. Your auto service manual will provide oil pressure; specs for at idle, and also part throttle conditions.

In some cases Diy car mechanics may be surprised that very low oil pressure is OK at idle by the manufacturer standard.


Low oil pressure readings

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engine bearings

The engine’s overall oil pressure can be compared to the blood pressure of the human body. In an automotive engine with a good oil pump and clear pick-up screen, the bearing clearances will determine the engine oil pressure.

If the clearances are worn, and well above the specifications low oil pressure will be the result.

Oil clearances can be adjusted by replacing bearings. There must be a gap for clearance between the outside diameter of the crankshaft journals and the inside diameter of its bearing.

This clearance allows for the building of an oil cushion. During an engine rebuild if there is little or no wear on the journals the proper oil clearance can be restored with the installation of a standard size replacement bearings.

However, if the crankshaft is worn to the point where the installation of standard sized bearings will result in excessive oil clearance space, a bearing with a thicker wall must be used.

Although these bearings are thicker, they are known as undersized, because the journals of the crankshaft are smaller in diameter than standard. In other words, they are under the standard size.


How to check oil clearances

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Plastigage

The proper lubrication and cooling of the bearing depend on correct crankshaft oil clearances. One of the easiest ways to take this critical measurement is with a specialized measuring tool called plastigage.

Plastigage is a fine plastic thread used to measure the clearance between the bearing and the crankshaft. The procedure for checking bearing clearance with plastigage is explained well in the products instructions.

This operation is much easier when the engine is removed from the vehicle, but it is possible to take these measurements with the engine installed if the oil pan can be removed while in the vehicle.


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how to use plastigage

One side of the package has markings for inch measurements and the other side has metric measurements.

After you take your plastigage reading in the oil clearance you will then compare the plastic threads imprint to those on the package markings.

Note that the plastic gauge will come in different ranges of sizes and clearances. Again, this is where you must know the specification to get the right stuff.

When performing this operation on crankshaft clearances I have always used the green color, as it falls within most of the vehicle’s parameters.

But again, check with your auto repair manual to see the exact specifications for your automobile.


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