What Causes Low Engine Oil Pressure?

Updated on August 10, 2018
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

There are several causes for low oil pressure.
There are several causes for low oil pressure. | Source

Here are the four most common reasons for low oil pressure in a car:

  1. Low oil level
  2. Worn-out oil
  3. Overheating
  4. Worn-out bearings

Oil may be the lifeblood of your engine. But without good oil pressure, your engine is in serious trouble.

Although low oil level is the most common reason for low oil pressure, there are other other preventable conditions that cause it, which car car owners may neglect. For example, with the new extended periods oil changes in modern engines, car owners are not only forgetting to check oil level between changes, but even to change the oil when it loses its efficiency.

And if you drive mainly in stop-and-go traffic, the oil usually needs changing before the recommended change interval. Most manufacturers' suggested oil change intervals apply to ideal driving conditions: driving at highway speeds for at least 20 minutes every day. But most vehicles are driven within the city, on short trips with frequent stops. Short trips don't allow engine oil to reach its operating temperature and reduce its service life.

Of course there are other things that can cause not only low oil pressure but high pressure as well.

What's in This Guide

This guide will explain:

  • what the oil pressure should be;
  • what low pressure means for your engine;
  • how normal oil pressure can be upset;
  • and some symptoms caused by low oil pressure.

Towards the end, you'll find some troubleshooting procedures that can help you determine whether your lubrication system is working outside its proper oil pressure parameters.

But first, let's take a look at normal oil pressure.

Index
I. What Should My Oil Pressure Be?
II. So What Does Low Oil Pressure Mean?
III. Low Oil Pressure Causes
IV. Low Oil Pressure Symptoms
V. My Low Oil Pressure Warning Light Comes On and off
VI. I Have Low Oil Pressure at Idle
VII. My System Has High Oil Pressure
VIII. Will Low Oil Pressure Cause a Vehicle Not to Start?
IX. How to Check Oil Pressure
X. Further Diagnostic Tests
Low oil level will drop oil system pressure.
Low oil level will drop oil system pressure. | Source

I. What Should My Oil Pressure Be?

Normal oil pressure may vary slightly from one vehicle model to another.

When there's proper pressure, you'll see the needle on the oil pressure gauge at about the middle of the scale, and you won't see the low pressure warning indicator light come on.

When your engine is at operating temperature, a typical system pressure may fall within a range of 20 to 30 psi at idle (140 to 200 kPa), and 45 to 70 psi (310 to 482 kPa) at driving speed.

For the exact specifications, check the vehicle repair manual for your particular make and model. If you don't have the manual, you can get an inexpensive, aftermarket copy from Amazon. Haynes manuals (get the one for your exact model and year) come with step-by-step procedures for many maintenance, repair and troubleshooting tasks. So the manual pays for itself after your first maintenance project.

System pressure allows oil to travel throughout the lubrication system.
System pressure allows oil to travel throughout the lubrication system. | Source

II. So What Does Low Oil Pressure Mean?

To properly lubricate, cool ,and clean critical components like crankshaft and camshaft bearings, pistons, and other moving parts, there should be adequate oil flow and pressure in the lubrication system. Together, flow and pressure allow lubrication of components under the valve cover at the top of the engine as well.

To do this, the oil pump has to push oil through narrow clearances under bearings and galleries. It is these narrow passages that build pressure in the system because they restrict the rate of oil flow. The faster the engine moves, the higher the rate of oil flow and the higher the pressure.

Too much pressure can be damaging to engine components, though. A relief valve close to the oil pump opens when pressure reaches a predetermined value, usually between 45 and 75 psi, allowing extra oil to drain back into the oil pan.

However, when the lubrication system can't maintain proper oil flow, or pressure drops when it should not, engine components are going to be starved of much-needed oil.

Using oil with the wrong viscosity for the application will upset system pressure.
Using oil with the wrong viscosity for the application will upset system pressure. | Source

III. Causes of Low Oil Pressure

One or more factors can contribute to a drop in oil flow or system pressure, for example:

  • Not enough oil in the system. This is perhaps the most common cause of low oil pressure. With wider intervals between oil changes in modern vehicles, car owners forget to check oil level between changes.

  • Aged oil that has lost its viscosity. This happens when car owners forget to change engine oil at the manufacturer recommended intervals or as necessary. This is another frequent cause of low oil pressure.

  • Using oil with a much lower viscosity than that recommended by the car manufacturer.

  • Engine overheating will cause oil to thinner, preventing proper pressure to buildup. This is similar to using oil with a much lower viscosity than recommended.

  • Too much clearance between the engine bearings and journals due to wear. This is common in high-mileage engines.

  • A worn-out oil pump with wider clearances between the rotors can cause a drop in the rate of flow. Although this is not as common.

  • Worn-out piston rings, cylinder walls, valve stem seals, or guides will allow oil to enter the combustion chamber and get burned, causing a drop in engine oil level and pressure. Usually, you'll notice some blue smoke coming through the tail pipe when this happens. Fuel can also dilute engine oil this way, causing a drop in pressure.

Valve clattering sounds can be a sign of low engine pressure.
Valve clattering sounds can be a sign of low engine pressure. | Source

IV. Symptoms of Low Oil Pressure

A few indicators can point to problems with oil system pressure:

  • Knocking or clattering sounds coming from the engine during operation. This is perhaps the most common symptom.
  • Oil pressure gauge reading lower than usual at idle.
  • Poor starting.
  • Loss of engine performance.

Modern vehicles come equipped with a low oil pressure warning light.
Modern vehicles come equipped with a low oil pressure warning light. | Source

V. My Low Oil Pressure Warning Light Comes On and Off

Most modern vehicle models come with an oil pressure switch that activates a warning light on the dashboard when a dangerous low oil pressure is detected.

For the light to come on, oil pressure has to drop below 5 or 10 psi at idle, depending on vehicle model.

Unless pressure drops suddenly, you may first become aware of knocking or clattering sounds coming from the engine. This may happen, for example, when not enough oil gets to the valve train or engine bearings.

However, a flickering warning light may also happen when a fault appears in the oil pressure switch or its electrical circuit.

The next procedure can help you check operation of the indicator light or oil pressure gauge, if necessary.

Most sending units or switches are located next to the oil filter. If you need help to find the unit, consult your vehicle repair manual.

To check operation of the warning light or oil pressure gauge:

  1. First, make a visual inspection of the electrical wire connected to the oil pressure sending unit or switch. Check for signs of damage.

  2. Check the connection. It should be tight and clean.

  3. Then, disconnect the wire from the sending unit

    • If the warning light remains on, the circuit is grounded and you need to find the fault.

    • Similarly, if you have a gauge, when disconnecting the sending unit, if the gauge reading remains indicating low pressure, the circuit is grounded or there's a mechanical problem with the gauge.

  4. After disconnecting the wire, ground the warning light or gauge by touching a metal component or an unpainted surface on the engine with the wire.

    Now, either the indicator light should come on, or the oil pressure gauge should read minimum pressure

    Otherwise, there’s something wrong with the indicator light or gauge.

    Some pressure sending units can be tested with a digital multimeter by checking their resistance. Consult your vehicle repair manual.

NOTE: On modern vehicle models, some manufacturers recommend using a special gauge tester to check for proper indicator operation. This is usually done on vehicles where the oil pressure system is controlled by the car computer. Consult your vehicle repair manual.

Whether or not your tests show the warning indicator light or gauge is working properly, it is a good idea to check system pressure. Head over to the section How to Check Oil Pressure below for the procedure.

Low oil pressure is more likely to appear during engine idle.
Low oil pressure is more likely to appear during engine idle. | Source

VI. I Have Low Oil Pressure at Idle

When the oil warning indicator light or the oil pressure gauge reads too low at idle, it can mean one of several things:

  • Low engine oil level
  • Engine oil is worn
  • Overheating is causing the oil to thin
  • Oil pump not working properly

Adding too much oil will upset lubrication system pressure.
Adding too much oil will upset lubrication system pressure. | Source

VII. My System Has High Oil Pressure

Although low oil pressure is the most common problem when there's something wrong with the lubrication system, high oil pressure can also happen.

High oil system pressure shows up when system pressure goes beyond the upper limit parameters in the system.

High system pressure may be caused by the following conditions:

  • Pressure relief valve sticking closed. This valve is located in the engine oil pump or close to it. The valve limits the maximum oil pressure allowed in the system and opens when the maximum allowed pressure is reached, usually between 40 and 60 psi, depending on model. Otherwise, pressure will rise. Debris or sludge or thick oil may clog the valve.

  • High-viscosity oil. Using a 20W-50 oil in a modern, low-mileage passenger vehicle, for example, can cause high oil pressure. Something similar may occur if the thermostat sticks open. The engine won't reach operating temperature, and the oil won't thin enough to properly do its job.

  • Restricted oil galleries or passages. Debris or sludge can clog passages and prevent engine oil from reaching crankshaft bearings and other system components.

  • Adding too much engine oil. Check oil level by reading the dipstick, and consult your car owner's manual or vehicle repair manual for engine oil capacity for your particular vehicle model.

  • Plugged oil filter or engine block bypass valve. This usually occurs when debris or buildup accumulate inside the bypass valve to prevent oil flow. Also, this can happen if you haven't replaced the oil filter during the last few engine oil changes.

Low oil pressure can prevent the engine from starting on some vehicles.
Low oil pressure can prevent the engine from starting on some vehicles. | Source

VIII. Will Low Oil Pressure Cause a Vehicle Not to Start?

Some vehicle models come equipped with a low-oil-pressure safety circuit.

The circuit connects to the car computer, and the computer that disables the ignition system when too much oil pressure is lost.

This is to prevent the engine from serious mechanical damage.

Check your vehicle repair manual, if necessary, to see if your model has this switch installed, and, if necessary, check oil system pressure using the following procedure.

The oil pressure switch or sending unit is usually located next to the oil filter.
The oil pressure switch or sending unit is usually located next to the oil filter. | Source

Warning!

When checking oil pressure with a test gauge, wear safety goggles to prevent eye injury from hot oil.

IX. How to Check Oil Pressure

You can use an oil pressure gauge tester to check the lubrication system in your vehicle.

Proper lubrication system pressure depends on:

  • Good operation of the oil pump
  • Certain system components being within specifications
  • Adequate clearance between bearings and journals
  • Clean galleries

The gauge can give you information about the condition of the oil pump, and the lubrication system in general.

If you don't have an oil pressure gauge, you may borrow one from your local auto parts store.

  1. First, locate the oil sending unit or switch. This is usually mounted next to the oil filter. If necessary, consult your vehicle repair manual.

  2. Unplug the sending unit's electrical connector. Then, remove the sending unit using a wrench.

  3. Connect the correct fitting to the gauge's hose to plug the gauge in place of the sending unit. You may also connect the test gauge in one of the oil cooler lines, if your engine uses one (usually found in diesel, turbocharged, or high-performance vehicles). If necessary, consult your vehicle repair manual.

  4. Start the engine and let it idle for 15 or 20 minutes so that it reaches operating temperature.

  5. Increase engine speed to about 1,000 rpm and check engine oil pressure.

  6. Increase engine speed to about 2,500 rpm and note engine oil pressure.

  7. Compare your readings to the specifications listed on your vehicle repair manual.

    • If oil system pressure is lower than specifications, you need to find out the cause.

    • If pressure is within specs, and the engine operates without knocking or rattling noises, engine oil has the correct level and is in good condition, and there are no overheating issues, then there may be a problem with the engine oil pressure sensor, oil pressure gauge or indicator light. Or the electrical circuit.

NOTE: Specific test procedures may differ from the one indicated here. Consult your repair manual, if necessary.

A worn oil pump can also cause low oil pressure.
A worn oil pump can also cause low oil pressure. | Source

X. Further Diagnostic Tests

Some common problems related to low engine oil pressure, and high oil pressure, are discussed here. As you can expect, knowing the exact cause of the problem will help you make better repair decisions and prevent further, unnecessary repair expenses.

Although it's not as common as the above causes, don't forget that a failed oil pump can also upset system pressure. A failed pump may leak internally; its relief valve may clog or become weak; the screen may clog; or its driver may fail. If necessary, you can install a new or rebuilt unit.

If the oil pump in your vehicle fails, make sure the problem is only with the pump itself. A worn out pump is rare and may suggest, especially on high-mileage engines, worn-out engine bearings as well.

Sometimes, more than one test, including a prelubricator test, may be necessary to pinpoint the cause behind a problem with the lubrication system.

Some car owners seem to have found a practical way to deal with clogged oil pump screens. Watch the video below for some tips about cleaning this device without removing the oil pan. These tips may work for you too.

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