Are there any common reasons for low oil pressure?

[Sterling27]

My truck is a 1990 5.2L 318

 

[goldeneyee]

OLD AGE...........

 

[reek]

off the top of my head:

bad pressure gauge, worn pump, clogged filter, worn motor, wrong oil viscosity, not enough oil.

 

[GTyankee]

BINGO
You got the Big 6

Start by taking out the oil sensor on the engine & screw a mechanical gauge in there, If the sensor was incorrect replace it

 

[Ron Boggio]

Mechanically:

Engine oil pressure is created by resistance to oil flow through the main bearings. As the main bearings wear clearances increase proportionally reducing the resistance to flow resulting in reduced pressure. The oil pump, as stated in its name does not create pressure. The pump generates oil flow, the main bearing clearances induce resistance to that flow, resulting in pressure. A function of Bernoulli's principle.

Keep in mind the idea of flow. Flow is directly proportionally to volume. The pump is designed to “move” a certain volume of oil eg. 1qt per 10 revolutions as an example. The oil pump will have clearances specific to its design as well. Unlike the main bearings these clearances create vacuum that pull the oil into the pump allowing the pump veins to move the oil along.

The pump pulls the oil from the sump i.e. the oil pan. This is accomplished by the pickup tube which has a screen at the end of it. Two considerations here, if the engine oil maintenance has been neglected excessive sludge will accumulate at the bottom of the pan and if excessive enough will plug the screen. Reducing the flow of oil into the pickup tube in effect starving the pump. Do keep in mind, the oil filter is in the oil flow path. The oil filter is the first line defense against physical impurities in the oil, it is also the first casualty of physical contamination. If the oil maintenance has been neglected it is possible to see it in the oil filter e.g. clogged, damaged, etc. Typically the oil filter will restrict flow before a plugged pickup screen.

Electrically:

The oil pressure is monitored electrically. The 1990 engine is a far less complicated design in that it is not computer monitored. The oil pressure sending unit controls the oil pressure indicator be that a light or gauge depending on which you truck is equipped with. In the case of a light, the sender unit is really a pressure controlled SPST (single pole single throw) switch. The sender is calibrated to “switch” when the oil pressure reaches a specific pressure e.g. 5lbs PSI for example. When oil pressure exceeds the minimum pressure, the electrical contacts open extinguishing the light. In the case of a gage the sending unit is a voltage divider i.e. the voltage to the oil gage will increases as the pressure increases causing the gage to read higher as the pressure exerts against the sender increasing the voltage to the gage.

Each sending unit will fall out of design specification and or fail over time.

Do not neglect the gage itself, for simplicity purposes a voltage gage is a voltage variometer. While a gage has relatively few moving parts they will fail for several reasons. There are test procedure to validate a gage is reading as designed. You will have to do a little research here.
Finally, do not neglect to validate the wiring. Two considerations here. First and most obvious, the connectors. Broken; bent; corroded; missing. Not as obvious the wiring itself. Cracked or missing sheathing. Is the wire uniformly flexible across the entire length? Should you find a spot that is stiffer than the rest it could indicate the wire overheated there resulting in a voltage drop. This can occur without melting the sheathing although typically it is noticeable.

A couple tips on narrowing down issue.

Observations:

Cold Soak start i.e. after the engine has sat overnight: Does the oil pressure start at a maximum pressure then as the motor warms it drops off and stays relatively low? Oil is viscous i.e. thick and thins as it warms. Thick oil will not flow as easily as thin resulting in a faux pressure initially. If this is the case main bearing are highly suspect.
Cold Soak start: oil pressure starts low and remains low i.e. unchanging suspect plugged pickup screen or sludge caked sump.
Warm engine: oil pressure makes wide pressure change with engine RPM suspect oil pump. Wide would mean almost or zero at idle and say 40 at 2000 rpm as an example.

These are just pointers and should not be considered definitive tests. They are simple observations to assist in dividing between electrical and mechanical. As was mentioned above removing the sender and attaching an oil pressure gage is a valid testing method. The information provided here is for educationally purposes only. Do your own research, validate everything.

 

[GTyankee]

sorry, but my eyes glazed over reading that
But i got the point

 

[Wallyb4]

Describe what you call low. For example my 97 Jeep cherokee with a 4.0L has about 12 -15 psi at a warm idle. That is normal for this type of engine. I have 193k on it.

 

[reek]

the 92 318 I just rebuilt runs about 60 at temp.

my 2017 Mustang GT runs pretty high at start, almost 80+ then goes to 50-60 once at running temp.