Low Oil Pressure Question/Answer

[M.D. Holloway]

Someone asked me today why their oil pressure has dropped after their oil change and if they should be alarmed about this. My first question was "Did you use the same weight oil?", they said yes. Of course, the oil viscosity will help dictate the oil pressure and even the same wieght oil may actually have a different viscosity if it is a different brand. The viscosity of 1 15W40 from one company may infact be slighly different from a different oil company. The pressure drop wasn't significant - well, let me just say pressure wasn't completly lost only reduced about 30%. The engine temp is fine and their appears to be no signs of wear or decreased gas milage.

I told him that the engine OEM has a target zone for the correct oil pressure as they do for compression and so forth. I also instructed him to find out what that target range is and to see if it is inline with the conditions.

But to his question, "should he be alarmed if it is too low", my answear would be yes, but a guarded yes and here is why (I think). The oil has to pass thorugh the bock and feed the bearings and establish a dynamic seal at the ring to liner site. Oil pressure is important for adequate flow through the block. A reduced flow (pressure) may result in more residence time thus improving the chance for increased engine temps, thermal breakdown of the oil and eventual oxidation. Pressure is important - high pressure is also a problem as is low oil pressure. Find out the proper pressure before judgement is passed...

Does this answer the question or is there something I missed?

[mackskibum]

Don't forget that oil pressure is directly related to the force required to breach the oil film on a babbett bearing for the crank or con rod. Low oil pressure at idle isn't necessarily a problem, but at high RPM or load it is. The big question is why... he could have a filter with a faulty bypass allowing oil through the circuit with no restriction- a problem. An oil filter generally is an equalizing bypass looking to maintain a specific maximum pressure drop across the filter- not a poppet that just dumps open. I would look there before anywhere else.

Just an idea.

[PRE-H20]

just a quick question....

what SHOULD the oil pressure show under load or high rpms?

[mackskibum]

On a 911, we look for 1 bar per 1K RPM, up to 4bar at 4K+ RPM- I believe this is correct. If not I'm sure someone will correct me.

On another make/model, it will be different- check in the respective service manual.