Oil Pressure during descent

[FredG]

Hi all,

I recently got a GI275 EIS installed.

I noticed that my oil pressure moves into the Yellow range (just below 60) during some stages of flights (descent), and wondered if that was normal. It is noticeable as the GI275 fires warnings as soon as it moves below 60 and I can't say that I noticed that before with the (quiet) steam gauges.

Thank you all.

F.

Edited June 16 by FredG

[N201MKTurbo]

What does your oil pressure read in cruise? If it is only varying a few PSI, I would just raise the oil pressure a little bit.

[FredG]

in cruise, it is at 65-68.

[N201MKTurbo]

That's not a lot of variation, but it is odd it drops on descent. Your RPMs don't really change. Your oil temp would normally decrease, which should raise your oil pressure. I can't think of any reason for it to drop on descent.

Do you reduce your RPMs on descent?

[Ragsf15e]

1 hour ago, FredG said:

in cruise, it is at 65-68.

What do you do with your power when you start down?

[PT20J]

Mine (IO-360-A3B6) reads 70 in cruise and doesn’t drop below 60 except at idle when hot. 

[FredG]

I’ll go run some test end of week and will report back more precise observations.

will keep you posted.

thanks for quick responses.

[marcusku]

On my J, I would get some fluctuation in oil pressure, and would show up first in a decent when the oil level was below 5.75 quarts or so.  As long as it was more than 6 quarts, there was no change.  I never did figure it out and simply kept the oil level between 6-7 quarts.

A couple of theories were tossed around and the one I most believed in was a leak on the inside of the oil pickup between the case and the pan.  The idea was when there was more than 6 quarts, that joint was below the oil level and therefore couldn't suck any air.  The oil pickup is on the back of the engine so when nose down, that joint would get exposed earlier.

[jlunseth]

Do you by chance reduce the power for the descent? 

[FredG]

4 hours ago, jlunseth said:

Do you by chance reduce the power for the descent? 

Yes, I do reduce power. Throttle back.

[jlunseth]

Solves it. When you reduce power you will reduce OP. Engine isn’t working as hard, oil pump is not working as hard. In addition to your reduction of power, the act of tipping the nose over and going downhill reduces the load on the engine. 

[TangoTango]

Isn't oil pressure affected by RPM, and to a lesser extent temperature? Throttle back to me sounds like @FredG is leaving the blue knob at the cruise setting.

With my IO-360A3B6D, I do notice a reduction in oil pressure down to about 60psi when I pull the RPM back (either with the blue knob or by dropping MP so much the prop falls out of governing range), but I do not notice a difference by changing MP alone. 

By the way, the oil pressure is easily adjustable:

 

60297-12[1].pdf (Page 109 - Oil Pressure Relief Valve)

[marcusku]

Right, throttle wouldn't have an effect on oil pressure, but rpm would.  Temperature has an effect too, lower temp-higher pressure at a given rpm.

In my case though, the pressure was jumping around like some air entered the system.  I thought it was the gauge at first but then I got a second JPI gauge and was seeing the same thing on both instruments. 

[FredG]

I went to see my mechanics. We'll do the adjustment to to increase the base oil pressure with that screw mentioned above.

 

[MikeOH]

11 hours ago, FredG said:

I went to see my mechanics. We'll do the adjustment to to increase the base oil pressure with that screw mentioned above.

 

I believe the photo above shows a pressure RELIEF adjustment screw; THAT will NOT increase the base oil pressure.  It will only limit the MAXIMUM pressure the system sees; e.g. on start up with cold oil.

[N201MKTurbo]

1 hour ago, MikeOH said:

I believe the photo above shows a pressure RELIEF adjustment screw; THAT will NOT increase the base oil pressure.  It will only limit the MAXIMUM pressure the system sees; e.g. on start up with cold oil.

It will change the oil pressure. It is what sets the engine’s oil pressure. The oil pump is a positive displacement pump. It could make infinite pressure without a relief valve. 

[MikeOH]

4 hours ago, N201MKTurbo said:

It will change the oil pressure. It is what sets the engine’s oil pressure. The oil pump is a positive displacement pump. It could make infinite pressure without a relief valve. 

Maybe I'm using the wrong terminology.  The OP is dealing with LOW pressure at reduced power settings. If the pump cannot create, say 65 psi, then the pressure adjustment can only be set to LOWER that pressure, not raise it.  I.e., only vent/relieve pressure.  I was taking issue that his mechanic is going to be able to 'adjust' the 'base pressure' up.  Since he has higher pressure at higher power settings, the pressure setting must already be set higher; adjusting it downward will certainly change his pressure, but NOT in the direction he wants.

[Slick Nick]

Has it always done this or is it something it just started doing? 

[N201MKTurbo]

The oil pressure will vary with temperature. As the temperature goes up the oil will flow through the regulator easier, reducing the pressure.  I suspect his oil pressure is right on the cusp of being too low, so increasing the pressure 10 PSI or so should keep it well out of the yellow at any normal RPMs. I concur that it is the correct solution.

[FredG]

On 6/28/2025 at 5:12 AM, Slick Nick said:

Has it always done this or is it something it just started doing? 

It is hard to say as prior to having the GI275 EIS which visually and orally tells you as soon as you get below 60PSI, I had the standard gauge bar and never really noticed variation.

I am hoping it is just a sensitivity issue and a slightly higher pressure by 5 or 8 PSI will suffice to keep it in the green.

Will keep you posted.