LOw Oil pressure

Help me understand what's happening with my oil pressure. The last three or four flights with the engine cold my oil pressure is coming up to the mid 50s at start up. Since I've been flying it, 300 hours, it always came up to at least 80 at start up then eased back down as the engine/oil warmed up. It's not a gauge problem because I have the Dynon D120 as well as a mechanical oil pressure gauge and both oil pressure readings match. I checked the oil level and it's good sitting right where it always does. In-flight the oil pressure comes up to about 75. I couldn't find much in the forms regarding Low oil pressures. Is this something I need to be concerned about and why the change? Thanks in advance.

Cylinder head temperature as well as exhaust gas temperature's are all normal throughout.

Warmer

If I had to guess, I'd say it's probably been warmer than normal the last few flights. Since the pressure hasn't changed once the engine is warm, there should be no difference in your engine.

-Andy

A good indication of engine bearing condition is hot idle oil pressure. Oil pressure down near minimum idle oil pressure spec likely means loose bearing tolerances but may not mean trouble "if" it has always consistently been that same pressure at the same rpm and oil temp. Cold start at 50 deg will definitely show higher oil pressure than a cold start at 75 deg or so. Maybe that's all you're seeing? I know my oil pressure is 48 to 53 psi at 900 rpm when oil temp is around 180 deg. As long as that number stays about the same, I can be reasonably sure the lower end is in good health as far as the bearings go. A new engines hot idle oil pressure will be higher or lower depending how the bearing tolerances ended up. The main thing is consistency with the same weight and brand oil. If it's within spec, you're probably OK. If it has been declining, could be the bearings, oil pump... are going away. Time to investigate.

Sudden drops in oil pressure that aren't attributed to oil type or temp are an indication that something has changed and usually not for the better. Assuming you haven't recently changed oil viscosity or type and assuming you have paid good attention to startup oil pressure over the last 300 hours and given the rapid onset of the condition, I would be looking at a problem with the oil pump or debris stuck in the oil pressure control (ball and spring).

In the Lycomings, the oil pressure is measured before the pressure control valve, whereas most autos have the pressure control in the pump housing and therefore measure pressure after the pressure control valve. In the Lycoming's case (a bad design), You never know the pressure that exists in the engines oil galleries and the effects that bearing tolerances or other issues have on that pressure. You can have 80 PSI at the sensor, pre-control valve, and have 10 PSI in the galleries, due to shot bearings. In this case you'd never know it.

With Lycomings, when your pressures drops, it is an indication that total volume from the pump or supply side has dropped. It is not related to anything happening after the pressure control valve, as the sensor doesn't measure that side of the valve.

The oil pump produces well in excess of the volume necessary to produce 75 PSI at 2500 RPM. However, it has nowhere near the same excess capacity at 800 RPM and that is why failing pumps first show up at low RPM. What you are experiencing is probably a sign that more investigation is necessary. It is possible the damage/wear event is over and everything will stabilize (your current pressures are not problematic) or it could be the beginning of an ongoing wear issue that was kicked off by something. You need to watch closely for further drops and act quickly if they happen. Be advised that these pumps can fail catastrophically and I would be concerned about the initial damage and likelihood of further failure.

Larry

lr172 said:

Sudden drops in oil pressure that aren't attributed to oil type or temp are an indication that something has changed and usually not for the better. Assuming you haven't recently changed oil viscosity or type and assuming you have paid good attention to startup oil pressure over the last 300 hours and given the rapid onset of the condition, I would be looking at a problem with the oil pump or debris stuck in the oil pressure control (ball and spring).

In the Lycomings, the oil pressure is measured before the pressure control valve, whereas most autos have the pressure control in the pump housing and therefore measure pressure after the pressure control valve. In the Lycoming's case (a bad design), You never know the pressure that exists in the engines oil galleries and the effects that bearing tolerances or other issues have on that pressure. You can have 80 PSI at the sensor, pre-control valve, and have 10 PSI in the galleries, due to shot bearings. In this case you'd never know it.

With Lycomings, when your pressures drops, it is an indication that total volume from the pump or supply side has dropped. It is not related to anything happening after the pressure control valve, as the sensor doesn't measure that side of the valve.

The oil pump produces well in excess of the volume necessary to produce 75 PSI at 2500 RPM. However, it has nowhere near the same excess capacity at 800 RPM and that is why failing pumps first show up at low RPM. What you are experiencing is probably a sign that more investigation is necessary. It is possible the damage/wear event is over and everything will stabilize (your current pressures are not problematic) or it could be the beginning of an ongoing wear issue that was kicked off by something. You need to watch closely for further drops and act quickly if they happen. Be advised that these pumps can fail catastrophically and I would be concerned about the initial damage.

Larry

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Great post Larry. Helps me have a better understanding of how stuff works.

Pressure sender

What Larry says doesn't make sense to me. The pressure relief valve just dumps oil into the sump when the pressure exceeds the set value of the valve. It doesn't matter where the sensor is since the pressure is the same before the relief valve and after the valve (the valve is "in parallel " with the rest of the oil system). The valve just bleeds off oil from the main gallery when the pressure exceeds parameters. The valve doesn't block oil from the pump to the rest of the engine.

Low oil pressure could be from a failing pump or from increasing clearances anywhere in the circuit.

-Andy

I'm betting there's a very small piece of debris on the pressure relief valve ball or seat. Seen it many times. It will have your exact symptoms.

lr172 said:

In the Lycoming's case (a bad design), You never know the pressure that exists in the engines oil galleries and the effects that bearing tolerances or other issues have on that pressure. You can have 80 PSI at the sensor, pre-control valve, and have 10 PSI in the galleries, due to shot bearings. In this case you'd never know it.
Larry

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Bull.

Andy has it right.

Paul 5r4 said:

Is this something I need to be concerned about and why the change?

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Check the screen and filter. If good....https://www.youtube.com/watch?v=d-diB65scQU

Can't quite agree with Larry. I deal with this daily. In most Autos oil pressure IS taken right after the oil pump before going on to the galley or in the primary galley feeding the whole system. The pressure will drop as the oil runs though the galleys as the oil finally reaches the bearings but only as a result of the resistance to flow. Doesn't really matter where in the system the pressure is taken or the pressure relief valve is located as long pressure drop is accounted for where it is taken. Lycoming oil pressure specs are typically higher than other engines for this reason. Oil cannot be compressed. So, Idle oil pressure is a good indicator of bearing condition if the ball is well sealed at the pressure relief valve and the pump itself is in good condition.

Take note what the pressure is at same temp and idle rpm. If OP continues to drop, don't run it. Investigate. It may save the crank and or case if it is caught in time.

Start with the easy stuff first! Relief valve.

My apologies. I misread the schematic a couple of years ago and it appears the relief is designed in parallel with the main gallery therefore, there is no downstream or upstream relative to the relief circuit.

Larry