Oil pressure too high

[Hot 'n' High]

Guess what, it all depends and it seems everyone is a bit right on this - I guess it is down to specifics for aircraft and engines. From a bit of browsing - and no apologies for length, nor the formatting (which is doing my head in!) but I have highlighted a few bits to help(?).

1. From the Lycoming O-320 Operators Manual:-

“3. LUBRICATION SYSTEM.

1. Non-Adjustable Oil Pressure Relief Valve – The function of the oil pressure relief valve is to maintain engine oil pressure within specified limits. The valve, although not adjustable, may be controlled by the addition of a maximum of nine STD-425 washers under the cap to increase pressure or the use of a spacer (Lycoming P/N 73629 or 73630) to decrease pressure. A modification on later models has eliminated the need for the spacers. Particles of metal or other foreign matter lodged between the ball and seat will result in faulty readings. It is advisable, therefore, to disassemble, inspect and clean the valve if excessive pressure fluctuations are noted.
2. Oil Pressure Relief Valve (Adjustable) – The adjustable oil relief valve enables the operator to maintain engine oil pressure within the specified limits. If pressure under normal operating conditions should consistently exceed the maximum or minimum specified limits, adjust the valve as follows:

With the engine warmed up and running at approximately 2000 RPM, observe the reading on the oil pressure gage. If the pressure is above maximum or below minimum specified limits, stop engine and screw the adjusting screw outward to decrease pressure or inward to increase pressure. Depending on installation, the adjusting screw may have only a screw driver slot and is turned with a screw driver; or may have the screw driver slot plus a pinned .375-24 castellated nut and may be turned with either a screw driver or a box wrench.”

I could not find anything on when adjustment would be needed, just a table of various Max, Min, Idle pressures.

2. These guys identify a general comment right at the start:-“Lycoming generally sets the operating pressures for cruise rpm on their factory-rebuilt engines to between 75 to 85 psi. Most new, rebuilt or overhauled engines require a slight adjustment of the oil pressure to finalize the setting once the engine break-in process is complete.”

They also discuss, at some length, the following under Oil Pressure Relief valves, including why adjustment may become necessary:-

“The older styles were adjusted by removing the housing and spring and adding or subtracting washers behind the spring to increase or decrease pressure. The oldest style housing was short and had an adjustment of zero to three washers maximum. The longer housing allowed up to nine washers maximum to increase spring tension. Each added washer increases oil pressure approximately 5 psi. On the externally adjustable models, one turn in (clockwise) increases oil pressure approximately 5 psi.

There are also springs of varying tensions and lengths which can be interchanged if the above adjustments do not yield the desired results. Some of the springs are color-coded to help differentiate them from one another. The most commonly used ones are the white LW-11713 springs (thick, heavy springs that are used to increase oil pressure at all settings), the 68668 (purple springs that are short and have much less tension than the others), and the 61084 non-color-coded spring that is standard equipment on most regulators.

One of the more common problems with the oil pressure regulators is with the seat that the steel ball contacts every time it closes. The seat is simply a machined aluminum section of the crankcase itself on most engine models, and over time it can become worn, especially if the ball is not contacting the seat dead in the center. If oil pressure varies excessively with engine rpm, especially at lower engine speeds, the regulator ball and seat may not be closing properly. Poor contact allows some of the oil to bypass back to the sump when it shouldn’t. If the cast aluminum seat has an irregular wear pattern in it, Lycoming recommends rigging up a makeshift tool out of an old ball welded to a steel rod that is thick enough to be struck with a hammer, then inserting the newly made tool squarely against the seat and giving it a couple of sharp hammer strikes to reform the seat, allowing a tighter fit between a new ball and the seat.

The field method of repairing a worn or non-concentric seat that most mechanics employ is to use the same tool mentioned above, but instead of striking it with a hammer, they use a tiny bit of valve grinding compound on the ball to re-lap the seat. Care must be taken to prevent the compound from getting into any of the oil passageways during the process, but overall this method tends to work well to reform the seat and regain a good seal between the ball and seat. Some of the earlier engines did have a replaceable seat insert that could be changed out and replaced if it was worn, but the most common seat is the cast aluminum type mentioned above.”

They also mention that the Pressure Transducer:-

“Some aircraft use an oil pressure transducer or sending unit that looks similar to the oil pressure switch used for Hobbs meter installations. It is a unit that has an oil pressure line piped into one side, and electrical wires connected to the other side. Pressure is converted to an electrical signal and wires are run to a gauge that displays the oil pressure reading. The oil pressure in most Lycoming engines is taken off the top rear accessory case. The oil pressure fitting has a reduced orifice in the outlet to the gauge. This helps prevent catastrophic oil loss if the oil pressure line or gauge begins to leak. Carbon or dirt can sometimes clog the orifice and cause an abnormally low oil pressure reading.”

They then go on:-“Troubleshooting oil pressure problems

Most oil pressure problems can be adjusted back to normal with the regulator or traced to a malfunctioning regulator or gauge. Sometimes, the trouble is a little more difficult to repair. The first step in correcting abnormally high or low oil pressure should be to double-check the pressure reading with a separate pressure gauge to confirm that the oil pressure really is too high or low. Check the oil temperature, too. Low oil pressures will produce increased oil temperatures, and vice versa; overly high temperatures thin the oil and can cause a lower-than-normal oil pressure reading.

Excessive internal engine clearances due to excessive wear or a bearing failure can become so great that the output of the pump is insufficient to fully pressurize the oil system. This is typically a worst-case scenario and lower oil pressure readings occur gradually over time. Excessive oil pump clearance between the impellers and the housing can also cause degraded oil pressure output. Oil viscosity plays a role in oil pressure as well. A slightly lower than normal oil pressure may be caused by using too thin an oil depending on where the plane is operated. A clogged suction screen or partially blocked passage between the screen and pump can also cause low oil pressure.

A higher-than-normal oil pressure reading, especially one that occurs suddenly, can be indicative of a blockage somewhere in the system, usually downstream of the pump. Oil pressure readings should be consistently monitored so that any deviation from normal operation can be detected and remedied quickly. Consistent, normal oil pressure from startup to shutdown helps assure that an engine will run reliably for a long time.” https://www.cessnaflyer.org/maintenance-tech/item/1149-hone-in-the-range-lycoming-oil-pressure.html

And there is a useful thread which includes:-

“I have a Continental (smaller O-300) engine, and I was incredibly surprised to learn just how sensitive the relief valve mechanism is. You "adjust" the oil pressure on the engine by adding or removing washers from under a spring, which keeps the piston/plunger (the sliding valve) pressed against a valve seat. My O-300 was running 10-15% above the minimum, so I thought I'd add a washer or two in there to bring it up just a little.

Two additional washers resulted in a startup pressure spike (80-100 PSI), so I immediately shut down, removed one washer, and tried again. This time it only went to 70-90 PSI, so I shut it down again and replaced the one extra washer with a thin washer. The thin washer still spiked the pressure, so I had to go back where I had started, with the same number of washers as it had, and the same 10-15% over minimum that I had.” https://www.flight-mechanic.com/reciprocating-engine-lubrication-systems-oil-pressure-regulating-valve/

So in summary:-

During the 50 hour run period, the oil pressure PRV may need to be adjusted.

That there are several different ways to adjust them, depending on make.

That the adjustment is usually quite sensitive.

That wear and tear of the PRV itself can necessitate some mid-life adjustment.

Any pressure transducers can get partially blocked so lead to low readings.

But the unanimous message is, ensure it is always in the correct operating range or snag it!

Finally, so far (a) it seems impossible to find any simple definitive statements and (b) seems everyone is right as even just the above supports many of the statements made throughout this thread. But - the one common theme everywhere, if it ain’t green, ground it!

I’m all Googled out here and my brain is now hurting so I will retire from this thread now! Finding a definitive answer is like finding the Holy Grail and I'm neither Indiana Jones nor a Monty Python! And more to the point, you'll be totally fed up with me .... if not from wayyyy before this post!

Oh, I have to say it - for "washers" you can read "shims" when adjusting things - well, generally!

Cheers, H 'n' H