Oil pressure relief valve
 

I have a new to me RV-3B that I'm having a little issue with consistent oil pressure. Start up it is at 85-90 then stays in that area until after climb out and put in cruise. As soon as you set cruise rpm you can watch the oil pressure start dropping slowly. Until it gets to around 45. Then it stays in the general area until throttle back to land then goes up again to the high 60s .
We have had the oil pressure relief valve apart to check for debris or something else but all looks good. We are wondering now if the relief spring could be bad or wrong and would cause this type of issue. Engine runs great with steady oil temperatures and cylinder head and exhaust temps all seem good. Any ideas on what to look at next would be greatly appreciated.

Don't always assume it's the actual oil pressure in the engine. Often it's the gauge, sensor, wiring or support parts. If you are unsure that the oil pressure reading is right you should connect another oil pressure gauge, preferably mechanical, to the system and see what readings you get. Then troubleshoot from there.

Connections
 

I have checked the wiring connections and ground. All is good.

That means nothing. The only way you know if you're reading the real pressure is to compare it to a good mechanical gauge. I've seen EFIS software cause erroneous readings, air bubbles in pressure lines, mismatched components, low voltage from grounds or miswired, bad sensors or gauges, etc. What you're describing doesn't sound like you're getting a true reading. The pressure will go down as the oil thins from heat but 10 ~ 15 lbs is normal. The pressure won't go up as the RPM's go down. The pressure bypass is just a ball and spring, it's only releases pressure over the set amount. Further testing is needed.

OIL PRESSURE INDICATION.
 

I AGREE WITH PREVIOUS COMMENT. ALTHOUGH OUR RV-6 FLYING SINCE 1994 WITH ORIGINAL VANS OIL PRESSURE GAGE & TRANSMITTER, DEVELOPED A SIMILAR PROBLEM. WE ENDED UP REPLACING BOTH GAGE & TRANSMITTER AND PROBLEM WAS SOLVED.

Assuming that you have seen this pattern several times, it seems less likely to be an electrical issue; Seems too related to engine conditions that should not affect electrical.

Do you by chance have a C/S prop? Do you by chance not dial back the prop control until after your climb? If yes, I would be looking at your prop / governor as the likely suspect for taking your oil volume and ultimately your pressure.

When problems appear and disappear in repeatable situations and circumstances, it is best to identify what else is happening / changing in those same situations and look for correlation to help identify causes or relationships.

pressure relief either works or it doesn't, short of a blockage on the overflow circuit. Can't imagine anything that would cause relief to operate properly in climb, but not in cruise. In the case of blockage it would only read normal or high, based upon volume. I can't think of anything that would cause the relief to "sometimes" read low. A failing oil pump, yes, but not a relief. Why don't you give us the RPMs that you are seeing in "climb" and "cruise." Then we can speculate on the possibility of a failing oil pump. With a worn pump, you can see oil pressure rise in high RPM and drop in low RPM. The tell tale is usually a low idle pressure and you didn't mention that.

EDIT: the fact that the pressure slowly drops after entering "cruise" keeps taking me back to prop/governor.

Larry

Cato fixed
 

Not a constant speed, fixed pitch Cato three blade. After doing a search on this through Google, it seems like I'm not the only one with this exact same issue. One person that had the exact same description as mine replaced the whole valve with a new one and problem solved. I find it hard to believe that out of at least three people with the same problem that only one replied back to the original post to let others know how they fixed it but that is the case. Someone also stated that the spring may not stay centered and that was his problem.

I use the internet A LOT to help me diagnose problems and look for similar symptoms. The problem is that it is very difficult to determine if the complete mix of symptoms are the same. Often you're just collecting data points to add to you troubleshooting.

Boiling down your problem, with the limited info provided. it seems that your problem appears as you enter the "cruise" state and disappears when you throttle back. You need to provide a bit more detail.

Given that you know the exact circumstances under which the problem appears and disappears, you need to sit down and think about what those situations mean and how the changes between those situations could affect oil pressure or sensor readings.

I assume going from climb to cruise means increasing RPMs. Think about that. Pump produces more flow. More flow through the relief. Crank spinning faster, etc. Cavitation in the pump is also possible, as it is something that often has a speed threshhod (only cavitates above a certain RPM). Have you checked your oil screen recently?

Good luck with this. Maybe others will have had a similar problem and can help. I still suggest you add more detail so that other can look for similarity.

Larry

Try this
 

There are 2 types of setups on the case where the oil pressure relief mechanism is installed. We will call them the old style and the new style. If an old style oil pressure relief valve is installed in a new style machined case, the ball will simply slide off to the side instead of being held in the center. When that happens, there is a major loss of oil pressure, especially when engine rpm is reduced. Most engine shops are aware of this and can get you the correct oil pressure relief setup.
I had a very expensive education with this. I bought an experimental champ in CA and observed the exact same symptoms that you describe. I initially thought it was a faulty gauge. Oil pressure was normal at times but always settled down to 40ish in cruise. One of the bearings got VERY hot due to lack of oil and in the end, ruined both the case and crank.

However, my oil pressure goes down during cruise and up when I throttle down in the pattern.