Oil pressure fluctuates

"I have a 95 Sea Ray Sundancer

"I have a 95 Sea Ray Sundancer with twin 5.7L Mercruiser inboards. The starboard engine runs a steady 40psi on the oil pressure gauge. The port engine starts out at 40psi and will stay there all day if I run at 800-1400rpm. If I go to a cruising speed 3200-3500rpm it will hold steady at 40psi for about 10 minutes then slowly drop to 20psi. When I reduce the throttle back to the 800-1400 range the pressure will drop a little more and then begin to slowly climb back to 40psi over the next 15 minutes. The pattern repeats every time. Any suggestions would be appreciated.
Lighthouse Point, FL"

"Don, The oil pressure gauges

"Don, The oil pressure gauges are electrical, running off a resistance reading from the oil pressure sensor, they are therefore subject to bad connections. You can put a mechanical guage on the port engine to assure it is not a low oil level or weak oil pump issue. Other than that, the electrical oil pressure sender can go bad, the gauge can go bad the wiring can go bad or some combination of the three can go bad."

Bad oil filter bypass valve?

Bad oil filter bypass valve?

Sending unit incorrect?(test pressure with mechanical gauge)Go from there.

"Don: you can check the gauge

"Don: you can check the gauge by disconnecting each sending wire on the gauges and swapping them. You may need a jumper wire for each. If the problem stays with the gauge after the wire swap, the gauge is bad. If the problem changes gauges the you need to check the sending units.

Just swap them between engines. Put the wires back at the gauges to the original connections. After you swap sending units, clean the connecting terminals on the units and wires. If the problem changes gauges, the sending unit is bad. If it doesn't change gauges, the sending unit is good. Do the least expensive next.

I would change the oil and filter on both engines before I went any further in case it is just a bad filter bypass valve like Charli stated.

Problem still there? Is there oil leaking around the valve cover of the problem engine? The oil returns could be restricted (sludge or gasket pieces) and the oil can't return fast enough to the pan. With the oil levels full in each engine, when it happens again, stop both engines right away and check the oil levels on the dip sticks; bad engine first. They won't read full immediately but should be near the min. level. They both should be the same if there is nothing wrong. If not, it sounds like a restricted return passage.

Pull the valve cover and clean the oil return passages. If same condition still exists, time to put a mechanical gauge on the engine--compare both engines. You may need a new pump or just the pickup screen cleaned--it may have some sludge or gasket material stuck in it or the pump is breaking down.

Good luck."

"Don C.: You posted "Oil

"Don C.: You posted "Oil pressure fluctuates" about a month ago and I am just curious if you solved the problem and how you did it. Thanks, Guy"