Need help with a gremlin. I have a 2002 Volvo Penta 8.1L Gi. We recently rebuilt the entire motor using a remanufactured long block from Bishop International. Other than the intake, oil pan, timing chain cover, valve covers, pulleys, etc., everything on the motor is brand new including the fuel system, starter, alternator, pistons, rings, bearings, oil pump, timing chain, plugs, belts, etc. When running the boat, everything is fine up to about 3000 rpm. If you run it above 3000 rpm for more than a few minutes, the engine rpm drops to under 2500rpm, beeps and the motor will go into guardian mode. Once at idle for a minute, the motor resets and runs fine until you take it past the 3000-3500 rpm mark. Temperature is fine with no signs of overheating. The boat has been run with a diagnostic computer attached and a manual oil pressure gauge. When the diagnostic computer reads low oil pressure and moves to guardian mode, the manual gauge shows tons of oil pressure; well into the normal 30-40 lbs of pressure. At a loss on figuring this one out.....

maybe have the wrong sending unit for the oil psi?

We have changed it…

Where did you install the pressure sender ? factory spot or did you move it

Factory spot.

I'd suspect a wiring issue with oil press sender. Why it's associated with RPM is odd.

Could try connecting a voltmeter to the sender, route/secure test wires to be safe and have voltmeter placed somewhere it can be easily seen with it at >3K rpm, then try to duplicate the issue. If scanner data doesn't follow the voltmeter readings, you have a wiring issue.

Which ECM? I helped a guy 2 years ago with a 38 ft cabin cruiser with twin 305 small blocks, think they were volvo penta. They had mefi4 ecms from factory, he had same exact issue as you, one engine went into alarm past a certain rpm and wouldnt rev no higher, dash showed good oil pressure. In his case it was NOT the oil pressure sender for gauge that you read, it was oil pressure safety switch thats simply N/C, pressure to open, like a late 70s gm car for the dash light. You start the car, switch opens, light goes out. Since it was a holiday weekend, I simply unplugged the one wire oil pressure switch and alarm no ;longer worked on that engine until he could get a switch the following week., Smitty