I have a fairly new engine,melin pro 604 cid.When i first started it
the oil pressure was at 75 psi according to electric guage at idle and buried the guage when i reved it.I have taken the boat out about 6 times,everything find.Started up the engine last week and oil pressure was at 60 psi cold and 45 warm but reved up to 60 psi.I put a mechanical guage on this week,now the electric guage reads 30 psi and the mechanical guage reads 20 at idle and drops to 15 when you rev it.Any suggestions before i pull the motor.

Pull the oil filter, cut it open and look inside. I suspect there will be bearing metal because I think you have a bearing-probably a main-going south. But that is a good easy first step.

soudns like what happene to me. I still haven't checked the bearings. Check your oil level thoigh. Mine was SUPER low (thank my ex-mechanic for that one, he said he "topped it off" before he have it back to me)

good luck.

I drained the oil,nothing on the magnetic plug.Installed new oil,
pressure still bad,pressure drops when revved,like sucking air.Motor only has 10 hours on it and never past 3500.Motor is all
new,lunati 4340 crank,oliver billet rods.Suspect problem with pump but don't know what effects filter bypass might have on pressure.There were 6 qts of oil in pan at time,never low.Don't know what the difference is with the merlin blocks and there priority oiling system.

Bearing metal is non-magnetic-look in the filter.

I removed filter and cut open,nothing in filter.Any ideas now2

If it drops when revved and there is no metal then there must be a restriction somewhere in the oiling system or the pickup tube has come loose and you are drawing air. Was the pickup stock or aftermarket, welded? You might try a new filter first to see if that may have been doing it. Greg P from this board had a similar problem and it turned out that moisture was blocking the filters on his engine. I am not familiar with the bypass set up on your engine so hopefully someone else might chime in on that part of it. The good news is no metal in the filter.

Have you tried to overfill the oil, just to see if the pressure increases???
Just try it at the dock.

As said earlier. The pickup can come out or become loose.
Is it welded???

If you still have low pressure, pull it. You don't have enough pressure to run it. All you're going to do is more damage.

My first test also would be to add a couple of extra qts. of oil. That way if the pressure does come up, it is because the extra oil will keep the pump supplied if the pickup did come off. But wouldn't run it long with extra oil, just enough to diagnose.

The pump came with the Steff's pan and the pickup is welded on.
I will try a new filter and more oil and see if there is a change.At
least it did not happen on the water and the presure has not dropped below 20 psi.