So here it goes...

Purchased a 2000 Fountain w/ 500 EFIs roughly 3 years ago. Boat had 199 hours at time of purchase. Ran the boat the first year without issue, but had some intermittent oil pressure concerns with the starboard motor. It's been so long, I truthfully don't even remember what I began chasing.

That winter I had both motors and drives out of the boat to redo the bilge, and do valve springs/seals. I decided to pull the pan on the starboard motor. We inspected a few rods/mains and all appeared normal. I decided to button it back up with a new Melling M77 and a new Mercury oil pressure sending unit.

Back in the boat the motors start and run flawlessly. Both make equal power, oil temps and water temps within 5-10 degrees motor to motor. The starboard motor makes 40-42lbs of oil pressure when at idle cold. The thing is, it makes 42lbs of oil pressure always. All except hot idle, the boat keeps the gauge at 40-42 lbs. I may settle into a hot idle pressure in the low to mid 30's, but never low enough to tell me the clearances are shot. As soon as I touch the sticks it's 40-42... no matter if its 2500RPM or 5000 RPM. I ran the boat 50 hours like that last year, and now that brings me to this spring....

What do I do? I'd hate to start a rebuild now, as I planned on doing both this coming winter. Motors have 299 on them now. Am I hurting this motor if it really is making only 40-42 PSI? Could I have an issue other than clearances? I'm a bit lost... and I certainly don't want a mid season failure, but at this point I don't even know what I'm chasing.... any and all advice... I'm all ears.

mechanicle oil gauge .. if youve ran 50 hrs w ith no probs id say youre good to go .

+1 I agree with using a mechanical guage on each engine to measure the real oil pressures as some dash oil guages and senders can be quite a bit off. If the mechanical guage shows both engine at about 50psi or higher I would say you are just fine. If that starboad engine still measures lower than the port engine mechanically measured and stays in the 40-45 psi range you are near the edge of good oil pressure for high rpm usage but still probably ok. pressure differential is probably just a cleatrances difference between the two engines port to to starboard. What oil and weights are you using and what is engine temperature when oil pressure is at 40psi?

Best Regards,
Ray @ Raylar

Raylar, last year while I was chasing the problem I tried probably 3 different oils, from Mercury's suggested, to Mobil 1, to Shell... nothing seemed to matter at all. Oil temperature as I recollect (I haven't boated since last year) is always 175-180 at the highest... fluctuates with water temperature... but never above 190 even when run very hard. I think it cruises in the 150-160 range.

Obviously its not an oil weight or type issue and those good temps are not the problem. I would do the mechanical pressure test and compare the engines on pressure that way also. I think you may just have a dash guage difference, but a mechanical test at the engines will show that if its the issue.

Best Regards,
Ray @ Raylar

Ray.. I hooked up a mechanical today on the motor in question for the seasons first fire. Of course I had an impeller lay over as soon as I fired, but I did notice pressure at 45-47 for the few seconds it ran. Im going to put an impeller in tomorrow and see what we've got. At least I know my electric gauge is a few pounds off so far...

Your pressure is fine, you put a high volume pump on and that is what you really want. The amount of oil you pump through the engine is more important than the pressure it is pumped at.
I run a nitro funnycar, at the start I have 120 lbs on the gage, after the burnout and staging the car its down to 30-40 lbs. the pump is still pumping the same way, but the oil has been comprimised due to blow by and heat.
If you had an oil problem you would have smoked the bearings out a long time ago.

So I took the sender out, and hooked up a mechanical gauge, and took the boat to the lake today. I can honestly say I'm more confused, but at least I know what I have...

First fire of the day boat makes 40-45 PSI... idled for about 10 minutes before getting on the sticks. Bring the boat on plane to 3200RPM and it's making about 50 PSI. As oil temps climb I see a steady 45-47 PSI and there it stays. Once I got the oil to temperature I came down, at idle I have 32PSI... this is a constant, every time I am at hot idle, the motor makes at least 32 PSI.

However, I'm a bit ignorant to how the oil is cooled, how an oil thermostat works (if my boat motor has one), and exactly where and how it diverts oil, but I found this interesting, and I'm sure some experts will know what's going on. If I start the boat and bring it on plane it will make 45-47 PSI at cruise and above, regardless of RPM. As the oil climbs to 180 degrees it remains at this pressure. Now I suppose there is a thermostat, because once the oil is a bit above 180 it will quickly cool to 130-140 and start the process over. The odd thing is that AS SOON as the temperature needle begins to fall the oil pressure drops and holds at 40-42lbs regardless of RPM. The oil pressure will remain the same regardless of temperature for the rest of the run. Now if I shut the boat down, let it sit for an hour, and start over again, the boat will make the same 45-47 PSI and as soon as the oil cools it will drop and stay there again. I can only assume something is opening and hanging open? I know there are oil thermostats in some motors, is this opening and diverting oil another place, thus requiring more volume from the pump?

Measure the other motor in the same manner and report those findings and numbers.
The 500efi engine has an oil thermostat that allows most of the oil to by-pass the oil cooler across the back top of the engine. when the oil temperature gets to 180 degrees and higher the oil thermostat channels a lot more oil to the cooler and brings the oil temperature down until the temp reaches 180 degrees again. There would be just a slight decrease in pressure created mostly from the combination of hotter oil and the routing thru the cooler itself and back to the engine.
I don't think what you are finding and describing is a problem and unless you have numbers on the other engine that are greatly different you do not need to worry or open things up.

Best Regards,
Ray @ Raylar

Ray... I guess my real question is... regardless of the "good motor".... am I "hurting" this one? Is it going to live, or have an untimely death with this oil pressure? I spend 99% of my time at 3200-3500 RPM... but is it enough for a quick 5000 RPM dash?