Correct. Checking psi over idle is useless on the trailer. Very little fuel needed to rev to redline without load.

Thanks for all of the input from everyone, picked up a new crankshaft sensor and going to put it in tomorrow and get to the water for some testing under load over the weekend, since it seems the general consensus is that a lot of my “trailer testing” isn’t really going to tell me anything real world.. so, I’m going to install the crank sensor and the take it from there. I’ll post my findings after some under load testing this weekend.

Well, now i am at a loss. Put in the new crankshaft sensor, tested (just on trailer) and power "available" still dropping to 60 % at 1800... I know it's without a load, but if it's not full power without a load, it certainly won't be in the water with a load. I purposely ran a scan on the good motor immediately after this one again today on the trailer, and all of the other parameters between the 2 motors are virtually identical, block pressures, manifold and water temps within 2 degrees of each other.. and the port motor never shows less than 100 % power available at any rpm... stbd motor drops as soon as it is warmed up. I guess all i can hope is that under load it will show a real. world fuel pressure problem, running out of things to check/change. What's even more odd, i can cruise all day at 3000 to 3500 and both run the same, other than a little more throttle on the stbd engine for the rpms to match over 2500.. I read a bunch of other threads on here about crankshaft sensors causing a complete power drop or even shut off in most cases, but i was still hopeful as my other symptoms seemed similar. I guess the boost pump is one of the few possibilities left, but i am not sure if that would even cause the scanner to show only 60 % available power..?

Something is changing that the PCM sees which in turn changes available power. The issue is what is causing it. If you had Diacom or other software which would record the sensors it would be easier to find. Not having that, have to tract sensors individually.

Check output of Alternator
Check output of 5V circuit like at the water pressure sensor
Fuel pressure should remain steady, if it is then there is no issue with boost pump. If its not steady then maybe, but could also be other things in the fuel circuit. Does the other motor show under 43 psi?

The "sensor" voltage always show 5.1, but are you saying that and even though both engines show virtually identical water pressure, the sensor could still be bad?

No, I mentioned the water pressure sensor because its a sensor I know uses 5V and the first one that popped into my head. If the 5V was low then this could be causing the issue, but with 5.1 then that isn't the issue.

Just trying to see if other things are changing, like maybe the ALT voltage drops around 1800

Had another thought, does your beeper work? Are you hearing 2 beeps when key is turned ON?

For power reduction mode there should be 2-beeps or 3-beeps being heard

Unfortunately, just got to the bottom of it... ecm is bad, I put bad ecm on good engine, immediately after scanning both, and problems followed the ecm... looking for a remanufactured one now, but no luck so far.

Send it off to boats unlimited and see if he can repair it

The ECM can be fixed, call John - 817.658.7721