3.0 MPI loss of power

Howdy all,
Recently purchased a 2015 185 Larsen with the Mercruiser 3.0 MPI. Ran great for the first 6 or so trips, plenty of power to plane out with 5 adults and hit close to 28-30mph, WOT right about 4800RPM, good hole shot, good fuel consumption, sounded good, etc.

Now, over the last 5 trips it's quickly gotten progressively worse in terms of power. It currently struggles to plane out with even 2 adults at WOT, struggles to get above 3500 RPM, and when/if it planes out it won't go above 4000 RPM/15MPH. Have not changed or added anything new, same propellor as before, etc.

No guardian mode beeps, no check engine lights on, single beep at ignition when we turn the key, idle speed seems high at 1000RPM (should be close to 780 per the label on top of the engine), oil pressure seems to jump up right after turning on motor and stays around 40, fuel is ethanol free 88 octane, oil levels all good, voltage is around 12V when off, around 14V when running, temp warms up to about 150 and stays in that area while running.
I hooked it up to the garden hose with the flushing attachment and was able to verify the RPMs go up above 4000, it hit 5k soon as I opened up the throttle.

I tried replacing the water separating fuel filter, distributer cap and rotor, and spark plug cables. Change was maybe a little better than before, but that might just be wishful thinking. I have not tried running any cleaners through the fuel, or anything like that.

Anyone have any thoughts, advice, etc? I've searched all over the place to no avail, hoping to find some answers that will help me get the family out a few more times before the season is over

I hooked it up to the garden hose with the flushing attachment and was able to verify the RPMs go up above 4000, it hit 5k soon as I opened up the throttle.

That tends to destroy the impeller and in neutral proves nothing
Check the fuel pressure or injectors clogging

Is the boat trailered?

Docksidemarineservices said:

Is the boat trailered?

Click to expand...

yes it is.

i did the plugs two days ago, found one that was wet and kind of gummy. The rest were dry and tan. Think I found my loss of power, hopping new plugs with the new distributor cap and rotor takes care of it. Think I’m testing it out today or tomorrow on the water

It was in gear and just for a second. Proves it’s not in guardian mode though, right?
you’re right about the impeller though, not a test I’ll be doing again now that I’ve read more into it and understand the cooling system a bit more. I chalk it up to new dumbness and inexperience, but it is probably time to replace the impeller regardless.