3.0 MPI loss of power/WOT RPMs

boater_stevo

Recruit
Joined
Aug 29, 2020
Messages
5
Howdy all,
Recently purchased a 2015 185 Larsen with the Mercruiser 3.0 MPI, Alpha 1 I/O. 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
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,001
Start with the basics
What did the plugs look like?
What is fuel pressure at the rail at idle thru WOT?
 

boater_stevo

Recruit
Joined
Aug 29, 2020
Messages
5
Just finished changing out the plugs. All were dry with some carbon build up except number 3, which was kind of wet/gummy.
I’ll have to go buy a fuel pressure gauge and check the fuel pressure.

hooked up the flushing cups and turned it on to make sure I did the plugs right, it idled kind of rough for 20 or so seconds and then idled more smooth than I’ve ever heard it. Rpm at idle dropped from 1000-1100 down to 900 now.
 

boater_stevo

Recruit
Joined
Aug 29, 2020
Messages
5
Also. Factory plugs were gapped at .040. New plugs now gapped at .035 per every manual I’ve found.
 
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