Every time I run my 42 Fountain with 525 efi's I am getting a lot of soot on the transom. Seems to be worse in the cooler weather. It is evenly distributed on both sides of the transom and runs up onto the back edge of the swim platform, does not seem to be worse on one side than the other. Does the ECM's need to be recalibrated? What else to look for? I hate cleaning the mess up after every run. Boat seems to be running very well.

What gas are you running? I think if you run anything higher than 87 octane, you get a lot more soot.

Run your engine serial numbers and ECM part numbers by your local MerCruiser rep. and see if there's a fix campaign or recall on your stuff. I'm sure you're not the first guy to have this problem and if ANYBODY knows what the fix is...MerCruiser does. Good Luck.

Gerrit,
I have the same boat as you, and I get about 1/10 the amount of transom soot with my 525's that I got with my 2004 Baja 33 Outlaw with 496 Mag HO's. That boat had a terrible soot problem, but the engines always started and ran flawlessly. I only use 87 octane, I add Star Brite enzyme water removal because of the E10 gasohol, and the 525's run flawlessly also. With very little soot.

O2 sensor???

Running a long time on the Rev limiters will create the Black soot, but if that is the case there is nothing you can do.

Early 525EFI's needed to be reprogramed by Mercury to eliminate the Soot you would get from not running on the Rev-Limiters.

I don't run on the rev limiters all of the time just some of the time. I run 87 octane marine fuel. I am leaning towards needing the ecm's reprogrammed.

I had to have mine reprogrammed. It was running very rich at idle. I still get a little bit of soot from idling, but nothing like before. Call the tech guy at Merc with your engines' numbers. He should be able to look it up quickly to see if it needs to be reprogrammed.

Try a hotter plug too.

Now what takes the soot of the transom? I have tried many products but take it all off?