I have an 01 496 Mag (NON HO) that has just over 300 hrs on it. Since I have had it (2 seasons) it has always left my transom covered with a greesy soot. As the water temprature rises it does seem to get better but still well beyond what I see on other boats running the same motor. I ran the boat for aprox 1 hr at 3500-4000 rpm and the transom was completly black. My merc. mechanic has scanned the ecu numerous times and every thing looks good as far a fault codes. Also at idle the air/fuel ratio looked within spec. Should I pay to have the mechanic take the boat out and check air/fuel ratio during mid-range and wot operation? I have already replaced the IAC due to abnormal air/fuel ratio at idle (last season). What would cause a super rich condition in mid-range rpm? The boat seems to run great other than it is a pain in the butt to clean the transom up after short hops from the launch to our party spot (11 miles)... I have pulled the plugs and they look normal. However I have not done a high RPM run/shut down to do a plug check as of yet. Any help is greatly appreciated.

Also my mechanic has verified there are no recalls/bulletins based on my serial number (I remember reading that some of the 496s required a reflash of ecm). He has told me the 496 ecm was programmed to run the motor a bit richer, however I believe what I am experiencing is not by design.

i have a 496mag HO in my 24' checkmate 240zt and same deal almost always has black soot. Pretty sure thats just how they run.

I would venture to say it's normal. My '01 HO did the same thing from day one until I had it reflashed just this winter, now transom is clean as can be. The only thing I found (besides the reflash) that helped was to run premium fuel even though the experts on this site say not to it worked for me. It won't completely stop it it just cuts down on it some. You are right though the early 496 ECM's do run very rich at mid range. Good luck.

Do you run premium fuel? I had a efi 454mag and I ran 91-93 octane in with the same problem. Switched to 87 and the problem completely went away.....just a thought. Same thing on a buddies boat.

Some soot is normal. Completely black after one hour is not. I suppose it could be the design of your transom and exhaust flappers blowing everything back. As mentioned, burn 87 octane. BTW, a quick fresh coat of wax on the transom after each washing makes the soot a breeze to clean off with soapy water.

Don't run premium, only 87. It will help

I purchased my boat new in 2004 (with twin 496 mag's), and briefly experienced the exact same horrible sooting issue during that first season in 2004.

After speaking to Merc, I realized they were well aware of the excessive sooting problem. Merc kindly sent me two new PCM's in exchange for mine. And they told me to run 87 octane.

No sooting since then!

I do run 87 octane. I have already found that the premiun burns slower so it was causing even more. Sounds like a reflash or new ecm is in order. But how do I attain either of these. How much does a re-flash cost? Also I saw Dynojet sells a fuel curve reprogrammer for the 496 mag. It sells for $599, would this do the trick? Would a reflash cost less? My boat is solid white, and it looks like my boat is a POS when I pull up with a black transom...

It is interesting that such similar engines have such opposite results, refering to 87 or 93 octane fuel. I switched from 87 after two seasons of use and ran it for another four on 93. After joining OSO I saw all the talk about how to never run 93 in a 496 so I switched back and the heavy soot came right back. Just my experience.

As far as the cost to reflash, mine was fee except for shipping because I was getting some weird codes that Merc. couldn't figure out so they reflashed it with the upgrades. Now I do run 87 with no problem. They seem to be pretty willing to reflash the older ones for a minimum amount. I would have your mechanic give them a call.

Here is a pic I took of the transom after a 15 minute run at 3500-400 RPM with a minute or two of WOT running. IMO this is way too much soot. Make my Velocity look like a turd!!