How can I eliminate black soot looking stains on back of boat. I try to clean continuosly but just after cleaning and a run up the lake the back of the boat is black again , under platform ect., boat runs great , no meck., problems ect. I think it is rich from factory. I read in some earlier threads about remapping the comp.,any sugestions? boat is a 98 32 fountain with stock 502 mpi's

thanks Joe

The condition you are talking about is what merc calls trailering effect on the early 496 merc came up with a comp fix and reprogramed the computers...
I havnt seen it to much on 502s..when you run the boat sitting still do you see a sheen on the water????? have seen the float in the vst stick and cause a run rich condition but you wiil see unburned fuel inthe water

Have the updated mapping and mixture program loaded into the puter...should clean it right up.

My '99 does the same thing, also my last 27', along with a buddy's triple 525SC 42'r. I wonder if alot of it has to do with the swim platform design trapping the exhaust. I get the black transom just idleing from the ramp to my slip, a 3 min junt even with the silent choice on. Doen't seem to be as bad of a problem on some friends PQ or Bajas, but they do get a little. I wash it down every evening while on my lift. That keeps the build up down considerably. Also scrub it mid summer and 2 coats of wax seems to help make cleaning easier. Good thing our boats sit so low, only seen when on the trailer.

Wouldn't that be a $350 or higher per motor mod? Thinking that was the going rate for basic ECM reprogramming.

usa 1 you are 100% right !!!!!! it happens while moving slow trapping the exaust gases under the platform thats why merc call it trailering effect...good call

I notice you are from Ontario. I've noticed more soot with my 454 Mag MPI in the spring and fall when the water is colder. Once the water warms I don't notice it as much. What's the water temp were you boat?

No I don't see a sheen on the water behind boat, the boat runs perfectly, the water is warm , and is still same condition.
I would like to know if the updating is an update from merc., or is a performance upgrade either way I would like to have it done.
Do I send away comp., or take it to a merc., dealer. Thanks for the help.

thanks Joe

Be careful here guys getting to technical. Some of this black sooting is additives the oil companies are adding to the fuels. We have seen this same condition in the exhausts and in our dyno motors after tear down. It seems to be agrevated when you use premium 91+ octane fuel where 87 or 89 octane will do. We know its a fuel problem, because when we lean out mixtures its still there and when we tried racing fuel it disappeared. Interesting Eh! They say they are cleaning up gas emissions and insted they increase particulate emissions! Sleep tight the politicians and oil companies are awake tonight!!

Ray @ Raylar

I always ran 93 octane in my stock 502mpi, and was informed I was tossing dollars in the lake. Switched to 87, boat performs just as well, and I do not have the "black soot" as bad as I use to. Still have to clean well after a weekend of boating, but not near as bad.