I am running the stock Delco plugs (the number you have listed in previous threads). The IAC was replaced 2 seasons ago.... The Map sensor makes sense, due to all of the oil flying around in the engine..... I will take a look at the MAP first.

Quick update....

The MAP sensor, like everything else, was coated in oil (due to the bad intake gasket). Mercruiser MAP was $140.00. I found that Autozone had a MAP for the 8.1L Vortec for $46. I figured fob $46 what the hell I will give it a shot. I had to epoxy a small spacer on top of the new MAP because it wasn't physically as tall,as the OE MAP. Since they are held to the manifold with a screw/washer on top, the new one wasn't able to be held in place without the spacer. I had my doubts, so I made sure to keep the original MAP just in case. Much to my surprise, boat runs great. There is no hesitation or miss upon throttle. And the soot has decreased even more. I think, since it is a dry manifold, it will take quite some time to clear all the left over oil out. Mthe throttl response seems a bit quicker as well. Midrange acceleration seems very strong.

Low-psi:

Glad my two suggestions and pointers gave you the fixes you needed and you now getting great results from that boat of yours you were about to sink.

Enjoy your boat, have fun and boat safe!

Best Regards,
Ray @ Raylar

I am noticing vitually no soot or black film on the transom at all if running the boat around in the 3500 range. However, when I run her up in the 4000-5000 rpm range is where I still see a bit. It is still a very greesy film. BUT, as I previously stated it is thinning out each time.... So with the water temps getting warmer and the oil in the intake thinning out.. The transom should stay fairly clean by the end of the season...........

RAYLAR, to your knowledge, is there any different between the Vorted 8.1L MAP sensor and the Merc 496 Mag MAP sensor? SO far the boat runs great but there was a physical height difference in the two. If it is just the casing that is different I will continue to use my adapted Vortec MAP for $46.00

This has been very good info from Ray who has posted the help to fix your problems so alot of credit has to go out to him.

To see if this MAP sensor is working correctly since it was not bought directly as a (per say) recommended mercruiser part or souce. I would scan the motor to check the voltage output values of the MAP sensor at idle and at full throttle.

Going off the top of my head I want to be clear about that.
Most Mercruiser ECM / PCM - MAP sensor voltage output readings at idle are 1 to 1.5 and at full throttle is in the 4 's. I would have to double check for the 496 Mercruiser PCM outputs to be spot on.

A Map sensor out of range and or a bad MAP sensor will cause a rich condition inwhich can add to the soot problem among other problems.

A Map Sensor is used as a load sensor plus it measures barometric pressure to report back to the ECM or PCM to adjust the fuel delivery along with spark curves and compensates for any altitude changes.

The ECM / PCM receives the MAP signal along with the RPM signal to select the proper injector pulse width and ignition timing thru out. Anyways MAP sensor voltage should be checked especially on this app even if you think its running great. It might even running better and cleaner with the proper MAP sensor installed but I have no clue if it is without checking the voltage outputs at idle and full throttle. Good luck

Oil Use
 

The information on this problem was great Ray really has a lot of knowledge on the 496's.
I have twin 496's in a 2001 Formula, I have a slight soot problem but what I didn't see anybody mention how much oil they are using. To give an example if I run to Atlantic City and back about a 3 hour run in the ocean. Running 3400 to 3600 RPM's I'm losing 2 quarts of oil in the Port engine Starboard is OK, so I think I must have the intake manifold leak. What I have also noticed and maybe it's part of the same problem I'm using 4 - 6 GPH more of fuel in that same engine then I am in the starboard side running at 3400 RPM's. I should also mention that the oil turns black an hour after after using the boat after an oil change.
The Starboard side, oil change to oil change stays clear like the day I did.
All stock new risers and manifolds this year, I wish I had read this tread before I could have checked for oil inside the replaced parts.
Any thoughts would be appreciated.

The sensors have the same voltage output.

Glad to hear you found the issue. Kudo's to Ray for recommending the intake gaskets and kudo's to lowpsi for seeing this through and sharing all the info. What a great thread.

One thing I can guarantee is that our tunes are not "richer" than stock. We use stock cal's for our baseline, and modify from there. With more coil dwell and more spark advance, the engines are more efficient and burn the fuel much more efficently, not less. The engines timing curve with our tunes are much closer to MBT.

Yeah, the problem is not entirely gone... Still a lot of soot when running over 3200 RPM... (Which is where the boat is most efficent). I still see a lot of unburned fule in the water behind the boat at start up. Although I believe the issue now maybe bad fuel injectors. I just haven't had a chance to have them tested.... It does loose fuel rail pressure quite fast when not running...

I have a 2003 496 (non-HO) and run 91 and have lots of soot on the transom, even on the sun pad vinyl. I switched to 87 and was better, but still getting soot buildup. I'd rather have it run a little rich vs. lean but the soot is annoying, especially on white vinyl.

Have you had Mercury look at the ECM? They can either re-program or replace it for less soot.