Try 87 octane. If you are afraid, try the 89. I've run all those mods with Whipple PCM's and more with ported heads and thinner head gaskets and still ran 87. For what its worth, the 120 stats made no difference for me whatsoever.

Never ping'ed.

I am currently running 87 octane....

I have considered this issue being oil instead of fuel. However that kind of berings me back to if it was an oil consumption issue (bad valve, seat, etc), wouldn't one side soot up? As you can see in the photo (and both tail pipes show it) the soot is consistant on both sides....... I don't really use that much oil. Maybe 1/2 quart between oil changes..... I assume that is about normal. I will try and monitor the oil consumption more closely.

Is soot caused by running Rich supper greasy? The soot I am experiencing is almost like tar. Soap and water will not clean it. It requires roll off. Also with this amount off soot, would my tail pipes smell of fuel after the boat has sat? I am thinking more and more that this is an oil issue.... What about crankcase pressure? Just a thought.....

Could a bad PCV valve cause this (if it is oil)? I guess I never put thought into looking to see that it has one....... Probably only a couple of bucks, will pick one up..

Yes I know the t-stat has nothing to do with raw water temps. That was my point. Somewher under normal operation there is a 45+ degree temperature gradient between the sea water circuit and coolant temp. Allow the coolent flow through the exchanger unrestricted and the engine runs too cold. Since we don't know how the OP is actually measuring the engine temps this could be the case. Is he using a gauge or a scan tool or system monitor? What was the actual raw water temp? The T-stat may not be the issue but the soot problem is temperature related IMO, I've seen it. Doesn't sound like you would have any problem with him trying the original 160 unit since they all run the same temp right?

I wouldn't have any problems with anyone doing anything but if the point is to save money and do the least amount of waste then it would only be bad advice to tell him to change it. The coolant does flow through that system unrestricted because that stat is more for warm-up and not so much cool-down.

I guess i fail to understand why the diff between sea water and coolant matters. The coolant is static for the most part. Sea water is flowing at rates that it cant' or shouldn't ever get very hot because of the constant supply of colder water.

If the engine (valves, seals etc) is known good, maybe try to find out if his ecm missed the service bulletin call where merc addressed this issue. I realize he's already flashed it so he's now on his own.

Don't forget the soot is also affected by the actual shape, size, of the boat. Some are worse than others because of swim steps, free board, + or - the amount of low pressure behind the boat when it's running..

Wouldn't hurt to have an 02 bung installed on the top of a tailpipe and check the fuel trim.

There is not enough coolant in the system to run "too cold". You can test that by running without a stat. The only way to make that stat change is to somehow add a significant amount of additional coolant (which is difficult, very expensive & pointless).

The volume of coolant is the only factor left with which to make a real difference.

Finally, the reason for your temp gradient is due to the system being closed. If it were raw water cooled then engine temp and exhaust water temps are almost same.

The gradient is what enables exchangers and coolers to work. There is nothing else.

If the 160 thermostat made no difference in the soot problem when you still had a stock ECM program and again after the first use of the Whipple reflash stage 2 reporgam , then the problem is just most likely some serious OVERFUELING caused either by overly increased fuel mapping in the ecm or a combination of this overfueling coupled with possibly the highly advanced timing curve mapping for the reflash. This is the added acceleration or power you are seeing at lower and mid rpms. The stock Mercury 496 timing curve is very Slow-Low and Lazy to help prevent detonation and such with the iron heads and to help control the idle. at lower rpms decelerations.
The partially unburned fuel soot can be condition that will show up when the quantity and timing of the fuel that's being injected into the engine is not completely burned efficiently in the cylinder. It ends up washing and mixing with some oil off the cylinder walls and it is expelled out the exhaust as an oily -sooty residue.
There is also a problem with some of todays fuel blends with the soot issue being worse for some than others. Have you tried using different brands and blens of fuels? The so called oxygenation and ethanol additives and such make some brands and blens of fuels sooty than others. I am not a chemist so I don't totally understand why or who but i have seen differences in this soot problem from fuels.
Lastly, measure your air fuel ratios underway and let us know what they are from lets say idle right up thru 5000rpms under loads.

Just some thoughts advice and information here.

Best Regards,
Ray @ Raylar

I don't have a way currently to measure the A/F mixture...... Here is a picture of a plug. They all look pretty much the same..... Going to replace while out.....

There is anti-seize on the threads.... Does the black ring at the base of the plug look correct? It looks like oil (to me.)
http://www.bradzgarage.com/velocity/IMG_0395.JPG

Does the Merc 496 have a PCV valve? If not how does it vent off crank case pressure?

I was burning about a quart every three hours. So I think your fine with half a quart between oil changes. I didn't mean to scare you. Hopefully you can get it figured out. Sounds like Ray is on to something with your ecm. Good luck.

How does your oil look and smell? If you are running too rich, the oil will get fuel/soot in it. My oil would turn black within a couple hours after an oil change and smell like fuel. If your fuel pressure is correct (~46lbs), then the PCM fuel mapping should be checked for rich spots (you would need to hook up an air/fuel guage to find out). Also you could have a sticking fuel injector but that would show on that cylinder's spark plug.

Would a bad sensor on the exhaust manifold cause this. The port side (I am ssuming) temp sensor appears to be broken. It is a small plastic plug that was all wobbly. I tried to remove it and it simply broke in half........ Going to get a new one now. What do you think?