BF225 return of the gremlin

Started out on a trip from Nassau to Highbourne cay Exuma with my son and his family for 4 days, I had Traveled about 5 miles and realized I had forgotten something at home.
I went back which obviously involved docking and 10 minutes at low speed and idle, when I attempted to get back up on plane mu gremlin which I have not seen in several trips returned, one engine would not rev beyond 3400 rpm (again), I shunted the test wires but no fault codes were displayed, I did the clear fault reset several times and got the double beep indicating there were no faults, but the problem remained.
My mechanic kindly met me at his shop ( he does not work on saturday) , checked for overheating with a heat gun, no issues there, checked that all cylinders were firing no issues there either took him out so he could see the problem, went back to his shop and he switched out the ECM, tried it again still would not go beyond 3400 rpm, the fact that the green light went out when the issue occurs had him stumped. He said he would have to do a more extensive trouble shooting, so we decided we would leave that for another day.

Now, heres the really strange part, I had figured out that if I revved the engine quickly in neutral, she would spin up to 5500 no problem, but if I tried it slowly she would quit at 3400.
So I trimmed that engine way out so she would have a lot of prop slip and reved to 5000 , gunned the other two and gradually trimmed the problem engine down, low and behold she ran fine, I spent 4 days in exuma covered 250 miles and never once had the issue again.
I do thing it may be related to the HO2 sensor, as I am getting a lot of soot out of the exhaust ports after each 2 hours or so running, on all three engines, they have always had a little since day one, but this is much worse than normal, there is no smoke of any kind while running at any rpm so I am also wondering if I might have leaking exhausts, anyone got any thoughts?

No suggestions here but I do have a question.... or three:
Do you know what the cause was the last time you experienced this?
Or, did it just go away on it's own then too?

Was it this same engine last time?

When was the last time you cleaned/replaced/serviced the VST and fuel filters on these engines?

Hopefully your tech has some good ideas.

Engines were just serviced including VST, fuel filters, HO2 cleaned, throttle bodies cleaned, I had this issue before the service for a while then it went away.
I have a suspicion it may be related to an exhaust leak as I am getting a LOT of soot around the exhaust ports, no smoke while running though.

Well, looking at the obvious....it is, more than likely, either something mechanical sticking intermittently or...it is something like a sensor going out of range intermittently.

On the mechanical side it seems to me the most prominent suspect would be the vvt (variable valve timing) mechanism. Not being experienced with this engine and it's specific components that is about as far as I would go as far as speculating on that side of the equation
On the electrical or sensor side of it, I would probably start with looking at the MAP sensor OR, as you have already said, the O2 sensor. I lean more toward the MAP because of the success you had with elevating the angle of the prop and then trimming in as the other two engines carried most of the load. The MAP is the load sensor for this engine and your "trick" was, in effect, changing the load and introducing it in smaller increments as you trimmed in.

I'm just offering this as food for thought in the hopes of getting the conversation started. Hopefully you will get it figured out before too long.

Good luck.

Jgmo, thanks for the thoughts, never thought of the MAP sensor, good thought and progression of analysis.

Like Jimmy, I would initially suspect the MAP sensor. However, a faulty or disconnected MAP sensor should give you a fault code "3." If you have the Helm Shop manual, see page 5-28 and 5-29 for testing procedures. If it happens again, pull any codes as soon as possible. These intermittent problems are the most difficult to track down.

According to the shop manual, to test the MAP sensor, disconnect the 3-prong connector to the MAP sensor. With key switch on, use a voltmeter to measure voltage from the brown/yellow wire to engine ground - it should be 4.75 to 5.25 volts. Then test voltage between brown/yellow to green/red. It should also be 4.75 to 5.25 volts. Then connect voltmeter positive to white/red wire and voltmeter negative to the green/red wire. It should also be 4.75 to 5.25 volts. There are further tests which require a test harness (which no one has) or Dr. H. (That test involves measuring the voltage between pin 11 and pin 9 on the 34 pin connector. It should be between 2.76 and 2.96 volts. If you really know what you are doing, you might try back-probing those pins to complete the test.)

Do I understand correctly that you are getting black soot at the exhaust ports on just the problem engine and not the other two?

CHawk, unfortunatly I am not getting any codes, so no guidence there, yes I am getting a LOT of soot on that engine and the center engine most out of the exhaust, but some between the pan and extension case. My mechanic is actually surprised that I have never had the exhausts changed, and suspects that is a part of the reason for the soot.
He just got a new Diagnostic machine and is gonna check it when he gets it set up.
FYI, kudos to him for coming to his shop when he was closed to help me out, I spoke to him yesterday to update him and reminded him to bill me for Saturday, he said there was no charge and was just glad to hear we had a trouble free trip! Cant complain about that, hes a good guy!

Well, maybe so, but I'm skeptical. If you have efficient combustion within specs, then you shouldn't have soot anywhere. The soot is coming from a burn that is too rich. There are quite a few things that can signal the ECM to adjust the burn to be too rich. These include timing, camshaft position (CMP) sensor, crankshaft position (CKP) sensor, barometric pressure sensor, MAP sensor, IAT sensor, coolant temperature, HO2 sensor, knock sensor, and so on. Hopefully, your mechanic's new diagnostic tool will unravel it.

Anyway, if I recall correctly those are 2003 engines, so replacing the exhaust tubes with the newer upgraded ones would be a very good idea. I think the entire kit is a bit over $100 on boats.net. See the last page of SB # 56.

Yep 2003's, got some othr issues to attend to very shortly, one has a slight oil drip due to corossion at the seal area of the oil sump, the problem baby has corossion in the same area but no leak yet, I figure to get them done at the end of the year, and do the exhaust done the same time.
They have just over 1000 hours and run great, and I am not prepared to do a repower as I plan to sell her in the next 18 months in anticipation of moving to Hope Town in the Abacos.
As to the soot yea I figure most is from too rich and I should start by changing the HO2's.

I would suggest testing the HO2 sensor first. They're fairly expensive to change out, especially on just a suspicion that it might be bad.

Dr. H will tell you whether the HO2 sensor is operating withing specs. Alternatively, you can use this procedure...

http://www.autozone.com/repairinfo/...sHowTo&pageId= 0996b43f80a01215&subtitle=test

Thanks CHawk.