Mermaid turbo 4 low power, black smoke.

[Bill W]

Evening All. I have a Channel Island 32 with twin Turbo 4s. These were factory reconditioned units, fitted in 2009 and rated at 170hp each.
Both engines will rev, no load to over 2800rpm. Under way, both are reasonably smoke-free (in Ford terms) up to about 1500rpm and the Morse control levers are in line with each other. Above this speed, the starboard engine begins to produce black smoke and the stick has to be pushed further forward than the port one to reach the same revs. At 1900rpm, the starboard engine will rev no higher and puts out some pretty hefty amounts of black smoke. The port engine will merrily rev to 2300 or more and remains clean.
Oddly, the starboard turbo seems to produce higher boost pressure, around .6 bar, compared to less than .5 on the port gauge. This I find confusing because black smoke usually means not enough air for complete combustion. I don't see how there can be good boost pressure but not enough air.
The boat had brand new stern gear fitted, including one new prop and one refurbished. I've been over the side and checked there is no fouling of either prop and they are both equally easy to turn manually.
If I can ever shake off the dose of covid I have just now, I'm thinking of starting with a look at the air pipework, charge air cooler, etc. Someone has suggested a possible raw water leak into the CAC, but I find it hard to see how that would produce these symptoms. I've done a fair bit of googling but no lightbulb moment. So, here I am asking for help from the very knowledgeable members I know frequent this forum.

 

[volvopaul]

Which side had the new prop ?
Black smoke is unburnt fuel , could be a faulty injector , you could be loosing boost pressure to the cylinders , check the inlet manifold gasket and associated hoses , pressure test the intercooler .
Could be advanced pump timing .

 

[Bill W]

Which side had the new prop ?
Black smoke is unburnt fuel , could be a faulty injector , you could be loosing boost pressure to the cylinders , check the inlet manifold gasket and associated hoses , pressure test the intercooler .
Could be advanced pump timing .

I don't know which is the new and which the refurbed one. The work was carried out shortly before I bought the boat.
I'll look at all the points you mention as soon as I feel well enough.
Losing boost pressure to the cylinders? Interesting. I can see that might occur with a bad inlet gasket. Would wrong valve clearances do similar?
I should clarify, I've only had the boat a month or so. The starboard engine has never made full revs in that time, originally topping out at 2100 and a bit of smoke, which recently dropped to 1900 with quite a lot of smoke.

 

[harvey38]

Did the S/B engine make full revs on the sea Trial prior to purchase, any mention of engine issues in the survey report?

 

[Bill W]

Did the S/B engine make full revs on the sea Trial prior to purchase, any mention of engine issues in the survey report?

I didn't get a survey report done prior to purchase. I did have chance to look at her in detail while she was in the boatyard having new sterngear fitted.
On sea trial I didn't try WOT, in hindsight maybe I should have, but she ran nicely up to 2000 revs and easily achieved the cruising speed I was looking for.
I got her as a project, to bring up to scratch over the next few winters, at a project price, so expected to be doing some bits while running her for the season.

 

[Bill W]

I’m out on the boat just now. I’ve taken the crossover pipe off from between the CAC and the inlet manifold. No sign of any water or corrosion, just a light film of oil. Looking into the CAC via its outlet, the tube stack looks in good Nick, with a light coating of oil. Second pic is the inlet manifold.

 

[Bill W]

I managed to get out to the boat again this afternoon. Took off the rocker cover from the offending engine and adjusted valve clearances, which were a tiny bit on the slack side. This was done to cross another possible cause off the list. Took the boat out for a short run. Starboard engine still not going above 2000 revs. Port engine happily making between 23-2400 revs. With both sticks right forward the port engine was on 2350 and the satrboard just short of 2100 but on throttling back the port engine the revs also fell on the starboard, back to 1900-2000. Still black smoke from the starboard engine above about 1500 revs.
Back on the buoy, I pointed a laser thermometer at each of the charge air coolers in turn, They were almost identical. Did the same with the gearboxes, negligible difference between them.
Next suspect has to be the props. I find it hard to believe that a professional marine engineer would fit non-matching props but it has to be ruled out.