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Old 19-05-2016, 07:22   #1
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Diesel losing power - black exhaust

Wife and I started on a four week cruise yesterday with high hope and no more than the usual anxiety. After motoring for abut for about 4 hours (no wind), I noticed the port engine on our Lagoon 37 TPI was losing power. The RPMs had declined from 2200 to about 1600 and pushing the throttle forward (just to see effect) caused to increase. I shut the engine down and then: 1) changed primary fuel filter, 2) changed secondary fuel filter, and 3) dove to insure that the propeller was not fowled by a line. When started, the engine (Perkins Perama M30 - 27HP) seemed to be OK while in neutral, but failed to attain normal RPMs while in gear, and the water exhaust had black clouds of soot.

We limped back to our marine on the starboard engine and now I am trying to figure out the problem. Lets just say I am not a diesel mechanic. Any ideas what might be happening?
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Old 19-05-2016, 07:29   #2
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Re: Diesel losing power - black exhaust

I'm not a diesel mechanic either but if it runs fine in neutral, with normal revs and no black smoke, something after the engine is putting a load on the engine to produce the black smoke.

I know. Not real helpful. Sorry.
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Old 19-05-2016, 08:23   #3
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Re: Diesel losing power - black exhaust

Ten bucks says you have a clogged/obstructed mixing elbow.
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Old 19-05-2016, 08:28   #4
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Re: Diesel losing power - black exhaust

Would it run ok in neutral, maintaining high revs with an obstructed mixing elbow?
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Old 19-05-2016, 09:34   #5
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Re: Diesel losing power - black exhaust

Before a diesel tech chimes in I just wanted to add a few questions.
How old is the engine? Saildrive or shaft? Gearbox ok? Are you able to turn the prop by hand with engine off and gear in neutral? Stuffing box ok and not too tight?
Cheers

Ps I too would have checked the filters first like you did, but since it didn't help I'm also interested in learning what's wrong.
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Old 19-05-2016, 09:51   #6
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Re: Diesel losing power - black exhaust

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Originally Posted by svHyLyte View Post
Ten bucks says you have a clogged/obstructed mixing elbow.
I am a mechanic, and my money is with this too.
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Old 19-05-2016, 23:54   #7
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Re: Diesel losing power - black exhaust

Black smoke could mean a burnt injector tip. Way too much fuel will effect power and create lots of black smoke.
Switch injectors between engines. See what happens. While they're out inspect the tips.
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Old 20-05-2016, 00:49   #8
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Re: Diesel losing power - black exhaust

My bet would be exhaust blockage (see elbow above) or a blocked air filter. If those are ok, then the engine is loading up for some reason. Have a look at the prop - a prop fouled with a rope or other foreign article can do it!
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Old 20-05-2016, 01:02   #9
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Re: Diesel losing power - black exhaust

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Originally Posted by Neptune's Gear View Post
My bet would be exhaust blockage (see elbow above) or a blocked air filter. If those are ok, then the engine is loading up for some reason. Have a look at the prop - a prop fouled with a rope or other foreign article can do it!
I'd check the other filters before doing anything expensive or difficult. A diesel with a clogged air filter, or even a "wet" paper air filter will make black "sooty" smoke under load. Depending on how bad the filters is plugged, it may behave perfectly normal under idle.
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Old 20-05-2016, 01:07   #10
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Re: Diesel losing power - black exhaust

Go with guy who says he's a mechanic. This engine is NOT turbocharged, right?



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Old 20-05-2016, 06:31   #11
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Re: Diesel losing power - black exhaust

OK, some additional information. The engine doesn't produce black smoke in the air - it produces clouds of black soot in the water. The prop is not fowled by rope or other and does turn freely when transmission is in neutral. I don't know if the transmission is a problem. I'll check the transmission today to the extent that I can. If it might be a clogged mixing elbow then I guess I would just take it off and inspect. Again, I am not a mechanic. Is this something I should be able to do without screwing everything up?
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Old 20-05-2016, 06:43   #12
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Re: Diesel losing power - black exhaust

How's the condition of the prop? Is it clean, scraped and free of growth and barnacles? Sometimes it doesn't take much to create an extra load on the engine and plug things up. You may also just need to wash out a plugged up turbo if it's turbocharged. It doesn't sound like the fuel mixture is getting enough air.

On a diesel, the problem can only be fuel or air.

One more thing to try, will it revv freely in neutral?
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Old 20-05-2016, 06:56   #13
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Re: Diesel losing power - black exhaust

Mixing elbow is usually four bolts and a hose that is clamped on, sometimes it can be tough to get to, but level of difficulty wise if you can change a fan belt, you can R&R a mixing elbow, there is a gasket on mine, I put a little grease on it and re-installed and had no leaks. Unless the elbow is clogged, there isn't much pressure in an exhaust.
Growth on a prop can absolutely cause this too
How many hours on the engine? Usually but not always a "bad" injector causes a knock at idle.
Majority of the time it seems what you describe is most often a blockage in the exhaust.
Worrisome thing to me is how do you know when an elbow is going to fail? Some when they rot away inside can let salt water back in through the exhaust and cause engine damage, depending on design, not all designs apparently can fail that way.

If there is a mechanic available with experience with your engine, it may be worth buying an hour or two of his time.

Filter is easy to check, remove it and if problem goes away, it was the filter, but I doubt it is the filter. Do you even have a filter, or do you have a screen that catches big pieces?
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Old 20-05-2016, 07:01   #14
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Re: Diesel losing power - black exhaust

I had the exact problem. Spoke to a lot of mechanics as well as people at my yacht club. Got everything from injector pump to engine is worn out. Ended up I figured out the problem myself there's a small screen on the air intake that was clogged up from who knows what I guess just years of building up I clean that out and everything's been fine since. The best I could describe it as the engine was just being choked out for lack of air. 2 simple and fast thing the check I would start there. Good luck
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Old 20-05-2016, 07:23   #15
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Re: Diesel losing power - black exhaust

Black smoke and soot are partially burned fuel. This typical occurs when the engine is overloaded, or not enough air is getting to the cylinders or excessive fuel is getting to the cylinders. It sounds like you've ruled out engine overload; prop turns freely, etc. Next check air intake (M30 usually doesn't have an air filter) and exhaust for blockage (mixing elbows clog regularly as mentioned above - how many hours does this one have on it?). If air intake and exhaust are clear; then you'll need to check injectors, injection pump, etc.

You might have to drain the coolant and remove the coolant tank to get at the mixing elbow bolts.
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