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Earl34Mainship 10-14-2016 06:37 AM

370 HP Yanmar black smoke
My 370 HP Yanmar ran fine until last week. It's a 2005 with 1100 hours. I noticed that it just didn't sound right - a little noisy and not the usual Singer sewing machine hum... Looking over the stern, I was seeing black smoke collecting on the transom and swim platform. The boat is currently on the hard because of Matthew. Thoughts about problem and fix?

C lectric 10-14-2016 07:15 AM

Prop damage, something fouling the prop, dirty air filter, lazy turbo.
-Since you are out the prop fouling may not be the case or would have been see.
- Prop damage not always visible. Damage can cause a sharp increase in engine loading.
-Dirty air filter can reduce the air available to the engine so the fuel is not burned fully producing smokier operation.
-lazy or seizing turbo. turbo can suffer damage from a poorly designed and set up exhaust system which allows sea water to back up into the turbo. It will rust which can seize, or partly so, the turbine but also the rust will increase the clearance reducing air delivery so producing more smoke. That might account for the change in sound.

The exhaust and turbo are going to take some work to examine and determine if that is the problem. The others can be checked while the boat is out although the prop will have to be checked by a prop shop as I've seen tweaks that are difficult to spot visually yet can cause trouble.

There were a few years, Mainship early production, that had real problems with exhaust designs and flooded turbos. Get yours checked.

Go to Seaboard Marine and do some exhaust system reading and then look at your system. All the info is applicable regardless of engine mfgr.

Tony's Tips - Seaboard Marine

Understanding Low Power Troubleshooting - Seaboard Marine

You might also look at This subject has come up many times. Although different vessels the theme is the same/similar, overloading of the engine and or exhaust system problems.

Just a few thoughts to get you started.

Earl34Mainship 10-14-2016 07:37 AM

Thanks C lectric.. I looked at the prop and there's no fouling... re the air filter.. there's a circular screen on top of the engine and if there's a separate air filter, I don't know about it... I can check the Yanmar books about a separate air filter. The turbo is way above my level of expertise... Thanks C lectric..

Marlinmike 10-14-2016 07:56 AM

Good diesel mechanic could easily go over your Yanmar, take measurements and give you a diagnoses.

How does the fuel bowl look, clear/clean or black and filled with debris?

Moonstruck 10-14-2016 08:00 AM

My guess is the turbo. The louder exhaust tends to make me think this is the problem. The spinning turbo breaks up the harmonics of the exhaust and acts somewhat to muffle the sound. Black smoke (unburned fuel) would be a natural result. Just my opinion. Could be wrong and often am.

sunchaser 10-14-2016 08:34 AM

Celectric mentioned exhaust elbow and salt water back into turbo. Dave Marchand will likely weigh in on this area, a sometimes troublesome installation for Mainship. A few good pictures of that specific area may be helpful.

BTW, any maintenance done on motor prior to issues arising?

Earl34Mainship 10-14-2016 09:06 AM

Marlinmike.. thanks for the reply.. the fuel filter bowl is clear.. noting in there that shouldn't be there.. The nearest Yanmar certified shop is $110 miles away each way.. but Frank is a Yanmar genius...

Moonstruck.. Leaving the dock was an easy reverse with the current so I'm hoping that no water backed into the turbo... Frank will know what's wrong.. thanks for your reply...

Also, for everyone's information, I'll post what the problem/fix will be.. thanks all..

DavidM 10-14-2016 09:15 AM

While I agree with all of the possible causes noted above, I don't think that this model of Mainship has an exhaust system problem. I owned one in the late 2000s. The exhaust from the turbo drops down to a water lift muffler, then up and out an underwater exhaust at the transom. To prevent water backing up into the turbo, there is an equalization line from the bottom of the lift muffler out the side of the boat. This limits the level that water can build up in the lift muffler and makes it unlikely that it gets back into the turbo.

But anything could have happened and it is easy to check the turbo. Have your mechanic unbolt the v-clamp holding the exhaust elbow to the turbo and remove it. Then you can look into the turbo. A good turbo should have no rust, or roughness, just a smooth coating of soot.

The air cleaner is a cylinder of foam over a perforated frame. Sounds like the foam is gone. Here is a long shot: the foam element got sucked into the inlet turbo, chopped up and blown through to the after cooler where it plugged the air side. This would cause the symptoms you now see. Or maybe the lack of a foam element let soot get sucked in which fouled the after cooler.

Also, if you haven't serviced the air cooler since new, it is past time to do so. Go to and look at Tony's tips for articles on servicing the air cleaner. Look particularly at the Cummins article as it gives more details than the Yanmar one.

If after checking the turbo and after cooler they are found to be good, then after launching in the spring with a clean bottom and prop, check for wot rpm using a phototach as the dash tach can be off. Should be at a minimum 3,300 rpm. If not then it is probably weight growth and you should repitch an inch or so down.


Earl34Mainship 10-14-2016 01:53 PM

Thanks David... I also had heard that some earlier Mainships had water in the turbo and often that was caused by being towed backward and the wash from the tow boat was Mainships.. I'll be at the boat tomorrow and can check on the foam filter. I've owned this boat since 2012 and I haven't had the air cooler serviced so I'll look into that. The bottom is relatively clean - a diver does that once a month, and I saw it on the hard and there isn't any growth.. I'll check for 3,300 rpm's at wot.. thanks for the suggestions David..

bayview 10-14-2016 02:51 PM

Bottom line is too much fuel, too much load, or too little air.

Too much load could be bent prop
Too much fuel could be injectors or IP
Too little air could be filter or turbo

Turbos are easy to check. Spin with your finger. check all air hoses and clamps for air leaks.

props require haul out

Just remove filter to check it with a light or run the boat with it off.

IP and injectors require pump shop but this wild be my last bet.

Junk on the prop or bent prop is where I would start.

Irish Rambler 10-14-2016 02:52 PM

My money's on the turbo.
Very easy to take it off and reinstall, shop around for a recon.

Earl34Mainship 10-14-2016 03:30 PM

Thanks Bayview... your suggestions are now on my check list.. I haven't hit anything with the prop and the boat is on the hard now.. no visible damage.. but worth investigating.. the filter is an easy check... I wash it occasionally, and it appeared to be OK the last time I washed it... easy check also..

Earl34Mainship 10-14-2016 03:33 PM

thanks Irish Rambler... I may have to move the boat to a marina once the hurricane debris is removed from the yard and I can check the turbo...

Narbonne.. beautiful city.. I picked up a canal boat there for a week on the canal du midi.. fantastic journey...

Ski in NC 10-15-2016 10:55 AM

If engine does not sound smooth at idle, I would not expect that to be caused by a turbo. If turbo is seized or rotted or otherwise damaged, engine should still run smooth.

I'd start with pulling rocker cover and checking valve gear. Easy to do, zero cost. Check for broken or bent pushrods. If ok, check valve lash. Also easy and zero cost.

Had one of these break a pushrod, symptoms were the same.

Codger2 10-15-2016 11:31 AM


Originally Posted by djmarchand (Post 488574)
While I agree with all of the possible causes noted above, I don't think that this model of Mainship has an exhaust system problem............

My money is on djMarchand! I've followed his writings for years and have found them to be excellent advice.

DavidM 10-15-2016 12:05 PM


Thanks for the vote of confidence, but I will always listen to Ski.


Earl34Mainship 10-18-2016 07:17 AM

Thanks Ski, Codger2 and David as well as others who offered expertise advice. The boat is still in the yard and it'll be another week before I can get to it. I'll post back what was wrong and what the fix was.. thanks again.,

C lectric 10-18-2016 06:07 PM

Do as Ski suggested. He's a pro, i,m just an owner.

but also check the boost pipe or hose between the turbo and the air manifold.
If a clamp has loosened, or a hose split, it may allow a hose to bleed air creating noise and at the same time the engine doesn't get all the air it needs for a clean burn. It does not have to come off the connection to cause trouble, just allow a leak.

Irish Rambler 10-18-2016 11:39 PM

Hi apologies for going a tad off thread.
We love Narbonne because of it's history, strategic position, Littoral weather pattern, access to the Med the Midi etc.good airport/motorway/train connections. We're glad you enjoyed your visit.
All the staff at the company you rented the boat from walked out because the company wouldn't release the funds to repair damage on the boats. there's a new untried crew but I couldn't recommend them to anyone on TF.

Pgitug 10-24-2016 08:33 PM

370 HP Yanmar black smoke

Originally Posted by Earl34Mainship (Post 488551)
Thanks C lectric.. I looked at the prop and there's no fouling... re the air filter.. there's a circular screen on top of the engine and if there's a separate air filter, I don't know about it... I can check the Yanmar books about a separate air filter. The turbo is way above my level of expertise... Thanks C lectric..

A quick check of the turbo is to take the air cleaner off and with a WOOD stick or dowel try to spin the turbo intake blades. If they do not spin free with very little effort you may have found your problem..

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