Black Exhaust discharge

[Teko]

My boat has a 40yr old Volvo penta md5 it's fairly clean and been well cared for. But when I run the engine too hard (there's no rev counter) I get a black coloured slick in the water behind the boat. I dont think its diesel because it doesn't give the rainbow colours you get when spilling fuel. Could it be oil? Although I dont seem to be losing any? Only other though was maybe it's carbon from exhaust been blown out?
I'm not a mechanic engine runs great and starts well too. Just worried it might need work doing.


Thanks

 

[greggron]

Have you antifouled and cleaned your prop this season?

 

[Teko]

No not yet she was ashore in June last year. I think it was doing it when the hull.was clean too

 

[black mercury]

Does it have a cold start lever you pull out for cold starts? If so make sure it is returning fully down at the pump after the engine starts. Depends on how hard you are running the engine. Full throttle operation especially if you are still tied to a pontoon is going to cause black smoke. At what point on the throttle travel does the black smoke start?

 

[Teko]

Hi black mercury. It does have a cold start and I should check it returns fully. It's only.when the throttle is opened up that it creates the black slick. But its definitely not smoke I get a little white steam/smoke.but.nomther colour the black.slick is in the water

 

[ANDY_W]

If it is any help, I have found that when my propeller has been fouled with weed the boat may only do 3 knots even with the throttle wide open. Looking over the stern, I have seen black soot like deposits from the exhaust in the water. Effectively, the weed is preventing the engine turning fast enough to properly burn the fuel. Once the weed is cleared all is well again.

Assuming the boat was not bought new by you and that there is no excessive fouling of hull and prop, I suggest the possibility that either the fuel system is delivering too much fuel or perhaps a slightly oversized prop was fitted before your ownership.

I should say that I am not a mechanic so my advice is intuitive rather than from authority!

 

[black mercury]

Hi black mercury. It does have a cold start and I should check it returns fully. It's only.when the throttle is opened up that it creates the black slick. But its definitely not smoke I get a little white steam/smoke.but.nomther colour the black.slick is in the water

Yes, check the cold start lever on the engine springs back after you start engine and the revs increase. Just because you push the hand lever in don't assume the lever on engine has returned. It should, but may need encouraged with a tap!

 

[VicS]

Hi black mercury. It does have a cold start and I should check it returns fully. It's only.when the throttle is opened up that it creates the black slick. But its definitely not smoke I get a little white steam/smoke.but.nomther colour the black.slick is in the water

If it starts easily, runs well and does not use excessive quantities of lube oil dont worry about it unless it starts to produce clouds of black smoke.

The injector probably just needs cleaning/ servicing. Has it been done every 3 years as specified in the manual ?

 

[jwilson]

Seen this many times on old Volvos, apart from the sooty water engines running well and reliably. As VicS says, possibly a poor injector spray, maybe too rich.

 

[Teko]

Thanks everyone doesn't sound too serious then. Plenty to think about with the above info! I'll report back with what I find

 

[Daydream believer]

If it starts easily, runs well and does not use excessive quantities of lube oil dont worry about it unless it starts to produce clouds of black smoke.

The injector probably just needs cleaning/ servicing. Has it been done every 3 years as specified in the manual ?

My Volvo MD 2020 has now done over 4500 hours (15 years) & runs well so I have taken casual advice from a couple of people to leave well alone. It does use a bit more fuel than i would like but smoke etc is OK & does not burn oil .
I have never noticed that about injectors in the manual. I still wonder if a set of exchange injectors should be fitted.
What is your opinion Vic?

 

[RobinBirch]

Maybe worth checking for blockages/delamination in the exhaust elbow/Vetus type house. If there is a restriction the engine doesn't breathe properly and as the load/fuelling is increased the unburnt fuel manifests itself as black sludge out the back end. If your 40 year old engine is connected to a 40 year old exhaust it is worth a check -somewhere along the timeline someone on start up will have forgotten to open up the sea water intake and given that exhaust a roasting.

 

[Homer J]

It could be all that’s described but more than likely it’s a dirty prop. Unburned diesel will leave a trail in the water exactly as described and often will not have the rainbow.

 

[jwilson]

My Volvo MD 2020 has now done over 4500 hours (15 years) & runs well so I have taken casual advice from a couple of people to leave well alone. It does use a bit more fuel than i would like but smoke etc is OK & does not burn oil .
I have never noticed that about injectors in the manual. I still wonder if a set of exchange injectors should be fitted.
What is your opinion Vic?

The 2020 is a relatively modern engine. All the ones I've seen and used that did the black soot in the exhaust thing were the old huge-flywheel jobs built from the late 1950s to maybe about 1980 - a surprising number still around and running. The one I owned was a 7 hp MD1 with a dynastart pushing an 8-ton sailing yacht, hence if it was running it was usually at a fairly high throttle.

 

[MAURICE]

Next time you go out (with clean prop and hull) let the engine warm up until normal temp. Then go full throttle for about ten mins. this will help burn away any deposits and should help clean the cylinders injectors etc.

 

[rich]

I would suggest that you fit a rev counter,

 

[Spirit (of Glenans)]

On modern diesel car engines there is a Diesel Particulate Filter (DPF) which collects soot and burns it off periodically. High rev driving on motorways aids the process and prevents complications requiring a costly Forced Regeneration, (don't ask me how I know this!). By the same token, low rev running of your boat engine allows a soot build-up in the exhaust, which can be prevented by running at high revs from time to time, the traditional "Italian Tune-Up".
What you are seeing is probably the byproduct of the soot being burned off when you run it at high revs.
Here is a link to an RAC article on diesel emissions:
https://www.rac.co.uk/drive/advice/emissions/diesel-particulate-filters/

 

[Teko]

Thank you all. You've put my mind at rest. I shall have to boat out to scrub off and clean the prop before the round the island race but it will only be dried out between tides but I will be on the wall at ryde next weekend so will look at the prop condition. A rev counter would be a good move. Not sure of the cost associated though.