M25xp - Grey smoke, unburnt fuel in exhaust

[kurt]

My '88 (1300 hours) has just recently begun giving off some smoke and I noticed looking behind the boat that fuel is coming out of the exhaust. I can see the black fuel in the water.   Boat starts fine, runs fine and is not overheating.  (I've previously sorted out the hx and am pretty sure that is not an issue.)

Should I just replace the injectors?  Check the valves for adjustment?  (Or hire a mechanic to further diagnose.)

Any insight appreciated.

Kurt

[Ron Hill]

Kurt : You aren't going to like my answer.  Your engine has not been run enough - you have less than 50 hrs/year over 26 years!!

I'd get a mechanic to look at your injectors/injection pump for excessive fuel (reason for black smoke)

A thought

[Noah]

Ouch! Not the answer I wanted to hear...with 550 hrs on my 1990 boat. I was thrilled that it had such "low" hours when I purchased..Ooops, maybe I need to rethink my bragging rights criteria.

[Stu Jackson]

and I noticed looking behind the boat that fuel is coming out of the exhaust. I can see the black fuel in the water.  

Kurt,

You didn't mention how much, nor how you know it is fuel. 

Does it occur just at startup?  If so, it could just be the crap in the muffler being blown out.

Fuel isn't black, but the exhaust gases are.

[kurt]

Hello Stu,

I guess I sail a bit too much (and motor a bit too little.) 

I started the boat yesterday and upon startup saw some black come out of the exhaust initially creating a bit of a black sheen on the surface of the water, then idled fine.  Lately though I see the smoke and black injected into the water only after the boat is well warmed up.  Am starting to think it is showing the symptoms of being slightly overpropped - I get to 2800 rpm max, flexofold 3 blade 10x15.  Next time out will try to examine the situation more closely at varying rpm levels.    Thanks for the advice/thoughts.

Kurt

[Noah]

A couple of comments/questions (consider the fact that I am not a mechanic). As I understand it, WOT (wide open throttle) should be approximately 3,000 RPM. So check your actual RPM with a handheld tach to confirm 2,800 is all you truly getting underway. And at WOT, what is your true speed over the bottom (GPS) in smooth water slack/no tide current? I am considering the Flexofold 3 blade 15x10 for my boat, so am interested in your comment about possibly being over propped...

[Les Luzar]

Noah has a good point about actual RPM vs tachometer reading RPM. There is the issue of optimum cruising RPM for your prop and engine. Based on your sail number, I would say you have a 1988 C-34 with an M25XP engine. This is what I have. With my fixed 3 blade Sailor Prop, my optimum cruising RPM is between 2,450 and 2,500 on my tachometer ( I am not sure of my actual RPM because I never tested it). My point however, is that in my case, if I increase the RPM above 2,500, my engine works harder, but I do not see any sustained increase in speed. I listen to my engine, observe the temperature gauge, feel the vibrations, and keep an eye on the exhaust from time to time. This way if I notice anything different, I can be aware of what is going on with my boat. Do this and then make up your own mind about your cruising RPM and engine load, and see if there is any difference with your smoke issue.

[KWKloeber]

My '88 (1300 hours) has just recently begun giving off some smoke and I noticed looking behind the boat that fuel is coming out of the exhaust. I can see the black fuel in the water.   Boat starts fine, runs fine and is not overheating.  (I've previously sorted out the hx and am pretty sure that is not an issue.)

Should I just replace the injectors?  Check the valves for adjustment?  (Or hire a mechanic to further diagnose.)

Any insight appreciated.

Kurt

Mis-propping the boat and loading up the engine can cause that, but that shouldn't be your issue!

ken

[MarkT]

The advice of not running your engine at full throttle is important. There is almost nothing to be gained in boat speed between 2500rpm and 3000rpm but fuel consumption and engine wear.Maybe a fraction of a knot.  If you are getting soot at 2500rpm then maybe you have a foul bottom and growth on the prop? If not then how smooth is the engine running? If it is not running happily then maybe a bad injector is your issue. That will lead to poor performance, soot and smoke but injectors are not cheap to replace or service. Investigate all your other options first. Changing injectors is a pretty simple job so if you decide to go that way then it is something you can likely do yourself.

Mak Tamblyn

[KWKloeber]

My '88 (1300 hours) has just recently begun giving off some smoke and I noticed looking behind the boat that fuel is coming out of the exhaust. I can see the black fuel in the water.   Boat starts fine, runs fine and is not overheating.  (I've previously sorted out the hx and am pretty sure that is not an issue.)

Should I just replace the injectors?  Check the valves for adjustment?  (Or hire a mechanic to further diagnose.)

Any insight appreciated.

Kurt

You can check to see if you have an issue with one cylinder, like a bad injector. 
If you have one bad injector, you may not notice it (rough running), but you can narrow it down with this procedure:

With the engine running and warm, crack open the 1st injector line nut, listen for a change in RPM and smoothness, record your observations, retighten, and move onto the next cylinder.   If you have an optical tach it helps.

If cyl A has a bad injector, you WILL NOT see a difference when cracking open that fuel line.

However if cyl A is bad, you WILL see a difference when cracking open cyl B or cyl C.

In other words, the cylinder on which you observe a "no change" -- is a bad cylinder.


Ken K

[kurt]

Thanks for the comments everyone.  A new datapoint - my boat use to rev to 2850 per meter on boat.  Now it can only go to 2400 and then I see the unburnt fuel pouring out via the exhaust.  At cruising rpm of 2200, maybe a bit of gray smoke but power output seems fine.  Forgot to check the idle max rpm.  Still thinking fuel issue - maybe the high pressure fuel pump?  Kurt

[KWKloeber]

Thanks for the comments everyone.  A new datapoint - my boat use to rev to 2850 per meter on boat.  Now it can only go to 2400 and then I see the unburnt fuel pouring out via the exhaust.  At cruising rpm of 2200, maybe a bit of gray smoke but power output seems fine.  Forgot to check the idle max rpm.  Still thinking fuel issue - maybe the high pressure fuel pump?  Kurt

Have you checked that all injectors/cylinders are hitting?

Ken K

[kurt]

Haven't done any diagnostics yet - just observing symptoms.  Besides a bit of gray smoke, all seems normal at start and cruise rpm of 2200 which is making me scratch my head.  I will check injectors and cylinder firing as suggested here soon and will update.  Thanks for everyone's advice/thoughts.  Kurt

[Ron Hill]

Kurt : Pull your injectors and take them to a diesel repair shop.  They will test them on the spray pattern and "pop" pressure.  You may want to buy new ones (injectors) or have the old ones rebuilt.

(Mainsheet Tech articles written on how to remove and replace injectors)

If that doesn't solve your problem then you may want to look at your injection pump.  Parts do wear out!

A few thoughts

[KWKloeber]

Ron,

I'm a big proponent of troubleshooting to pinpoint (or at least potential eliminate) causes before starting to arbitrarily pull components or replace parts.   That can be a disaster because you may never know what the real cause was, not mention the unnecessary hit on the pocketbook. 
And then you may get a false sense of security (unless it happens again because you missed the root cause.)

Not exactly Ockham's Razor, but.... - Take the easiest route first.  It takes 10 minutes (including getting two wrenches out of and returning it to the tool bag) to test if you have a bad cylinder/injector!  And then go deeper from there if necessary.

Ken K