M25 Blowing smoke at half throttle

Kevin A Wright

Member III
Ok I'm at my wits end and looking for ideas.

I had to replace a broken water flange below the thermostat housing which required pulling the head. While we were at it replaced the valve guides. After buttoning her up, she runs perfectly in neutral and will run right up to 3000 rpm. But once put in gear runs rough and starts blowing huge clouds of black soot at about 1500 rpm.

Pulled the exhaust system to the muffler and checked for obstructions, none. Checked the injectors and one was bad so replaced all three. No change. Had a diver clean the prop and make sure it was clear. No change. Found failed motor mount so shaft was binding a bit. Replaced and realigned. No change. Rechecked valve clearances. No change.

I've run out of ideas. Any thoughts out there on what I should look at?

Kevin Wright
E35 Hydro Therapy
 

fool

Member III
Hello Kevin,

Engine overload? What's the transmission looking like these days?

M25 with Hurth transmission? What vessel size? Note: you're investigating the "transmission woes" thread ahead of this suggestion.

Max
 
Last edited:

Christian Williams

E381 - Los Angeles
Moderator
Blogs Author
The common wisdom on this is Internet-ready. I have no idea how to apply it, but there are a lot of diesels out there and we're not alone.

 

Kevin A Wright

Member III
I'm starting to wonder about the tranny, but not sure how to check it. It is acting like too much load which is why I checked prop for fowling an shaft alignment. PO replaced the tranny probably 10 years ago and it's not a Hurth. Just not sure how you go about checking that. Again was running fine before I started working on it. So has sat idle for about 2 months. Oh, and I also changed the air filter with no results.

Kevin Wright
E35 Hydro Therapy
 

fool

Member III
I doubt anything you've done to improve the engine would cause transmission failure, just bad timing.

A photo to ID the transmission or PO records might help. Are you able to do a bit of boat yoga? While you're under there at physically impossible angles check the transmission oil level, as well as appearance and smell.

The Hurth would have you check the level with the dipstick "out". Without knowing which transmission is in there I can't tell if the dipstick should be threaded back in or left thread out to check the ATF level.

Should you decide to change the transmission oil avoid pulling drain plugs out of the inaccessible underside. Better perhaps to use a suction extractor through the fill tube. Dexron III ATF is likely recommended and a replacement for earlier ATF fluid types.
 

Kevin A Wright

Member III
Heading to town to check the tranny now. I have to drain and pull the heat exchanger to access the dipstick so it doesn't get done too often, but I did it about 2 years ago and drained and replaced with new ATF fluid while I was at it. The tranny is a german make I believe and starts with Z if I remember right. Have all the records and manuals for it, but on the boat of course.

Kevin Wright
E35 Hydro Therapy
 

Kevin A Wright

Member III
Tranny is a ZF 10M. Fluid looked a little dark but not abnormal for 125 hours since last change. Changed it anyway just since it is a bear to get to (have to remover the heat exchanger to get to the dip stick). Still blowing soot at 1500 RPM and won't rev higher than that under load. Can turn the shaft easily by hand when in neutral since realignment.

I've got the local diesel mechanic working with me on this and he is running out of ideas as well. Kind of frustrating since I had planned to leave tomorrow to move the boat to a new slip in Port Orchard. Have already given my notice to Port of Port Townsend so just have till month end to get under way. And don't really want to pay for 2 slips any longer than I have to.

Kevin Wright
E35 Hydro Therapy
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
One place for friction on that shaft would be the cutlass bearing, IMHO. I believe that you have eliminated any shaft drag on the side of the frp shaft alley.

A diver might want to check it for free turning, after first disconnecting the four bolts between flange and trans. Mark the orientation of the two parts, first.
Kind of a long shot, but maybe.......
 
Last edited:

Dave G.

Member II
It sounds like it could be a compression problem if it doesn't have power under load. My guess is something went wrong with the replacement of the head and or gasket. Black smoke is unburned fuel caused by lack of complete burning also symptom of low compression. Also check that your decompression lever is operating correctly and in correct position.
 

fool

Member III
How full is your diesel tank?
Weird question to ask for black smoke... the fuel tank vent may be blocked and inhibiting return fuel from the engine, a case of filling the tank until fuel goes into the vent. This symptom is rated rare by Peter Compton, but rare occurs more often than not at all.

Along those lines, the return line should not be restricted. The recommended test is to disconnect the return line close to the tank and put it into a bucket to see if fuel is returning when the engine is started. Use caution and proper PPE. If memory serves this is still on the high pressure fuel side of the system and could inject fuel under the skin, use caution is worth repeating.

Two seating washers under the injector? A possibility since the injectors were replaced. I’ve done this with oil filter gaskets before...it doesn’t work to have two where one is advised.

We‘re digging deeper into the weeds as you’ve checked the more common causes. I’m trying to figure out a way to test if your transmission is causing an overload. Any chance you can turn the engine over mechanically? Release the compression lever to take the engine out of the equation and shift into gear? If the transmission and propulsion assembly turn freely we can probably look elsewhere. If the broken engine mount put strain on the propulsion assembly (everything other than the engine) could that mis-alignment be the cause of a transmission woe? How do we (ie you) test this hypothesis?
 

Dave G.

Member II
The transmission & alignment can be checked for friction/binding by just putting in neutral and rotating prop shaft by hand. If you can rotate it fairly easy by hand then I would think you need to look elsewhere for the issue.
 

Kenneth K

Sustaining Member
Blogs Author
How full is your diesel tank?
A blocked fuel vent could lead to fuel starvation once the consumption of fuel leaving the tank starts to create a vacuum, but just running the electric (if you have one) fuel pump (mine comes on as soon as the engine panel os powered) is all that's required to "activate" the fuel return line.

The electric fuel pump provides a flow of fuel to the high pressure injector pump. If the engine is not running, all the fuel in the lines is recycled to the return line. If the engine is running, a small quantity of what's in the fuel line is consumed by
the injector pump and a lesser quantity is thus recycled through the return line.

The return line IS pressurized, but it's low pressure, i.e., whatever the electric pump puts out. The fuel return line is a convenient way to drain your fuel tank. I pumped out 5gal of diesel in under 5 min using mine.
 

Dave G.

Member II
A blocked fuel vent could lead to fuel starvation once the consumption of fuel leaving the tank starts to create a vacuum, but just running the electric (if you have one) fuel pump (mine comes on as soon as the engine panel os powered) is all that's required to "activate" the fuel return line.

The electric fuel pump provides a flow of fuel to the high pressure injector pump. If the engine is not running, all the fuel in the lines is recycled to the return line. If the engine is running, a small quantity of what's in the fuel line is consumed by
the injector pump and a lesser quantity is thus recycled through the return line.

The return line IS pressurized, but it's low pressure, i.e., whatever the electric pump puts out. The fuel return line is a convenient way to drain your fuel tank. I pumped out 5gal of diesel in under 5 min using mine.
Kenneth thanks for the tip on using the return line for fuel drain. Never thought about it and that's a keeper !
Not sure where Kevin the OP has gotten off to. I'm guessing he solved it or is busy taking the head back off ?
 

fool

Member III
The transmission & alignment can be checked for friction/binding by just putting in neutral and rotating prop shaft by hand. If you can rotate it fairly easy by hand then I would think you need to look elsewhere for the issue.
Check‘s okay in neutral according to post #8. Neutral wouldn’t overload when in gear, which is the symptom. How can you test a transmission‘s function when it is in gear without engaging the engine?
 
Last edited:

fool

Member III
A blocked fuel vent could lead to fuel starvation... {SNIP}

The return line IS pressurized, but it's low pressure, i.e., whatever the electric pump puts out. The fuel return line is a convenient way to drain your fuel tank. I pumped out 5gal of diesel in under 5 min using mine.
Agreed, a blocked vent could lead to fuel starvation, not what we’re concerned about with black smoke. I was thinking along the lines of back pressure preventing the return of fuel to the tank, which could cause black smoke. Again rare, but on Peter’s checklist of potential culprits.

Good to know on the return line pressure, should have remembered these tubes are sometimes rubber and held on with hose clamps. One wouldn’t do that with a high pressure line. If they were high pressure clearing back pressure from a blocked vent wouldn’t be much of an issue. Doesn’t hurt to use PPE and caution anyway.

https://boatdiesel.com/Store/item-detail.cfm?ID=Books-1 for Peter Compton’s Trouble Shooting Marine Diesels. If you already have Don Casey’s Complete Illustrated Sailboat Maintenance Manual look no further, Peter’s book is included in this guide.

Nigel Calder’s Marine Diesel Engines; Maintenance, Troubleshooting, and Repair doesn’t say much about transmission troubleshooting other than adjustments to the cable and clutch. Tons and tons of other info, especially transmission types and gasket replacement, just not much on the transmission trouble shooting side...
 

Kevin A Wright

Member III
Well the verdict is in. Low compression in cylinder 1. Looks like I've got a bad ring. Ugh.

So now starts the question of tear down and rebuild, find a rebuilt block,, or repower with new. Not really what I wanted to hear.

Kevin Wright
E35 Hydro Therapy
 

fool

Member III
Ow. Sorry to hear this verdict. Would have gone to this possibility right away if it were white smoke, go figure. I trust you’ll keep us informed of the chosen solution to this conundrum.
 

Dave G.

Member II
Question Kevin, was the engine pulling load fine before you R&R'd the head & replaced the valve guides ? If so I would think the odds of a ring going bad during that process is remote. I would check adjustment on decompression assembly first. If that checks out then pull the head and check that the valves are seating correctly in the head & springs are in tact. If you have a valve hanging open it would give you low compression reading. Also check the gasket and head to block surfaces for defects and debris. It doesn't take much of a tolerance issue to cause a problem in a diesel. Is the smoke definitely black ? Bad rings usually produce white/blue smoke due to oil getting sucked passed the rings into combustion.
 
Top