So the voyage of a Sea Scout donation of an above mentioned Hunter has a new wrinkle that I am hopefully looking for some guidance on. I was bringing her up the Chesapeake Bay this past Saturday and Sunday. Had a successful motor all day Saturday, stopped for the night, got up next morning continued to motor, had been running the engine for about 6 hours when the engine appeared to throttle down without touching the throttle and a little black smoke was seen. I keep going (at this point at the Bay Bridge) and am able to limp my way home for an additional 5 hours with the throttling down continuing to occur to the point that it is running just over idle speed (no idea on RPM because tachometer does not work). As I am making my way up the river the black smoke is getting more voluminous and I look back to see a trail of unburned diesel fuel on the water. So I actually touch the throttle at this point and pull back on it to actually align the throttle position with what I hear as the Idle speed and the trail of unburned diesel fuel stops, but I am still limping along. So I have an eventful landing with basically no propulsion, she is tied up in the slip, and now won't start since Sunday. I have been reading and researching and one article I read says "no throttle response and black smoke" is NOT a fuel issue. Another article says it is probably a clogged mixing elbox. Another article says a clogged water intake into the mixing elbow. Other articles say, getting the mixing elbow off is a real pain in the butt. I know no one call foresee what the problem is, but before I start ripping things apart, making the issue worse (although how can it get worse if the engine won't start), breaking things, or the like, I was hoping to get some sort of confirmation or advice or guidance that I need to maybe go in a particular direction. Thank you in advance for any advice. PS: The starboard rear quarter of the hull (behind the exhaust) is black with soot.
1981 Hunter Yanmar 2QM15
- Thread starter scruffy95
- Start date
I don't have experience with the QM but Diesels are Diesels and I do have experience with Diesels. Black smoke is partially burned fuel. Raw fuel out the exhaust is either unburned fuel left over from the partial burn on both cylinders or 1 cylinder not burning anything.
Partially burned fuel causes:
- intake air restriction (air filter clogged)
- exhaust restriction (mixing elbow).
- engine overload (Prop match or prop fouling)
- too much fuel being delivered (injector nozzel leaking)
One cylinder not burning anything:
- lack of compression (burned valve, hole in the piston, broken piston rings)
- carbon can become dislodged and block a valve open
- too much fuel (injector nozzel)
If you had a problem with one cylinder the engine may run on one only. If it won't start I would look at the air flow. As Kappy says, probably the mixing elbow. Long engine runs can cause carbon to dislodge and block if the engine is not normally run for long periods of time.
Good luck.
Partially burned fuel causes:
- intake air restriction (air filter clogged)
- exhaust restriction (mixing elbow).
- engine overload (Prop match or prop fouling)
- too much fuel being delivered (injector nozzel leaking)
One cylinder not burning anything:
- lack of compression (burned valve, hole in the piston, broken piston rings)
- carbon can become dislodged and block a valve open
- too much fuel (injector nozzel)
If you had a problem with one cylinder the engine may run on one only. If it won't start I would look at the air flow. As Kappy says, probably the mixing elbow. Long engine runs can cause carbon to dislodge and block if the engine is not normally run for long periods of time.
Good luck.
So a couple of other pieces of information for this diagnosis:
It really looks like I have come to the right place to hopefully find some answers. I am just glad I didn't have any youth onboard and until I can definitely resolve this issue and point to a definitive problem, I guess we will be a landlocked Sea Scout Ship...the trials and tribulations of a donated boat....
- Inside of the air intake, there is only a metal frame of what I am guessing was a filter that no longer exists
- I have not changed the prop and the donor of the boat does not believe her husband had changed the prop
- If the prop was fouled, would I have still been able to run for 5 hours
- There was no throttle response other than for it to stop the discharge of unburnt fuel into the exhaust
It really looks like I have come to the right place to hopefully find some answers. I am just glad I didn't have any youth onboard and until I can definitely resolve this issue and point to a definitive problem, I guess we will be a landlocked Sea Scout Ship...the trials and tribulations of a donated boat....
Lots of good info already advised. Maybe to help you actually actuate, here is a link to the complete service manual for the 2QM15 engine and transmission. Suggest you download to your computer and save.
http://j30.us/files/2qm15_servicemanual.pdf
http://j30.us/files/2qm15_servicemanual.pdf
1/ check the manual, your engine may lack an air filter. 2/ i second jake's excellent post of what all could be wrong. i add that if you run for a couple hours at the hotter temps, it can really play havoc with valves that maybe have not had their timing set for decades. if the valve timing is off, the proper amount of fuel is not all burned. when the exhaust valve opens, the unburned fuel has to escape out with the exhaust gas. 3/ change the oil and fuel filters, get a mechanic to set the timing and see if that fixes the problem. 4/ if you have to take off the exhaust el on a yanmar, check the threads; often they are reverse thread : right makes loose but left makes tight.
It is difficult to give an exact diagnosis from the other end of a digital line so I gave you generalities that could cause the reported complaint. Based on the fact that you ran the engine, and I assume that it ran well, all day on Saturday and for 6 hours on Sunday, the hard mechanical failures like pistons, rings and valves are probably unlikely but can not be discounted completely. After running for 6 hours on Sunday is when the issue appears to have developed and you ran for an additional
5 hours in a state of poor running. I hope that I understood this correctly.
When a diesel engine runs the exhaust contains a high amount of carbon and in a marine engine the exhaust is cooled quickly at the mixing elbow. This rapid cooling causes the carbon to come out of the hot gasses and be deposited on the exhaust piping if there is not enough air flow to carry it out through the exhaust system. The carbon deposits usually accumulate at the mixing elbow when the engine is run at low throttle for extended periods. This is one of the reasons why a marine engine should be run at about 80% of the maximum throttle. Carbon can also build up on the inside of the engine at the top of the pistons, on the exhaust valve and on the inside of the exhaust manifold. The carbon that has been deposited can heat up and glow red hot, a little like coal or charcoal but more brittle, when the engine is run hard for a period of time. When the engine is shut down and cools the glowing carbon cools and contracts, sometimes causing fractures in the structure of where it is deposited. When it heats up again it can be dislodged and cause a blockage in the exhaust path or jam a valve open.
When you lost engine speed and the exhaust turned black I believe that something changed and restricted the exhaust gas flow. The engine is a pump and when the air cant flow properly the back pressure slows the engine rpm. The governor is still set at a high throttle position and the fuel system still pumps the fuel required for a higher engine rpm. By reducing the throttle position you cut back on the over-fueling to the point where it matched the air flow and reduced the amount of smoke and raw fuel.
This is just a practical theory based on your description. I would remove the mixing elbow for inspection first.
You stated that there was a metal frame inside your air cleaner enclosure but no filter. That metal frame should hold a foam filter. If the frame is there but the foam is missing then it probably broke down and got ingested into the engine. Suggest that you remove the air cleaner enclosure and vacuum out the intake casting ports. Could be some pieces of foam blocking your intake valves. If you decide to remove the exhaust elbow, remove the four fasteners on the flange plate and remove the assembly to a vice to break it down. Note that the other end exits to sea through a water lift muffler (hopefully). Don't sink the ship. These engines are notorious for needing to be bled of air from the fuel lines as the return line goes back to the secondary filter instead of the fuel tank. That engine will give you a great education on diesels.
Let's start with something simple. Although you do need to replace the foam cone air filter. Have you checked the prop? You might just have wrapped an errant piece of line or something else on the prop and shaft while underway.
Was this issue resolved?
I’m running into a similar issue on my 2qm15. Losing top end RPM, noticed I couldn’t rev above 2,000, next run the top RPM was 1600, last run was 1,200.
Lots of black smoke and unspent fuel out the back. Plenty of water.
I’m running into a similar issue on my 2qm15. Losing top end RPM, noticed I couldn’t rev above 2,000, next run the top RPM was 1600, last run was 1,200.
Lots of black smoke and unspent fuel out the back. Plenty of water.
kappykaplan - Thank you, that sounds a lot easier than rebuilding or replacing. For the check I'm planning on just removing the mixing elbow from the exhaust manifold and visually inspecting for clogs. Does that sound right?
I was looking at potential other issues including the high pressure fuel pump, injectors, intake or even the oil rings on the piston(s) wearing out, all of which seem much more complicated than the mixing elbow.
I was looking at potential other issues including the high pressure fuel pump, injectors, intake or even the oil rings on the piston(s) wearing out, all of which seem much more complicated than the mixing elbow.
I replaced the exhaust elbow last spring, and turned a relatively easy project into a debacle. I learned that I should have removed the exhaust manifold with elbow attached and then proceed to attempt removing bolts, rather than squeezing into small engine compartment draped over engine and trying to loosen cooked in bolt. One bolt broke off and I tried to drill out broken bolt with extractor kit. That became a disaster and I stopped my attempts after almost destroying manifold. I searched high and low for a replacement manifold for the 2qm15 and found one in France for $350 plus shipping (heavy), but decided to try my luck with a fabricator down the street. Fabricator worked his magic (cast iron not easy) and done in a week. Replaced exhaust hosing also.
Thanks all! It ended up being both the manifold and mixing elbow that had failed. The diesel shop found extreme corrosion and clogged water jackets while attempting to separate, clean and save the parts. Thankfully a friend had a low hour 2qm15 on a dolly in his garage so we were able to locate the replacements, clean them up and install in a couple days. Aside from having to slide the engine over a bit on the mounts to remove the manifold it was all pretty straightforward. It fired right up after install with full power band, no smoke, more water and a now louder, unobstructed exhaust sound that will definitely take a little bit of getting used to.
Photos of the clogged parts below:
Photos of the clogged parts below: