Yanmar 3GM excessive smoke on cold start

YVRguy

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Jan 10, 2013
466
Hunter 34 Vancouver, BC
When we bought our Hunter 34 we noticed before the purchase that the engine produced a lot of smoke when being started completely cold (IE, hasn't been used in over 24 hours). The PO explained that this particular engine lacks a glow plug and therefore all heat in the cylinders has to be generated by the starting process cranking the pistons and creating compression. Therefore initially there is unburnt fuel creating a huge cloud of smoke. Later our mechanic told us we had one or two scored cylinders and therefore a small compression leak -further cause of the smoke.

Last year we bit the bullet for a complete engine rebuild and now the cylinders are pristine. Unfortunately this made almost no difference to the problem of the engine smoking on cold startup. Is this simply a characteristic of Diesel engines (or specifically 3GM's) that lack a glow plug feature? It's embarrassing to start our boat sometimes in our high-end marina and maddening that a complete rebuild failed to solve this problem.
 

dmax

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Jul 29, 2018
495
O'Day 35 Buzzards Bay
Diesel engines do not emit a lot of smoke on startup when running correctly. My current engine has glow plugs but I had a 2GM Yanmar previously and it did not smoke on startup, cold or warm. What color is the smoke? White is generally water/steam, bluish is oil and gray would be too much fuel. Take a good look at your engine oil, see if there's any evidence of water, anti-freeze or fuel in it.
 

leo310

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Dec 15, 2006
455
Catalina Catlina 310 Campbell River BC
cycle the engine with the fuel off to heat the cylinders first (5-15sec) then add fuel to start.
 
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Jan 4, 2006
3,907
Hunter 310 West Vancouver, B.C.
A properly tuned Yanmar 3GM and my 22 Y.O. 2GM20F does not smoke after the first puff out the exhaust. NONE ! And this includes one start at -5 deg.C.

The PO explained that this particular engine lacks a glow plug and therefore all heat in the cylinders has to be generated by the starting process cranking the pistons and creating compression.
Pure unadulterated bull:poop: ! The compressed air needs to be hot, not the cylinder walls.

Sorry, but there is still a problem with your engine that was not addressed during the rebuild.

Just out of curiosity, who did the re-build ?
 
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Jan 7, 2011
2,920
Oday 322 East Chicago, IN
My 2GM20F doesn’t smoke, but it belches a little unburned fuel on a cold start. Once the engine is running, no smoke.

Greg
 

YVRguy

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Jan 10, 2013
466
Hunter 34 Vancouver, BC
My 2GM20F doesn’t smoke, but it belches a little unburned fuel on a cold start. Once the engine is running, no smoke.

Greg
This is our situation exactly but it "belches quite a bit of smoke initially. Enough to make anyone standing on the dock nearby to be uncomfortable. But once started that's it for the day. Any subsequent start that day is instant. In summer even next day is very quick.
 

YVRguy

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Jan 10, 2013
466
Hunter 34 Vancouver, BC
Diesel engines do not emit a lot of smoke on startup when running correctly. My current engine has glow plugs but I had a 2GM Yanmar previously and it did not smoke on startup, cold or warm. What color is the smoke? White is generally water/steam, bluish is oil and gray would be too much fuel. Take a good look at your engine oil, see if there's any evidence of water, anti-freeze or fuel in it.
It's definitely excess fuel being burnt off. This ONLY happens at cold start and once the engine turns over it runs 100% smoke free. There is no issue with water in the fuel or oil.
 

YVRguy

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Jan 10, 2013
466
Hunter 34 Vancouver, BC
cycle the engine with the fuel off to heat the cylinders first (5-15sec) then add fuel to start.
We have tried this suggestion but we've been a bit timid doing it for very long. I'll experiment with trying it for a bit longer.
 

Tedd

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Jul 25, 2013
467
Between Boats near Vancouver, BC
If none of the other suggestions works, it's worth checking for a missing valve stem seal. It's unlikely a seal would stop working so soon, but it's possible that one got missed during the rebuild.
 
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AndyBC

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Dec 31, 2020
8
Hunter 28 Vancouver
My 30 yr old 2GM20F gives off white smoke for a few seconds after the first cold start of the day, and then smoke-free for the rest of the day
 
Jun 2, 2011
345
Hunter H33 Port Credit Harbour, ON.
It's definitely excess fuel being burnt off. This ONLY happens at cold start and once the engine turns over it runs 100% smoke free. There is no issue with water in the fuel or oil.
Are you trying to start the engine with the fuel lever at idle? If so, try to set the fuel at about 3/4 to see if it fires up quicker. The theory is, the fuel rack will be at full throttle with the governor weights stationary but as the weights start to spin during engine cranking the fuel delivery goes to idle, obviously with the throttle lever in the idle position. When the engine is cold the injectors are injecting only enough fuel to keep the engine at idle speed, which is good when the engine is warm. When the engine is cold it requires extra fuel to get the burning process started. By setting the fuel lever at 3/4 it puts tension on the governor and holds the fuel rack at full fuel during the starting phase. Be ready to pull the fuel control back to idle as soon as it starts.

It sound counter intuitive in that the white smoke indicates unburned fuel, which is true, but it is a situation where the fuel is only partially burning. You need to stoke the fire to get it burning quickly.

Some of this explanation concerns the inner working of a diesel engine fuel control and governor system. Try not to get too wound up about the mechanics.

Best of luck.
 
Nov 6, 2006
9,226
Hunter 34 Mandeville Louisiana
Couple of things to check, the injection timing and the injectors themselves..
 
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NYSail

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Jan 6, 2006
2,742
Beneteau 423 Mt. Sinai, NY
Had a 3HM35f and now a 4JH4E..... no smoke at all. It’s not the lack of glow plugs. Did they change / service all injectors as part of rebuild?

Greg
 
Jun 8, 2004
2,563
Catalina 320 Dana Point
When I first got mine (~450hrs) it smoked some on cold start, had a mechanic (long retired, unfortunately) who insisted I add Cetane booster. That was over 10 years ago so I kind of forgot that it had stopped smoking shortly after. Think it got better on the first tank, and gone with the second.
Couldn't hurt to try before tearing down your newly rebuilt engine.