Smoking Yanmar

[Dean Strong]

The Yanmar 2GM20F in my 1990 Hunter 30 is driving me crazy.

In 2007 the mixing elbow coked up, I pulled it, cleaned it, and reinstalled it. Since then, the engine starts smoking after about an hour at cruising RMP (2850-2900, 80% of max rating), and leaving an ever present soot stain on the hull after of the exhaust port.

At idle there is always a slight soot discharge visible on the water surface (on return to port).

I've run tests to make sure the 3 bladed prop is the right size. I get 4100 rpm in neutral, and 3750 in trials, achieve hull speed of 6.7 knots.

At the end of last season, I had a mechanic pull and rebuild the injectors, and adjust the valves. At the same time I pulled the mixing elbow again, it was half blocked, and replaced it.

The engine runs at normal temperature--I can always touch the mixing elbow even after hours of hours running, our cool Northwest waters keep it just warm to the touch.

The engine runs well and consumes very little oil between annual changes. I had the oil lab tested this spring and all is normal. The engine has 700 hours on it. I never idle for prolonged periods or run at low RPMs; always at 75% to 80% of max rating.

Its not the elbow, not overheating, not injectors, not burning oil, not overpropped, but still emitting unburned fuel.

I think about just running it, but I know the elbow is getting clogged, and slow but steady damage may be occuring to the pistons and cylinders.

Diesel experts, please tell me where to go next with this.

 

[woodster]

The Yanmar 2GM20F in my 1990 Hunter 30 is driving me crazy.

In 2007 the mixing elbow coked up, I pulled it, cleaned it, and reinstalled it. Since then, the engine starts smoking after about an hour at cruising RMP (2850-2900, 80% of max rating), and leaving an ever present soot stain on the hull after of the exhaust port.

At idle there is always a slight soot discharge visible on the water surface (on return to port).

I've run tests to make sure the 3 bladed prop is the right size. I get 4100 rpm in neutral, and 3750 in trials, achieve hull speed of 6.7 knots.

At the end of last season, I had a mechanic pull and rebuild the injectors, and adjust the valves. At the same time I pulled the mixing elbow again, it was half blocked, and replaced it.

The engine runs at normal temperature--I can always touch the mixing elbow even after hours of hours running, our cool Northwest waters keep it just warm to the touch.

The engine runs well and consumes very little oil between annual changes. I had the oil lab tested this spring and all is normal. The engine has 700 hours on it. I never idle for prolonged periods or run at low RPMs; always at 75% to 80% of max rating.

Its not the elbow, not overheating, not injectors, not burning oil, not overpropped, but still emitting unburned fuel.

I think about just running it, but I know the elbow is getting clogged, and slow but steady damage may be occuring to the pistons and cylinders.

Diesel experts, please tell me where to go next with this.

some one correct me if i am wrong here ....but 4100 rpm's seems a little high for that engine...i have been under the impression that 3600rpm's max is the norm......as for the smoke....leaking valve guides or timeing comes to mind but dont go messing with that until it has been confermed by some one else.....

regards

woody

 

[kloudie1]

Next look for some kind of intake air blockage.. air cleaner, something in air cleaner cover or snout , intake valves too loose.. What you are discribing sounds like unburned fuel and if the fuel side is OK, then ya not getting enough air.
4100 might be a little high for the unloaded max speed, probably should be more like 3750, but that is not the problem here and is not something that needs to be changed since it is basically to keep the engine from scattering while unloaded. 4100 limit is ok for that..
EDIT: Bill, on the other post has a good point as well.. the exhaust side.. may be restricted.. look for a kinked or collapsed hose or a muffler full of gunk .. or a rat in the outlet??

 

[sailaway78]

Air Intake

If you have not looked at the air filter, do so. Those little foam Yanmar filters, will self destruct after several years. You can find a K&N that will fit exactly into the Yanmar filter can. The only time I have had the problem with visible soot in the water, is from a prop loaded with barnacles. As for the 4100 RPM, look at the tag on the front of the engine. I'm thinking the max for this engine is 3400. My 3 GM is 3400.

 

[tkanzler]

I seem to remember that my 3GM was 3400 rpm continuous, 3600 rpm intermittent (27 hp), but I don't have the manual. I did find this (below) on some web site. It says the 2GM has the same cont. and int. speeds. For what it's worth.

Beyond that, I don't have a clue as to why it's loading up with carbon. Any chance the altitude is very high, like Mexico City? I've had to do some tweaking on constant-power controls on hydraulic power units used at high altitudes due to the thin air. Again, FWIW.

http://www.ayesail.net/sailing/Hunter/YanmarManual/pages/09.html

 

[kloudie1]

Manual says 3400 continuous, 3600 1 hour rating and 3750 (+50, -0) as the governor limit.. That 3750 (which he reports as 4100) is in neutral with the speed control lever wide open.. That is the limit that keeps the engine from scattering should it become unloaded.. as if a prop shaft broke at cruising speed..

 

[Chuckbear]

Dean, If you have been running the engine at lower RPMs, even 2800, for a long period of time, there is a possibility that the cylinder walls have glazed. The Yanmar is a high RPM engine and it must be run at the higher end to prevent this. The only way to be sure is to have a mechanic remove the head and check the cylinder walls. The bad news is, there is no fix for this. Chuck

 

[Dean Strong]

Its not air intake

Thanks for these ideas. It isn't the air intake side . . . forgot to mention I replaced the air filter too.

There is no kink in the exhaust line, but I suppose the muffler could be clogged or partially blocked. Anybody had experience with that?

 

[tkanzler]

Manual says 3400 continuous, 3600 1 hour rating and 3750 (+50, -0) as the governor limit.. That 3750 (which he reports as 4100) is in neutral with the speed control lever wide open..

FWIW, I found that the electric tach on my 3GM was a little squirrely, and couldn't always be trusted. If I hadn't sold the boat this spring, troubleshooting that tach would have been high on the pre-lauch project list.

If I were the OP, I'd check it directly off the crankshaft with a portable tachometer, at several speeds. Not that the engine speed is part of the problem, but without good data, it's hard to troubleshoot anything.

 

[Dean Strong]

I had an interesting conversation with the service manager (Don) of my local Yanmar dealer today.

I have also been wondering if this smoke problem is related to being over propped (one mechanic has now suggested that I'm under propped, and the engine isn't reaching operating temperature).

For all the talk of cruising speeds and propeller matches, Don said the Yanmar tech manual says there is only one key factor that matters--that the engine reach exactly the rated max (one hour rating) rpm at wide open throttle (WOT) under a load. If it does not, increase pitch, if it goes higher, decrease pitch. That simple. Yanmar cares nothing of theoretical hull speed, or even the actual speed of the boat at WOT, only the rpm at WOT matters.

The 2GM20F is rated 3400 rpm continuous/3600 one hour, so the simple test is to run the engine at WOT. It should reach exactly 3600 rpm if the prop is correctly pitched. Rpm should be measured with a laser tach, and not the Yanmar tach, which is unreliable.

He said the engine would go higher in neutral with no load, and was not surprised that I obtained 4100 as mentioned above.

Black smoke is to be expected at WOT. Yanmar says to back off 200 rpm from WOT; the smoke should clear, and the engine can be operated for any length of time at that rpm (hence the continuous rating vs. one hour rating) Don suggested backing off 400 rpm for normal cruising. Backing off more may decrease fuel consumption.

On this and other SailboatOwners.com forums, I have read often that 80% of max rpm is the proper cruising speed, so I have cruised at 2750 to 2900 rpms. Another popular site, Yanmarhelp.com says 2900 for the GM and 4JH series. But according to Don, 3200 would be fine, but never less than 2720 (80% of continuous rating).

I hope to do a sea trial on Friday to finally determine if my prop (3 blade Michigan Sailor) is well matched. Meanwhile, what say you?

 

[Ted]

For all the talk of cruising speeds and propeller matches, Don said the Yanmar tech manual says there is only one key factor that matters--that the engine reach exactly the rated max (one hour rating) rpm at wide open throttle (WOT) under a load. If it does not, increase pitch, if it goes higher, decrease pitch.

I think Don got it backwards. You would need to decrease pitch to raise rpm's and increase pitch to lower rpm's.

 

[Bill Roosa]

Couple of things:
the state of the hull does effect the calculation. A dirty bottom will slow the boat and cause your WOT test to yield "over propped" results. The engine and prop are operating at max power but at a lower speed similar to going up a hill in a car. The prop sees a lower than (clean bottom) normal water flow past it and so can't get to as an RPM. So if your results yield a over propped indication I'd also check the bottom to rule that out as the cause before I went and "fix" the prop and have an under propped situation with a clean bottom.

 

[Dean Strong]

My mistake, not Don's.

 

[Steve Dion]

You should have your mechanic adjust your governor.

I would also agreee that you are probably under-propped. Our Hunter 31 (same motor) had a 15 x 12 (2 blade) which was over-propped for the engine. We move down to a 15 x 9 (2 blade) CDI prop which worked much better.