Yanmar 2GM20F RPM issue

cspaniel · #1 07-02-2011

Hi, After two weeks of trying to figure this one out I am out of ideas, and need help. (sorry for the long post)
I have a 1987 2GM20F on my new to me sailboat with probably 1500 to 1700 hours on it. After working great for almost four weeks, 2 weeks ago it started to develop a problem that I cannot figure out what it is (neither the mechanic apparently).

The engine starts and idles great, no issues at all, no smoke, nothing. In Neutral, RPMs hit 3700-3800 with the throttle full.

Now the problem, in gear, the engine runs great between 800 and 2400 RPM, after that, I can push the throttle for another inch and nothing happens, then it jumps to 2900-3000 and that is the max it gets with full throttle. A little bit of black smoke (very little) when it hits that high RPM.

Background:
Boat sat for a year before I got it.
Before commissioning (May this year), we change oil, change oil and fuel filters (both primary and Racor 500) and impeller, and installed a new mixing elbow. Also installed new air filter.
When this issue started to happen I check the fuel filters, they look like new, no water in the Racor filter.
Mechanic came and check everything and said everything looks good. Then when the problem didn't go away, he thought the issue was with the valves and he adjusted them. None of this helped.

The propeller was never fouled during all this time. I dove and checked it myself as well as the shaft, no issues there. Propeller is original, has not been changed.

Today, after the mechanic adjusted the valves, the issue persisted. I decided to give the bottom a clean (I thought it wasn't bad) and after that the engine worked great, no issues at all. We motored for almost 1.5 hours (no wind at all) and all that time it work great, was able to hit max RPM and kept it cruising at a stable 2800 RPM.

Well, after sailing back (wind picked up) I started the engine, lowered the sails and gave it a try to make sure the problem went away, nope, same thing, between 800 and 2500RPM, fine, then push throttle for 1 inch and nothing happened, then a bit more and RPM jumped to 2900-3000 and a bit of black smoke. Why???? why is it intermittent????

Before I continue to spend money with the mechanic, it would be great to hear your suggestions. I am really at a loss here.

Thanks for any advice.
-Abe

ps.
-Tank vent is not clogged
-Stuffing box leaks about 10-15 drops a minutes, keeping the temp about 25 to 30 degrees above water temp (95-100 degrees)
-It is not a problem with the throttle cable, it does the same when disconnected and push it from the engine. BTW the control at the engine moves smooth with or without the cable connected.
-Stop cable goes all the way in, no problem there.

svHyLyte · #2 07-03-2011

From what you are describing, the issue sounds more like a throttle linkage issue rather than an internal engine issue. We had a 2GM20F on our former boat and it was very reliable and predictable. Throttle, shift and fuel shutoff linkages sometimes were problematic. I suggest you trace out the entire linkage system and ensure that all is well with that. In particular check the last mount for the cable before the connection of the wire to the throttle.

FWIW...

shogan50 · #3 07-03-2011

Yes, first check the linkage. In fact, you might disconnect it and try to duplicate problem by manipulating the power lever, perhaps at the dock. If you can't reproduce, probably linkage. If you can, probably governor. I have no idea what the governor on your engine looks like, but mine (Beta 14) got sticky like that after getting some water in the crank case (the full saga in my blog below). I had to disassemble the injector pump and use a brass wire brush and a stone to remove small amounts of rust. My beta has an inline injection pump and this isn't as scary as it sounds. I probably wouldn't attempt this on one of the rotary ones.

cspaniel · #4 07-03-2011

Thanks guys for the help. Definitely not linkage issues, disconnecting the throttle cable at the engine end, and pushing the throttle directly causes the same issue.

shogan50, could an issue with the governor be intermittent? works great for a few hours then again act up?

Could a small air leak cause this issue? but not kill the engine?

Thank you again!

shogan50 · #5 07-04-2011

In my experience yes, it was intermittent. If you have an inline style pump, like the Kubota, and are comfortable taking things apart, it might not be too bad. In my case it wasn't actually the governor itself, but the input slide from the governor on the injector pump. It had developed some corrosion. I had other issues, but that one amounted to removeing the pump (4 bolts, 2 pipes, 2 hoses) and partially taking the pump apart. I took the slide cover off, stoned the slide and put it back together. In my case I replaced the phillips head screws with class Allen sockets, likely of a higher grade and made sure to use locktite. I could to the whole thing again in 2 hours.

As for air, it is possible, but it sounds more like a sticky governor.

cspaniel · #6 07-04-2011

Shogan50, thanks again for your help, although I dont feel confident to tackle this one by myself ( I am new to diesel engines), at least I can tell the mechanic about it (he is baffle by this issue apparently!!) and learn in the process...he will be checking the governor hopefully in a few days.

another interesting aspect of this issue is that in reverse, the RPMs are as smooth as they should be, no problem at all, only in forward gear!!!! does this give any hints? this is driving me crazy!

Abe

shogan50 · #7 07-05-2011

With most props, the load curve is quite a bit different in reverse than in forward. Some transmissions also have different ratios in forward and reverse. If there was a sticky spot in the governor, I could see where it might act this way. My reasoning is that the sticky spot isn't associated with a power setting, but a fuel flow setting. It probably takes less fuel flow to hit that RPM range in reverse. Unlike in a gasoline engine, you don't actually set the "throttle" position. You set the governed speed. The governor sets the power level or what would otherwise be "throttle" position for you, but it does it by pulling with a relatively delicate spring. If the mechanism is sticky, it could do what you are seeing.

cspaniel · #8 07-05-2011

thanks you, this makes complete sense now.

if I can only get the mechanic to check this! hopefully soon.

cspaniel · #9 07-11-2011

I have read hundreds of forum post all over with similar symptoms and rarely ever the OP posted how or if the problem was solved, so here it is if anyone has a similar problem.

72 hours before my mechanich was going to work on the governor and, suggested by him over and over, rebuild the injector pump, I decided to do a few minor things and see what happens:

1) I changed the secondary filter (the whole thing, and filter element was like new), including the copper washers. I had very small leaks, bleeding screw was not in good shape, and there was always very little air when bled.
As part of this I adjusted all fuel hose connections just in case.
Bled the system

2) A diver cleaned teh bottom: he is charging me half since it was in apparently great shape. Few very small barnacles in the keel and in the propeller nut(2 in that location) nothing else

3) (my favorite) I took the fuel pick up tube out and it had the screen on it, with little black slush stuck on it, but in no way clogged completely. Took the screen out (didn't put it back) clean the pick up tube as well as the fittings and fuel close valve.
As part of this I noticed that the fuel line coming out of the pick up tube was slightly crushed, maybe a 1/16 inch where it comes out through a hole in the floor and bends into the primary filter. I fixed that as well.
Bled the system.

After these, the engine works like it did, great RPMs all around, smooth, 2 days now, so I am crossing my fingers.

Which one was the solution? I don't know but one of this have done the trick for now...

crzhors · #10 07-23-2011

Howdy,This is one place where the vacuum gauge on your racor would be a great help.I've made myself a "emergency tank" from an old outboard tank,with a supplyline with squeezeball and return fitting too. you can eliminate parts of your fuel system while troubleshooting. good job 'fixing' your prob, forensics.