Yanmar 3GMF still can’t get above 2200 Rpm question ?

Jan 24, 2017
666
Hunter 34 Toms River Nj
l have been looking into this issue for two seasons and still have not been able to resolved it.
A little background: I have a 1983 hunter 34 powered by a yanmar 3GMF Diesel 25 Hp and I am the original owner. It is now 38 years old and all original. Engine is in excellent condition and has been well maintained over the years. No engine hour gauge but I estimate engine hours to be approximately 3000 - 3800 hrs.
Fuel pump and injectors replaced 2003, all regular routine yearly engine maintenance done, fuel/oil filters changed, fuel inspected and stabilized etc. replace mixing elbow 4 years ago, and throttle cable replace two years ago.

Engine runs absolutely perfectly, starts on the fist crank, no smoke, excellent water flow, no issues at all except the rpm issue under load. The fuel filters both have less then 20 hours on them and have always been clean.

Normally I would be able to reach hull speed of 7 - 7.5 knots any time of the season at full throttle and now 5.5 - 6.2 knots with no wind or current. Every season the boat is hauled no barnacles and very little slime if any. Prop is the same I have had for thirty years and always clean.

No apparent fuel issues, engine doesn’t appear to be starved of fuel and tank was emptied and new diesel added to rule out bad fuel. Manually able to engage the throttle to max throttle and cable appears to be functioning properly. Just cannot throttle up past 2200 rpm.

I read that the tech gauge on these boats are problematic and sometimes inaccurate however if this is an instrumentation issue then ground speed would not be effected.
Other post have reported rpms of 3200 - 3800 and ground speeds of 7.5 knots which is about what I remember it normally running at.

any suggestions would be appreciated.
 
Apr 22, 2011
865
Hunter 27 Pecan Grove, Oriental, NC
You are able to get only 2200 rpms in neutral,, is that correct?
 
Jan 24, 2017
666
Hunter 34 Toms River Nj
in gear tied to the dock or out on the water, however I don’t think I have tried to go that high of rpm in neutral. Maybe?.
Why do you ask?
 
Jan 24, 2017
666
Hunter 34 Toms River Nj
I have adjusted the governor to the maximum that the lock nut allows and the new cable doesn’t seam to have any slack in it. Other then the new cable is much smoother to operate, really no difference between the old cable and new one.
 
Apr 22, 2011
865
Hunter 27 Pecan Grove, Oriental, NC
in gear tied to the dock or out on the water, however I don’t think I have tried to go that high of rpm in neutral. Maybe?.
Why do you ask?
If you are only able to get 2200 rpms in neutral, then you can eliminate any component of the drive train as a problem.
 
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Jan 24, 2017
666
Hunter 34 Toms River Nj
Spoke to a diesel mechanic and he has given me a list of things to check
He is 85 percent sure that it is a fuel issue and not mechanical

Non mechanical:
1. Fuel return from transfer pump, fuel flow.
2. He thinks that with the low sulfur fuel that the fuel lines could be bad.(partially clogged, inner core breaking down)
3. Clogged pick up tube.
The fuel lines are thirty eight years old, so I’m going to change them regardless.

Mechanical:
Bad fuel pump
Governor
Things he didn’t think it was
injectors
compression issues
mixing elbow
Going to do some hopefully quick easy fixes and hopefully they fix the issue.

Thanks for all the suggestions
 
Nov 22, 2011
1,192
Ericson 26-2 San Pedro, CA
Spoke to a diesel mechanic and he has given me a list of things to check
He is 85 percent sure that it is a fuel issue and not mechanical

Non mechanical:
1. Fuel return from transfer pump, fuel flow.
2. He thinks that with the low sulfur fuel that the fuel lines could be bad.(partially clogged, inner core breaking down)
3. Clogged pick up tube.
The fuel lines are thirty eight years old, so I’m going to change them regardless.

Mechanical:
Bad fuel pump
Governor
Things he didn’t think it was
injectors
compression issues
mixing elbow
Going to do some hopefully quick easy fixes and hopefully they fix the issue.

Thanks for all the suggestions
To rule out the pickup tube and crap in your tank: Get yourself a small plastic gas can. Just stick a (preferably new) fuel line into the can and hook it up to your engine's lift pump. Then reroute your return line to also go into this temporary tank. See what happens.
 
Jan 24, 2017
666
Hunter 34 Toms River Nj

got onto the boat today to do some of suggestions from the group.
1. got rpms to about 2600 in neutral still not great and rpms in gear 2200.
2. Removed fuel discharge return line from last injector back to the fuel tank and basically no fuel flow, only a few drops. Diesel mechanic said I should have a steady flow back to the fuel tank.problem
So fuel circulation issue appears to be the problem.
3. checked for fuel from tank to fuel feed lift pump and all appears good.
4. Checked all bleed points to the injectors including the nozzle body and all appear to get fuel.
5. Checked the top nozzle spring nut and got
NO Fuel . Problem!

Fuel in tank appears goodpicked up some fuel from bottom of tank all clear no sediment.
Replacing all fuel lines because they are 38 years old and even if they are still good, I’m going to replace all of them just to eliminate possible issue.
Replacing primary and secondary filters even though they have less then 20 hrs on them, again to eliminate possible issue.
Plan to rig a external fuel can with new diesel and run it to primary filter.
If the issue still exists.

diesel mechanic suggested replacing the fuel feed lift pump. He said that inside the pump is a diaphragm that may have started to fail. If it has not completely failed he believes that it will produce enough fuel to the injection pump to allow the injection pump to operate enough for the engine to run however not supply sufficient fuel to get to the correct rpms under load. Therefore starving the engine of fuel to the injectors.

hopefully this works
 
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Apr 22, 2011
865
Hunter 27 Pecan Grove, Oriental, NC
A couple of years ago, another forum member was concerned with only seeing a few drops of fuel from the injector fuel return line. I responded with this post:

"Your fuel return amount looks to be about the same as what comes out of my 2gm20f. Not much. I was concerned and did some research. I read that some diesel fuel systems are designed to have a much greater return fuel flow than the gm series yanmars. The large amount and rapid flow of un-injected fuel in the injectors is used to cool the injectors. Apparently this is not necessary in the yanmars."
 
Nov 22, 2011
1,192
Ericson 26-2 San Pedro, CA
A couple of years ago, another forum member was concerned with only seeing a few drops of fuel from the injector fuel return line. I responded with this post:

"Your fuel return amount looks to be about the same as what comes out of my 2gm20f. Not much. I was concerned and did some research. I read that some diesel fuel systems are designed to have a much greater return fuel flow than the gm series yanmars. The large amount and rapid flow of un-injected fuel in the injectors is used to cool the injectors. Apparently this is not necessary in the yanmars."
Same with my little 1GM.
 
Jan 27, 2008
3,045
ODay 35 Beaufort, NC
Do you have an electric fuel pump like a cylindrical facet style pump? If so there is a filter in the bottom of them that you remove by unscrewing the bottom of the pump. See if it is clogged if you have one.
 

RoyS

.
Jun 3, 2012
1,739
Hunter 33 Steamboat Wharf, Hull, MA
I have adjusted the governor to the maximum that the lock nut allows and the new cable doesn’t seam to have any slack in it. Other then the new cable is much smoother to operate, really no difference between the old cable and new one.
Suggest ignoring the cable. Disconnect it and operate governor lever all the way to the stop. Do not adjust the stop. What do you get for rpm in neutral with lever at the stop?
 
Jan 24, 2017
666
Hunter 34 Toms River Nj
Roy s,
Boat is on the hard for the winter, so unfortunately can’t do anymore tinkering until spring thaw.
If I recall correctly I believe I was able to get rpms to about 2300 by manually forcing the throttle body arm without the cable attached in forward and was able to achieve about 2600 in neutral.
I believe at idle the rpms are about 1100 but I’m not 100% sure going by memory, which is not what it used to be.
 

RoyS

.
Jun 3, 2012
1,739
Hunter 33 Steamboat Wharf, Hull, MA
Here is another test that you can try. With engine running, loosen injector fuel inputs one at a time. You need only crack the connection to disable the particular injector temporarily. This test is to determine if one of your three cylinders is not working. You should see the engine skip and slow down when each single injector is disabled. You will have to experiment a little to find the most telling RPM.