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Low engine RPM

Apr 12, 2019
13
Hunter 31 Cindy Lou Demopolis,AL
I have a 1987 Hunter 31 with the 2GM20F engine.The problem I am having is I can't get the RPM up past 2400 at full throttle while unloaded.The throttle cable that was on it separated inside the steering housing and I replaced it with a 12 foot cable I bought on this site but the old broken cable wouldn't get past about 1600 RPM.I replaced the two fuel filters and while trying to prime the engine I found a bad electric fuel pump beside the fuel tank.I replaced that pump and it primed up right away and I did get it up to about 2600 rpm by pushing on the throttle control arm on the engine.It idles at 600 rpm and cranks with just a touch on the start button.I do have the repair manual and real good with gasoline engines but just get by on a diesel up until this problem.Engine has 1229 hours and runs like a top but just won't get up to the rated rpm of 3825 rpm unloaded in neutral.Does anyone think I can adjust the governor lever and get more rpm?This problem happened all at once and I am afraid to adjust anything because of this suddenly developed problem.Anyone have any ideas?The trouble shooting section of the repair manual says to replace the fuel filters which I did and the only other item mentioned that the fuel pump plunger may be worn.I would like to hear from someone else that had this problem before throwing money and time away for something I probably won't need.Thanks in advance for any ideas.
 
Jan 11, 2004
59
Hunter 31_83-87 Middle River, MD
Are you certain that the throttle cable is moving the fuel lever properly? Also, I thought wide open throttle on this engine is 3600 rpm.
 
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Oct 22, 2014
10,172
CAL 35 Cruiser Portland OR, moored EVERETT WA
You might want to look further at fuel. The lines may be clogged with goo from the tank if the fuel was old. What did the fuel filters look like? I had bad filter crud. Looked like coffee grounds. When I looked at the fuel lines I found this goo that looked like snot. Pulled it out fuel flowed all was better.

The pick up tube may also have an issue (a clogged filter/screeen) limiting the flow of fuel.

Diesel engines are simple tools. They need fuel compression and air. With all three in the correct supply they go vrrrooooommmm. Fuel is the common that causes problems.
 
Jun 11, 2011
1,196
Hunter 41 Lewes
Have someone else helm the boat, remove the throttle linkage and push the throttle by hand. If it still doesn't get to the proper WOT RPM, I would look at the fuel supply system. If it does get to full throttle then you need to look into the shifter box and also check that no one changed something near the engine that might alter the throw of the throttle cable.
 
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Apr 12, 2019
13
Hunter 31 Cindy Lou Demopolis,AL
You are correct in saying that someone did something to the throttle system since I found the broken and separated cable which I did replace.After reading all of the other people's problems after doing a search I believe I will do as you suggest and get someone to help me on checking whether the throttle is wide open.Also I might drain the fuel I put in last winter and see if fresh fuel will help.Thank you very much for the fast reply uncledom.
 
Apr 12, 2019
13
Hunter 31 Cindy Lou Demopolis,AL
Are you certain that the throttle cable is moving the fuel lever properly? Also, I thought wide open throttle on this engine is 3600 rpm.
According to the Yanmar service manual that came with the boat no load max speed is 3825+50 and minumin low speed is 850.Glad you asked that question because my no load idle is about 500rpm and I need to raise it up by 350 which will help.
 
Apr 12, 2019
13
Hunter 31 Cindy Lou Demopolis,AL
You might want to look further at fuel. The lines may be clogged with goo from the tank if the fuel was old. What did the fuel filters look like? I had bad filter crud. Looked like coffee grounds. When I looked at the fuel lines I found this goo that looked like snot. Pulled it out fuel flowed all was better.

The pick up tube may also have an issue (a clogged filter/screeen) limiting the flow of fuel.

Diesel engines are simple tools. They need fuel compression and air. With all three in the correct supply they go vrrrooooommmm. Fuel is the common that causes problems.
Filters looked like new but I replaced both just to see.I am going to drain and clean out the less than a year old tank and pickup with sending unit.
Use an optical tach to ensure reading the correct RPM. Then check the exhaust mixer
I ordered a tach following a link from 2013 on a search I did for low throttle,pickup and screen and fuel tank are less than a year old.Fuel filters looked like new but I replaced them just to see.
 
Jan 7, 2011
1,507
Oday 322 East Chicago, IN
Are you thinking you have a problem with the cable or something else?

Was the engine running correctly before you replaced the cable?
If so, check the engine RPMs by manipulating the throttle lever directly on the engine. If it gets the desired RPMs, the you need to adjust the cable.

If she won’t make the desired RPMs that way, then you have to start verifying each component..
You mentioned replacing a fuel pump...is it the right flow and working?
Fuel filters clean ?
Someone mentioned the exhaust elbow...could be plugged up and causing back pressure.

One step at a time...

Greg
 
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Apr 12, 2019
13
Hunter 31 Cindy Lou Demopolis,AL
I have a 1987 Hunter 31 with the 2GM20F engine.The problem I am having is I can't get the RPM up past 2400 at full throttle while unloaded.The throttle cable that was on it separated inside the steering housing and I replaced it with a 12 foot cable I bought on this site but the old broken cable wouldn't get past about 1600 RPM.I replaced the two fuel filters and while trying to prime the engine I found a bad electric fuel pump beside the fuel tank.I replaced that pump and it primed up right away and I did get it up to about 2600 rpm by pushing on the throttle control arm on the engine.It idles at 600 rpm and cranks with just a touch on the start button.I do have the repair manual and real good with gasoline engines but just get by on a diesel up until this problem.Engine has 1229 hours and runs like a top but just won't get up to the rated rpm of 3825 rpm unloaded in neutral.Does anyone think I can adjust the governor lever and get more rpm?This problem happened all at once and I am afraid to adjust anything because of this suddenly developed problem.Anyone have any ideas?The trouble shooting section of the repair manual says to replace the fuel filters which I did and the only other item mentioned that the fuel pump plunger may be worn.I would like to hear from someone else that had this problem before throwing money and time away for something I probably won't need.Thanks in advance for any ideas.
I thank everyone for the good ideas for things to check so now I will be busy working for a week at least to check all of this,but I really appreciate the good knowledge on this site and I should have joined when I first bought parts a couple of years ago because several heads working on a problem are a lot better that just one.I have written down all of your checklist and will get to work on Monday.Thank you all very much.
 
Apr 12, 2019
13
Hunter 31 Cindy Lou Demopolis,AL
Are you thinking you have a problem with the cable or something else?

Was the engine running correctly before you replaced the cable?
If so, check the engine RPMs by manipulating the throttle lever directly on the engine. If it gets the desired RPMs, the you need to adjust the cable.

If she won’t make the desired RPMs that way, then you have to start verifying each component..
You mentioned replacing a fuel pump...is it the right flow and working?
Fuel filters clean ?
Someone mentioned the exhaust elbow...could be plugged up and causing back pressure.

One step at a time...

Greg
I will double check the new fuel pump to see if it is rated the same or better gpm than the old one.Also that exhaust elbow is a good thought since when the engine cranks I get a puff of black smoke come out of the cockpit drain beside the engine.Thank you very much and I have made a list and will go over it until I find the problem.James
 
May 24, 2004
5,882
CC 30 South Florida
The function of the governor is to maintain steady RPM at varying loads. It is not advisable to try and obtain maximum rated engine RPM with no load as you are fighting the governor. Your idle RPM at 600 is too low, I prefer the 750 to 850 range. Whenever there are issues of RPM the first thing to do is verify the tachometer accuracy. At under 1300 hours I would not worry about the high pressure pump plunger without other symptoms. Check the fuel pump output amount to determine any fuel flow obstructions. In a diesel engine the quality of the fuel could affect RPM, make sure the fuel is fresh. Adjust the new cable so that within the travel of the throttle lever the lever at the engine moves fully. Check the air filter and intake tube for restrictions or obstructions. Check the valves lash.
 
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Oct 22, 2014
352
Pearson P303 #221 RockPort Maine
I have a 1987 Hunter 31 with the 2GM20F engine.The problem I am having is I can't get the RPM up past 2400 at full throttle while unloaded.The throttle cable that was on it separated inside the steering housing and I replaced it with a 12 foot cable I bought on this site but the old broken cable wouldn't get past about 1600 RPM.I replaced the two fuel filters and while trying to prime the engine I found a bad electric fuel pump beside the fuel tank.I replaced that pump and it primed up right away and I did get it up to about 2600 rpm by pushing on the throttle control arm on the engine.It idles at 600 rpm and cranks with just a touch on the start button.I do have the repair manual and real good with gasoline engines but just get by on a diesel up until this problem.Engine has 1229 hours and runs like a top but just won't get up to the rated rpm of 3825 rpm unloaded in neutral.Does anyone think I can adjust the governor lever and get more rpm?This problem happened all at once and I am afraid to adjust anything because of this suddenly developed problem.Anyone have any ideas?The trouble shooting section of the repair manual says to replace the fuel filters which I did and the only other item mentioned that the fuel pump plunger may be worn.I would like to hear from someone else that had this problem before throwing money and time away for something I probably won't need.Thanks in advance for any ideas.
Mr. Kirby, I have a 2GMF13 with the original "Pump Lifter". If I'm reading you correctly you have an "ELECTRIC PUMP" that are feeding fuel to your engine. If this is correct and it was installed by the other owner, then everyone's ideas here are correct! However, I would start from the very beginning.
1. You must first identify the readings on your RPM's are correct. You might have the wrong Tach gauge installed or he might have settled for what he could find on the market. Without this test, you'll never know where you're going next. All the efforts before this one are fruitless I'm sorry to say. Only! move to 2 after checking #1.
2. Verify FUEL PUMP PRESSURE from the electric pump IS REALLY 5-10 PSI at the HI PRESSURE PUMP! anything less then 5PSI change your fuel pump. See when the engine calls for fuel and its starving you can not exceed the max-fuel supplied. It could also damage the HI pressure pump to your cylinder injectors. Please supply us with the fuel PUMP name, type and/or Part numbers Thank you!
3. Confirm the flow from tank to HI Pressure by allowing fuel to go into a diesel can for free flow with no restrictions to flow. If 1,2,3 are good go on to # 4.
4. Now check High-Pressure Pump and confirm Max Pressure at injectors and not the return to fuel tank end. If the pressure is low. then High-Pressure pump may need servicing. IF all 1,2,3,4 is good. I would at this point refer you to contact a Yanmar service tech. You have done all you can do at this point. Jumping around here and there is good only if, you know the system was running correctly from the beginning. You need to see a professional diesel tech In My Opinion. Once you have the tech working... you can learn a lot from him too!

Hope this helps, Capt Rob
 
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Apr 12, 2019
13
Hunter 31 Cindy Lou Demopolis,AL
Capt Rob, I did order a tach yesterday so it will be a few days before it gets here so I guess I will wait until then to start.The previous owner did install the electric fuel pump and I ran the boat up the ICW from Port Charlotte to Port Ritchie with the engine running just fine and it would rev up to 3400 rpm cruising but I backed off to 3000 rpm as per the Yanmar manual.I replaced a bad as in it separated throttle cable inside the helm just a couple of days ago which picked up the rpms by maybe 3 or 4 hundred.I think I will take your advise and wait until I get my tachometer and not do anything until I verify my true rpm.The electric fuel pump was exactly the same brand as the one that burned up and blew a 3 amp fuse.I had just changed the fuel filters and it wouldn't prime until I verified the pump was bad and had the blown fuse then it primed to the secondary fuel filter bleed port in maybe 30 seconds with the new pump.I am sorry but I can't verify the gpm on my new pump but when I installed it the rpm went up some to 2600 where I am right now.I am docked at the marina in Demopolis,Alabama 217 miles up the ICW from Mobile Bay and don't want to risk going out of the harbor because the river is above flood stage.Thank you Sir for the great check list and I will post more info when I have it since I will be going down to the boat tomorrow and maybe the new tach will arrive this week.The more I read about what to check I suspect the in boat tach right now.Thank you again, James
 
Oct 22, 2014
10,172
CAL 35 Cruiser Portland OR, moored EVERETT WA
Adding a new tach is cool but also a challenge.

Did that this winter and we tested it out in the Dock. We were trying to find the right calculation to get the correct electrical pulse to rpm ratio. Looked ok.

Then I took the boat out on the water and the High RPM was way off. Showing 4,000 while not at anticipated speed based on previous experience.

Now I am going to secure the boat in the slip. Then I’ll power the boat up and start the compare process Optical to tach Calculation again at various throttle settings.
 
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Jan 4, 2006
2,517
Hunter H-310 West Vancouver, B.C.
Someone mentioned the exhaust elbow...could be plugged up and causing back pressure.
Check this first. Removal is not too bad and the gasket is cheap. Maybe you'll find something like this if it hasn't been serviced in the last five years:

Dirty Sectioned Mixing Elbow.jpg

Watch the threads if you disconnect the elbow from the riser:

Mixing Elbow Assembled With Directions 2a.JPG


Let us know what the problem was.
 
Sep 30, 2008
1,429
Catalina 310 Quincy, MA
Capt Rob, I did order a tach yesterday so it will be a few days before it gets here so I guess I will wait until then to start
A new tach might not tell you much until it is calibrated. You could easily calibrate the old one in the meantime to determine how accurate it is or was. An optical rpm reader is inexpensive and easy to use. When I ordered my new tach, the reader was offered for sale at the same site.
 
Oct 22, 2014
352
Pearson P303 #221 RockPort Maine
Capt Rob, I did order a tach yesterday so it will be a few days before it gets here so I guess I will wait until then to start.The previous owner did install the electric fuel pump and I ran the boat up the ICW from Port Charlotte to Port Ritchie with the engine running just fine and it would rev up to 3400 rpm cruising but I backed off to 3000 rpm as per the Yanmar manual.I replaced a bad as in it separated throttle cable inside the helm just a couple of days ago which picked up the rpms by maybe 3 or 4 hundred.I think I will take your advise and wait until I get my tachometer and not do anything until I verify my true rpm.The electric fuel pump was exactly the same brand as the one that burned up and blew a 3 amp fuse.I had just changed the fuel filters and it wouldn't prime until I verified the pump was bad and had the blown fuse then it primed to the secondary fuel filter bleed port in maybe 30 seconds with the new pump.I am sorry but I can't verify the gpm on my new pump but when I installed it the rpm went up some to 2600 where I am right now.I am docked at the marina in Demopolis,Alabama 217 miles up the ICW from Mobile Bay and don't want to risk going out of the harbor because the river is above flood stage.Thank you Sir for the great check list and I will post more info when I have it since I will be going down to the boat tomorrow and maybe the new tach will arrive this week.The more I read about what to check I suspect the in boat tach right now.Thank you again, James
James, Thanks for your reply. Changing the tach is good, However, you must really validate the sensor pickup too and assure the right voltage and wires are correct for this application. Some people I've heard used BUTT-CONNECTORS to splice in wires to add a few feet / inches only to affect the current/voltage drop across there gauges. You must NOT take any shortcuts if you want to make this right! Follow the harness wires to ensure no added components are present(Resistors/filter cap's too!
PLEASE validate your fuel pump 12VDC DIESEL TYPE ONLY AND PSI and GPM! and NOT a gas type. Gas type impeller will not work but, give the impression it is. IF you are not sure on any item just REPLACE IT! save yourself time or guesswork that ends up costing you continuously money and grief.
Ralph Johnstone stated check your elbow for your elbow being clogged this would also lead to a higher engine temp as a result of poor water flow out of your heat exchanger as well. A quick way to tell is to during an idle condition VALIDATE that you have good water flow exhausting out of the exhaust with little or no smoke many symptoms of poor flow means you have to start from scratch in this area. From your seacock to filter to heat exchanger... etc
1. this is an easy test to check the flow from the heat exchanger output is unrestricted. Easy remove the hose from elbow to engine and extend a hose overboard or in your bilge and start your engine IDLE only to see that you have a good flow rate? Y or N if Yes put the original hose back on and continue. IF NO! bad rate. proceed to Walmart and buy a .22 gun cleaning kit, CLR cleaning solution and new rubber gaskets for the heat exchanger there are a total of 4 gaskets cheap, not bad. First, remove the front cover that has the hoses going into the heat exchanger with a flashlight look into the heat exchange to see any white buildup? Y/N None then put the hose back on and move on! YES then remove front cap and use the gun cleaning kit to clean the tubes. start with copper bristles dipping into the CLR and clean the tubes all the way to the end. That's each tube. Now inspect the cover if corrosion or the little V shape notch is filled in then soak in CLR and clean good. carefully poor CLR into all tubes to allow CLR to clean the back one while in place. Allow extra 2hrs for the cleaning process to be completed. put back together WITH NEW SEALS and start your engine and verify water output to the exhaust is improved? Y/N Yes the move on! NO replace your ELBOW its clogged.
What I decided to do is after using my engine in salt water rather than having it sit with salt in it I shut the valve for salt and use my freshwater tank to flush all salt out before I shout down my engine. FOOD for thought!
 
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Apr 12, 2019
13
Hunter 31 Cindy Lou Demopolis,AL
I do have an optical reader on order and it should be here later this week,I was just calling it a tach because I was thinking in terms of a electronic tachometer.James