Registered users don't see ads

Low idle 3gm30f

Discussion in 'Engines and Propulsion' started by GABristol29, Feb 11, 2019. Add this thread to a FAQ

  1. GABristol29


    Joined Dec 19, 2018
    22 posts, 4 likes
    Bristol 29
    Kate US Lake Lanier
    I have gotten my new to me 3gm30f running after it sat for 15 years. All I have done is replace the primary racor filters, the secondary filter, air filter element, oil and oil filter. I am pulling fuel from a 3 gal gas temporary tank until I finish cleaning the old monel tank. With the throttle lever at idle position tach shows 2000 rpms, steady rpms but seems to shake a little too much. Smooths out and looses shake at 2300 and sounds great all the way to 3500. Seems to have more in her but did not push her over 3500 for now. I went below and adjusted the throttle stop bolt/screw to get below 2000 rpms. I got to 1400 but anything below would cause the motor to stall out, first shake badly, then stall. My suspicion is that she is fuel starved at low rpm, or my tach is wrong, with my gut telling me the former. I am inclined to buy a photo tach to rule out the later, but my bet is that the tach is not the issue. Thanks for all feed back. Matt

  2. Davidasailor26


    Joined May 17, 2004
    1,615 posts, 387 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    My guess would be the tach. Someone would've really needed to play with the idle to get it all the way up to 2000. Also a diesel isn't like a carbeurated gas engine with high and low speed jets. If it can get enough fuel at higher speeds it should be able to do so at lower speeds too.

  3. Skipper


    Joined Oct 9, 2008
    1,665 posts, 306 likes
    Bristol 29.9
    US Dana Point
    1. If you are running outside the normal fuel supply system, have you then omitted the lift pump? If so, the injector pump might be struggling to get fuel at low rpm.
    2. Some of those old Yanmars have the bottom of the throttle as the stop position. That is, put the throttle to the bottom to stop the motor.
    3. Motor has been sitting. Fuel system clogs may lurk where you haven't yet looked, plus injectors may need cleaning.

    Last: you said "gas" can. I'm sure there's diesel in the can :)

  4. LeslieTroyer


    Joined May 20, 2016
    2,502 posts, 1,142 likes
    Catalina 36 MK1
    US Everett, WA
    How are you measuring RPM?? Tachs run from the alternator can be way off. Get a $30 optical tach at harbor freight and check your real RPM before too much adjustment.

  5. GABristol29


    Joined Dec 19, 2018
    22 posts, 4 likes
    Bristol 29
    Kate US Lake Lanier
    Leslie, tach is measuring at flywheel, not alternator, but I still plan to check with laser.
    Skipper, I am running the fuel from an actual temporary plastic gas tank (with diesel :)), return fuel line is directed to main monel tank for now. The lift pump is in the loop (but it is possible that is not doing its job at low rpm...). Re. engine stop, it is not attached to throttle - it is separate and, from what I can tell, stops fuel flow to stop engine. Injectors likely need cleaning - is that a big job?
    Thank you all.

  6. mermike


    Joined Nov 21, 2012
    27 posts, 4 likes
    Yamaha 33
    US Port Ludlow, WA
    I'd say pull the injectors and have them rebuilt. I just did mine and she went from trying to jump off the motor mounts to purring like a kitten. An extremely loud and clattering kitten, mind you. But so much better.

    To reinstall your rebuilt injectors, you'll need:
    3 124950-11450 GASKET, CHAMBER
    3 24341-000260 O-RING 1A S-26.0
    6 23414-080000 GASKET 8, ROUND

    Torque the pipe joint bolt to between 112 and 147 inch-lbs. Don't screw up the conversion from newton-meters like I did or you'll also need 3 105582-59150 BOLT 8, PIPE JOINT, and a bolt extractor.


    Last edited: Feb 11, 2019
  7. GABristol29


    Joined Dec 19, 2018
    22 posts, 4 likes
    Bristol 29
    Kate US Lake Lanier
    Thanks Mike, checking on those parts now. And thanks for the conversion warning - a mistake I would make. Matt

  8. jon hansen

    jon hansen

    Joined May 25, 2012
    1,798 posts, 1,400 likes
    john alden caravelle 42
    us sturgeon bay, wis
    simple. it's either injectors of fuel pump assuming the compression is normal.
    it's cheeper to start with injectors
    just had my fuel pump rebuilt last summer. it was the second time for my 50+ year old perkins 107. ain't cheep, but what is?

  9. GABristol29


    Joined Dec 19, 2018
    22 posts, 4 likes
    Bristol 29
    Kate US Lake Lanier

  10. Claude L.-Auger

    Claude L.-Auger

    Joined Dec 14, 2003
    1,057 posts, 80 likes
    Hunter 34
    CA Lake of Two Mountains, QC, Can
    Good info here. But do find a way to get accurate RPM readings before you spend money on anything. That engine has not been started in 15 years ! It could be gummed up from the lift pump to the HP pump to the injectors, including the fuel lines !
    Please refrain from pushing it neutral ! Engines are made to work under load. With the correct prop and everything in good order, those 3GM30F will rev to 3400 or 3600 under load and that is normal. Higher or lower means wrong prop or something else causing the problem. I would get a second jerry can and run the return line to it to see if there is something in the fuel being returned as the engine is brought to normal operating temp. Good luck

    Don Crowther likes this.
  11. GABristol29


    Joined Dec 19, 2018
    22 posts, 4 likes
    Bristol 29
    Kate US Lake Lanier
    Claude - thank you. Good thought to rig a second jerry can and have a look. I will check RPMs too.

  12. kloudie1


    Joined Nov 6, 2006
    8,257 posts, 722 likes
    Hunter 34
    US Mandeville Louisiana
    I second Claude's statement.. The Yanmar spec is 850 rpm for idle.. The tach is notoriously inaccurate, so an optical tach check is necessary..

Noflex Sewage Treatment
Treat sludge and odor problems in holding tanks.
Merriman pedestal control head
Finally, an aluminum replacement for this YS Merriman part.
Low prices, custom fit, no-wait estimates
Teak companionway doors
Amazing artisan quality, custom made to fit your boat.