Why lower RPM's after engine maintenance?

[CarolinaDelt]

I have a 4JH3 44Hp Yanmar diesel that recently had some engine work done. Now though, my rpm's in forward gear only get 2300 whereas before they easily went beyond 3000. In neutral, the rpm's get beyond 3100 so it's only an issue in forward gear. Any thoughts why?

Maintenance work I had done included a new muffler, damper plate and all filters.

Appreciate insights!

[David M]

Given you changed out the fuel filter(s), there may still be some air in the Diesel fuel lines. Have you bled the injectors just in case?

[Ostinato]

What about reverse?

[David M]

There is less resistance in reverse for the same RPM because a propeller is less efficient in reverse. It moves much less water in reverse. Therefore the maximum speed in reverse could possibly remain the same for this reason.

[GILow]

Originally Posted by CarolinaDelt

I have a 4JH3 44Hp Yanmar diesel that recently had some engine work done. Now though, my rpm's in forward gear only get 2300 whereas before they easily went beyond 3000. In neutral, the rpm's get beyond 3100 so it's only an issue in forward gear. Any thoughts why?

Maintenance work I had done included a new muffler, damper plate and all filters.

Appreciate insights!

+1 to air in the system, but also check it is not something simple like the control cable has been loosened or otherwise come out of adjustment, and is no longer creating full motion at the injector/governor control end.

Don't ask me why I thought of that one.

[Captsteve53]

Sounds like a fuel starvation issue as you get full RPM @ no load But the first question is there any sign of black smoke while in gear? if so this would perhaps mean dirty prop/dirty hull, if no black smoke then leads to Fuel starvation issues:


1) Air intake issues (dirty air cleaner)
2) Blocked or partial blocked fuel line or return lines
3) Lift pump under performing
4) Air in the system from filter installation/perhaps still sucking air
5) Is the new muffler the correct size or as original
6) Has the riser been checked recently (partial blockage) corrosion issues
7) How old is the riser? /replace if over 5 years
8) Have you checked RPM with laser Tach
9) Un likely but injectors or Fuel injector pump issues

Cheers Steve

[Ostinato]

True about less resistance in reverse but still I'd want to know it. So far most people think this is a fuel related problem but it may not be. It may be transmission or shaft/prop related and that needs to be checked out too.

[CarolinaDelt]

Originally Posted by David M

Given you changed out the fuel filter(s), there may still be some air in the Diesel fuel lines. Have you bled the injectors just in case?

Thanks ... we did bleed them. What confuses me is that in reverse, she moves fine. The rpms get to 3000 just fine. So it's only in forward. My technician is stumped ...

[Sailmonkey]

Originally Posted by CarolinaDelt

Thanks ... we did bleed them. What confuses me is that in reverse, she moves fine. The rpms get to 3000 just fine. So it's only in forward. My technician is stumped ...

Reverse requires less fuel to attain full speed (less load). If this has only occurred after the filter change, there is a fuel restriction preventing full fuel delivery when fully laden.

[Tkeeth]

Mine acts that way when the prop is dirty

[gjordan]

Any chance that they put the wrong filters into the system? Check the numbers on the filter itself to see if it is correct. A very fine mesh/micron rating that might work fine with a small diesel, might restrict flow a little on an engine your size. Probably will turn out to be a problem with the new exhaust but check out all things. Let us know what you figure out. Good Luck. Grant.

[1Sunseeker]

You might check the throttle cable has not got pinched. That happened to me.

[1Sunseeker]

Let us know outcome please.

[Oceanride007]

Other than checking to see if prop is clean and exhaust is not more smoky, I'd let it go, sure check any limits on Fuel rod / morse cable but unless you have increased fuel consumption or noticeable drop off in speed, don't throw more money at the Diesel Mechanic, there are plenty of other jobs to throw money at. Sure there may be a issue but diagnosis will become more apparent in time.