Yanmar 3YM30 reduction of RPMs while motoring

[Jm621]

I have a 2009 Jeanneau 36i and the engine has performed flawlessly until today. After motoring for about 30 - 45 minutes the RPMs started dropping from 2200 to 1400 and I am at a loss as to why. I changed the fuel filter, engine oil and filter, and gear oil about 3 months ago and have had low amount of hours since then. I also checked the throttle cables but they were unobstructed.

Could the drop in RPM's be from the engine self bleeding air or could it be from adding too much gear oil? My only other thought was when this happens on my gas outboard (dingy outboard revs down and back up) it means I am low on fuel, but according to my fuel gauge I have half a tank of diesel.

Appreciate any help. Thanks!

[safetypeter]

Hi Landseer,


A quick could be to remove the air filter momentarily and try it.  If it is clogged up with dust etc the amount of air reaching the inlets would be reduced, this would cause the engine revs to drop.

[kev]

Have you picked up a rope around the prop?

Can you easily turn the shaft by hand? (engine OFF !!! and out of gear) If you get resistance it suggests a rope tangle.
NB cheap underwater camera kept on board is useful on these occasions.

Then try turning the shaft in the opposite direction by hand to unwind the rope or you could try motoring in reverse for a short distance to see if the rope unwinds. Failing that it's a dive or breadknife on a stick.

Good luck!
Kev

[safetypeter]

Sept 14, 2014 9:16:19 GMT safetypeter said:

Hi Landseer,


A quick could be to remove the air filter momentarily and try it.  If it is clogged up with dust etc the amount of air reaching the inlets would be reduced, this would cause the engine revs to drop.

[safetypeter]

Sept 14, 2014 2:09:33 GMT Jm621 said:

I have a 2009 Jeanneau 36i and the engine has performed flawlessly until today. After motoring for about 30 - 45 minutes the RPMs started dropping from 2200 to 1400 and I am at a loss as to why. I changed the fuel filter, engine oil and filter, and gear oil about 3 months ago and have had low amount of hours since then. I also checked the throttle cables but they were unobstructed.

Could the drop in RPM's be from the engine self bleeding air or could it be from adding too much gear oil? My only other thought was when this happens on my gas outboard (dingy outboard revs down and back up) it means I am low on fuel, but according to my fuel gauge I have half a tank of diesel.

Appreciate any help. Thanks!

Sorry,

Just getting the hang of posting comments, but something else you need to try is to check your fuel flow, basically the fuel will flow into the filter chamber, any extra demand on the flow may lead to a shortage, this could indicate a small blockage in a fuel line, running a thin wire through could clear any blockage.  Secondly, as the engine is cooled by sea water, check the cooling water lines and through hulls are clear. any reduction in cooling could result in slight overheating and therefore a tightening up of the internal components.  I hope you get it sorted.

[rxc]

Sounds like fuel blockage to me, so a check of the entire fuel line would be in order, including the filters and expecially the dip tube into the fuel tank. My boat was delivered with some plastic shavings in the fuel tank that accumulated in the dip tube and blocked fuel flow. Very difficult to figure out, but once known, not too difficult to fix. And it took a few episodes to flush all the bits out.

[sailbleu]

Landseer,

Do you add biocide to the fuel ?
Check the pre-filter again , if you see things that should not be there you could be a victim of what is called the fuel bacteria . That nasty stuff also gets through the filters into the fuel lines and in most cases will stop the engine.

Regards

[MartyB]

SOunds like what I am going thru, and a dock mate a year ago. JUNK in the fuel line.

I've drained the tank, literally sucking out everything at the bottom. Amazing the little dead BB sized fungus or something that can clog the fuel inlet or line. One even managed to clog the pump line we were using.

The dock mate, had a person clean the tank, then went line by line thru the system, and found a 6" long something in a line beyond the filters! He does nto know the exact place, but I am going to swag the return line to the tank.

My sons and I are going to take one of there small compressors they use at work, and blow out the line from the tank to first filter, then the line from the last injector to the tank, and see if that helps. If not, then a go through the remaining lines, pumps, injector will have to be done.

Hope this info helps some.

Marty

[electricmonk]

Before you get too carried away and start to dismantle everything revisit the fuel filter, its probably 1. Not purged of air, 2. Not seated properly so it lets air in - so take it off and refit it with a dab of oil on the rubber seal, or 3. its a faulty filter.

IF that doesn't solve the problem look for other places that air could be getting in (that have recently been disturbed) then start a witch hunt for debris in the fuel line.

[sailbleu]

Sept 15, 2014 13:00:40 GMT MartyB said:

SOunds like what I am going thru, and a dock mate a year ago. JUNK in the fuel line.

I've drained the tank, literally sucking out everything at the bottom. Amazing the little dead BB sized fungus or something that can clog the fuel inlet or line. One even managed to clog the pump line we were using.

The dock mate, had a person clean the tank, then went line by line thru the e use bi, and found a 6" long something in a line beyond the filters! He does nto know the exact place, but I am going to swag the return line to the tank.

My sons and I are going to take one of there small compressors they use at work, and blow out the line from the tank to first filter, then the line from the last injector to the tank, and see if that helps. If not, then a go through the remaining lines, pumps, injector will have to be done.

Hope this info helps some.

Marty

Yes Marty , do it properly or else you'll be in trouble very soon again.
Those terrible creatures thrive in the zone between fuel and water.
That's why it's vey important to keep water out of the tank , even the smallest amount , and of cours add biocide !!!
It's all because of the addition of bio fuel that this problem becomes a big thing. I know of people that are hit severall times . And most every time the engine stops on a very critical moment entering a port and all.
If you are hit , clean the tanks ( and all the lines) completely , add a shock amount of biocide the first time you fuel up , continue to use the biocide , make sure the tanks are full during the winter , and be very sure no water can seep into the tanks.

Regards

[electricmonk]

Sailblue and Marty, if your problem is the dreaded diesel bug then you have my sympathy. A couple of decades ago I had a similar problem, we ended up removing the tanks TWICE for steam cleaning and the whole fuel system was dismantled flushed and cleaned with what was in those days the strongest biocide available - but the bug came back! Then I found FuelSet a product made by liquid engineering of Australia, we added the recommended double dose to a heavily infected tank of gunge and the problem never reappeared while we owned the boat (a further 7 years) it does what is says on the packet. I have no connection with the company except as a very satisfied customer, all my cars , boats and boilers have had the treatment on every fillup since and no problems - EGRs', flues and exhaust elbows stay clean as well.

[MartyB]

I have not had this issues with 3 trucks, bobcat, trackhoe.......but the boat.....eeee gads...

Not sure if this issue is a bug, or possibly some debris, ie rubber when I had to replace the deck fitting. The rubber tube got chewed up, so it may be some of the debris from this.....or, I did have some water at the bottom of the tank. Not what I would call a lot, but an 1/8 to 1/4 of a liter is probably enough to cause issues in some shape or form. The dock mater had a 2mm by maybe 6 cm long pile of something in a line. I have figured out, when an issue occurs with diesel, it does not take much. I have been using biocides etc for the last 20+ years with the diesels I have owned.

I have been using biocide/algaecides that are known to work here in the states. Not sure I could get what is found in Australia or Europe per say. It may be available here, but the brand is probably different. The marina I fill up at, also uses biocides etc from the get go for ALL of the fuels, ie gas and diesel. I feel sorry for the gas users, they HAVE to have a 10% ethanol mix.....boy is that bad on the system. I've had nothing but issues off and on with my small engines, ie less than 15hp 4 and 2 strokes for my landscape biz.

Kind of a good bad with fuels. On one had they are cleaner, but in the process, the issues if they get dirty etc are way worst than before.

Marty

[MartyB]

Finally got the motor back together.

One of my sons and I initially blew out the inlet, and return fuel lines. Got some junk out of the lines, ie water and other stuff. Put back together, still had the stall out issue. SOOOOOOOO.......

Off came ALL the lines, initial fuel lift/pump, rubber hoses etc. Then went out and got a new fuel pump. Finally got it along with new filters again, rubber hoses, all the copper washers for all the copper lines, bolts etc......I could pump the fuel with the NEW pump by hand, vs not with the old one. The old one the lever that goes into the engine block and does the pump action off the cam shaft, was really loose. About 5-7 mm play up and down, and 2-3mm side to side in my hand with very little pressure on the lever. New one was solid as a rock per say sitting there at rest in my hand.

Once I got things bleed, she started up easy, seemed a bit quieter, let her idle for 3-5 min, then hit the throttle to 3/4 or so, ie cruise speed full throttle, let her run for 45 min with a nary a hiccup or cough! Will know more this Saturday with a 1.5 hour motor to a race. May get out later this week to motor outside marina and see if it is really fixed. But fast throttle idling int he slip in the past has caused the die down issues to occur with in 15-20 min. Figure things overall are fixed. Now to add a bit more biocide to up the amount, to make sure any residue of old fuel is gone, and run thru on Saturday to and from the race. Should burn up 2-3 gals of my 7 gal tank.........

marty

[sailbleu]

Good thing you finally came through Marty.
Dont let the bed(diesel)bugs bite you :-)

Regards