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Old 07-01-2013, 15:40   #1
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Yanmar rpms dropping

Hi all,
I have a Yanmar 4JH2E. It's been a great engine in the time I've had it, brought me safely from California all the way to Melbourne Australia, until very recently.
Just as I was entering Bass Straight, the revs faltered from 2000 down to 1500 and then back up again. It didn't do it again for a couple of hours but then it started to happen more frequently until it got to the point that I had to drop the revs back to 1500 where it seems to continue happily.
I just change both the primary and secondary filters, the secondary having a fair bit of crud in it, but it's made no difference.
My thoughts are still that it's not getting sufficient fuel flow, but I'm throwing this one out to anyone who might have other ideas.

Chris
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Old 07-01-2013, 15:55   #2
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Re: Yanmar rpms dropping

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Originally Posted by Sampson View Post
Hi all,
I have a Yanmar 4JH2E. It's been a great engine in the time I've had it, brought me safely from California all the way to Melbourne Australia, until very recently.
Just as I was entering Bass Straight, the revs faltered from 2000 down to 1500 and then back up again. It didn't do it again for a couple of hours but then it started to happen more frequently until it got to the point that I had to drop the revs back to 1500 where it seems to continue happily.
I just change both the primary and secondary filters, the secondary having a fair bit of crud in it, but it's made no difference.
My thoughts are still that it's not getting sufficient fuel flow, but I'm throwing this one out to anyone who might have other ideas.

Chris
Been there done that !! Try removing the fuel lines and cleaning them. I used small diameter stainless multistrand wire, strong enough to get the crud out but flexible enough to get around the curves. I now dose with biocide and bulk filter the tanks every six months.
Good luck.
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Old 07-01-2013, 15:59   #3
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Ok, thanks for that tip!
Did you clean the lines all way through or just after the filters?
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Old 07-01-2013, 16:03   #4
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Re: Yanmar rpms dropping

Check the fuel pickup dip tube and any valves/check valves in the system. That would be the most likely cause of a fuel restriction if filters are clean.

David
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Old 07-01-2013, 16:04   #5
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Re: Yanmar rpms dropping

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Ok, thanks for that tip!
Did you clean the lines all way through or just after the filters?
I had to start at the tanks (including valves, also full of crud) and work through to te racors. The PO had installed magnetic fuel treatment devices in the lines so the removal and cleaning of the lines was easy. (short lengths) No more problems since, touch wood.
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Old 07-01-2013, 16:10   #6
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Ok this all sounds reassuring. I have new fuel line on board so I'll just change it all over rather than trying to clean them.
I'm glad no-one has mentioned the injector pump yet!
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Old 07-01-2013, 16:30   #7
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Re: Yanmar rpms dropping

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Ok this all sounds reassuring. I have new fuel line on board so I'll just change it all over rather than trying to clean them.
I'm glad no-one has mentioned the injector pump yet!
You will probably find crud in the tank as well. Simple fuel polishing is a very worthwile exercise, and neither difficult nor expensive.
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Old 07-01-2013, 16:40   #8
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So far, the lines have been ok, good in fact. There is a fuel flow meter installed half way between the racor and the lift pump, is it possible that this might be gummed up?
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Old 07-01-2013, 17:31   #9
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Re: Yanmar rpms dropping

Sampson

I have a 4JH4E and had a similar problem when the engine was only a year old. I checked it out and found a small cut in the fuel line that was letting air in the system. It got worse the higher the RPM's. The cut was underneath the hose clamp and had actually been caused by the clamp. I cut that out and everything worked fine. I also replaced all of the hose clamps with the more expensive AWAB clamps.

If you fuel tank is dirty I recommend cutting access holes in the tank and physically scrubbing the tank. I don't think that fuel polishing cleans the tank well.
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Old 07-01-2013, 17:53   #10
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Hmmm well I have replaced the fuel lines between the tank and the lift pump, and at 2000 in neutral it was fine, just cruising now and same problem still exists. Could it be the lift pump?
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Old 07-01-2013, 18:03   #11
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Re: Yanmar rpms dropping

It could be the lift pump. It could also be injectors. Possibly could be the cooling injector elbow. Was there any smoke? Leave the HP pump till everything else is checked.
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Old 07-01-2013, 18:09   #12
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I don't think it's the injectors, I cracked each one individually and each had a significant impact, Di I'm confident they're working fine. I've just done an oil change so although I can't tell by looking at the oil, I know I'm not making any either by the level, so the lift pump "should" be ok, but possibly a little bit blocked??
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Old 07-01-2013, 18:18   #13
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Re: Yanmar rpms dropping

Did you change the secondary filter? Did you change the fuel line from the lift pump to the injection pump. An easy way to check if it is a fuel leak is to install a piece of clear fuel hose in the sustem and see if there are any bubbles. If it is clear you have eliminated that possible problem. If the lift pump is good then the next area that has to do with RPM is the governor. I don't know enough about them to make a logical comment. My guess is that it is fuel getting into the system somewhere.
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Old 07-01-2013, 18:56   #14
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Do you mean fuel or air getting into the system?
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Old 07-01-2013, 22:56   #15
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Re: Yanmar rpms dropping

Sorry I meant air in the fuel.
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