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Old 14-08-2016, 06:08   #1
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Diesel slow to accelerate?

I've got a 4JH2TE Yanmar in my Hylas 46, probably 4600 hours. It seems now slow to speed up, when I advance the throttle lever. Symptomatic of ?
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Old 14-08-2016, 06:29   #2
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Re: diesel slow to accelerate?

That's a classic symptom of a turbo problem. Is it slow to rev in neutral, or in gear or both? The turbo could just be carboned up and a cleaning would fix it, it could be a leak on exhaust or pressure side, or the bearings could be going bad. I don't have experience with that exact turbo so some else may have more specific advice. Your symptoms definitely point toward a turbo issue.

Or you could have a majorly fouled prop or a rope around the shaft, but my money is on the turbo.
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Old 14-08-2016, 06:34   #3
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Re: diesel slow to accelerate?

Your engine may also be suffering from a blocked fuel filter restricting fuel flow.
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Old 14-08-2016, 06:39   #4
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Re: diesel slow to accelerate?

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Your engine may also be suffering from a blocked fuel filter restricting fuel flow.
Good point. I was assuming regular maintenance has been done, but that could be a bad assumption.
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Old 14-08-2016, 10:01   #5
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Re: diesel slow to accelerate?

I think I would first look at the fuel filters first,
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Old 14-08-2016, 10:23   #6
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Re: diesel slow to accelerate?

Could be filters ... make sure you check primary and secondary!
Good luck
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Old 14-08-2016, 11:03   #7
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Re: diesel slow to accelerate?

First check your prop and bottom for marine growth. After you are sure of no growth, then look for other fuel or turbo related issues.

Our boat grew a three inch fur coat of marine growth in just three weeks of staying in one place.
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Old 14-08-2016, 11:08   #8
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Re: diesel slow to accelerate?

I would also look to the air supply and make sure that their is not restricted by not just filter but air intake to the machinery space.
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Old 14-08-2016, 11:21   #9
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Re: diesel slow to accelerate?

A question we have not asked ... do you mean the BOAT is slow to speed up or the actual ENGINE is slow to speed up?
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Old 14-08-2016, 11:36   #10
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Re: diesel slow to accelerate?

Check the discharge elbow for carbon buildup. That can cause a diesel not to rev.
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Old 14-08-2016, 11:42   #11
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Re: diesel slow to accelerate?

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Originally Posted by SteveInMD View Post
Good point. I was assuming regular maintenance has been done, but that could be a bad assumption.
I made the assumption you had the boat and this became a problem until this.

Diesels by nature are not going jump up like a gas engine, are you sure you have a problem?
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Old 14-08-2016, 11:44   #12
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Re: diesel slow to accelerate?

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Originally Posted by briblack View Post
I've got a 4JH2TE Yanmar in my Hylas 46, probably 4600 hours. It seems now slow to speed up, when I advance the throttle lever. Symptomatic of ?
I would check the static things first... push your throttle to the full throttle position then go and check on the engine to see that the full throttle is opening the injection pump to full throttle. Then bring the throttle back to low... Check your log to see when the filters were serviced last. If in doubt change the secondary... if your primary only a fuel/water separator? Then start the motor up and increase throttle and take a stetescope to listen to the turbo winding up. You can usually tell if the revs go to maximum. Then go for a ride and see how the engine performs under load. Let us know what you find... Roger
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Old 14-08-2016, 11:57   #13
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Re: diesel slow to accelerate?

My bet is still on marine growth, it doesn't have to be significant to make a big difference.
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Old 14-08-2016, 12:23   #14
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Re: Diesel slow to accelerate?

Have to say it could be any of the above but without some more info it's almost impossible to say:

- Slow to rev or slow to move ? (ie. what happens without prop engaged?)

Answer that and we can start looking at whether it's a fuel/engine/prop/hull problem

although I have to say my first question will always be:
- when did you last have the hull/prop cleaned, if I don't clean my hull (mid atlantic coast) every 1-2 weeks she feels like a slug when trying to move
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Old 14-08-2016, 12:35   #15
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Re: Diesel slow to accelerate?

That's a lot of options! Let's see if we can help narrow it down a bit. Here's what I am thinking...

Unless you have a filter vacuum gauge, it might be time to do a filter change, especially if you have been in rough water recently since that can stir up tank sediment. However a clogged filter is most likely to first start showing problems under high fuel use condition such as motoring near hull speed. A filter vacuum gauge will eliminate the guess work on filter condition.

Diesel engines generally don't accelerate as well as gasoline engines, especially older diesels. The turbo greatly improves diesel acceleration. That's why most diesel cars are turbocharged. If your turbo is not working properly acceleration will if poor, even under low load situations (like in neutral). If the engine wont accelerate (rev) well in neutral it is not bottom growth or prop causing the issue. Also if revving in neutral is a problem it's likely not a filter issue because it's not a high fuel use situation (unless the filters are almost totally blocked). More info on the acceleration problem would be helpful. Revving the engine to near max rated rpm in neutral to test this is not bad for your engine.

You can test the idea of lack of air by temporarily removing the filter if it has one and opening the engine compartment while the engine is running to see if that improves the problem. Many marine engines only have a screen and not a filter. Usually there is not much dust in a marine environment.

It could be a clogged exhaust elbow. Often, but not always, you will have reduced cooling water flow with a clogged elbow. How does the cooling water flow look? Normal? Low flow would be a sign it's the elbow. Also, if you have not done it in the last year or two, it is time for the elbow to be removed and inspected. They often will develop cracks when they get old which can leak exhaust in to to cabin.
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