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Old 14-07-2017, 13:24   #1
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Yanmar 4JH4 rev limit

Hi everyone,

I have a 2007 B. Oceanis 46 with a 75 Hp Yanmar 4JH4 engine with around 1100 hours on it. For some time now it tops out at 2100 RPM. Not under load it revs to max easily. New oil, new oil filters, new diesel filter, new impeller. Any idea what could cause this? Turbo issues perhaps?

Thanks for your help!
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Old 14-07-2017, 13:33   #2
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Re: Yanmar 4JH4 rev limit

What did it used to top out at when motoring?
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Old 14-07-2017, 13:35   #3
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Re: Yanmar 4JH4 rev limit

Not sure, as I never maxed it out. I have always been conservative and used 2000 RPM...
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Old 14-07-2017, 14:05   #4
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Yanmar 4JH4 rev limit

It could be turbo, but unless this is a new problem I'd bet dirty bottom or prop and likely overpropped, a lot of people overprop it seems
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Old 14-07-2017, 14:17   #5
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Re: Yanmar 4JH4 rev limit

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Originally Posted by N4585K View Post
Not sure, as I never maxed it out. I have always been conservative and used 2000 RPM...
That is not conservative, but much too low. Normally these engines should max out at 3000, so at 2000 the load will be under 30%. Definitely not good for the engine. Turbos especially are very unhappy at these loads. Diesels should be run at minimum 60% load, this would translate to about 2500RPM.

There can be many reasons for the low revs, first I would check prop for growth, air filter, feed pump?
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Old 15-07-2017, 04:51   #6
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Re: Yanmar 4JH4 rev limit

Does it smoke at any particular speeds? For example, what color smoke does it make at wide open throttle? Typical colors are blue, white and gray to black. The color (or lack thereof) can tell something about why the RPM is low.

Also, what speed through the water does the boat make at max RPM? Is that close to hull speed?

Some owners think they are saving fuel by using an over size and/or over pitch prop that produces hull speed at way less than max RPM. This is an error in thinking but it is a quite common belief.
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Old 15-07-2017, 08:10   #7
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Re: Yanmar 4JH4 rev limit

No smoke whatsoever at any speed. Engine sounds and runs really well. Max speed now is 7.3 kts, theoretical hull speed is 9.1. Bottom is perfectly clean, prop and keel have some growth. Antifouling is one year old. The prop is a Maxprop folding prop...
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Old 15-07-2017, 08:30   #8
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Re: Yanmar 4JH4 rev limit

Try it with a clean prop before you do anything else.
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Old 15-07-2017, 08:38   #9
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Re: Yanmar 4JH4 rev limit

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Try it with a clean prop before you do anything else.


Yes, spotlessly clean.
Good news is with the maxprop you can change pitch so if it's overpropped it doesn't mean buying a new prop.
When I bought my boat, it was so overpropped it would only turn 2300 RPM, but would hit 8 kts with a theoretical hull speed of 7.7 kts.
Engine RPM limit is 3600, but even full throttle at 2300 RPM it didn't smoke.
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Old 15-07-2017, 08:52   #10
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Re: Yanmar 4JH4 rev limit

Good news, thanks everyone! Will get out the scraper when I'm in a bay...
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Old 15-07-2017, 20:42   #11
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Re: Yanmar 4JH4 rev limit

I have just fitted a new 4JH4 to replace a 4JH2 (hydrolocked--interesting cause covered by insurance)). In taking out the old engine the turbo filter was found to be carboned up. All other filters had been checked on purchase of the boat but the turbo wasn't. Your problem sounds more like a prop or drive issue (if the prop is not badly fouled, have you checked for fishing line or the Maxprop not folding/unfolding properly?) Just out of interest the manual for the new engine recommends revving over 3000 for 10 minutes or so every dozen hours to blow out carbon deposits.
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Old 16-07-2017, 05:33   #12
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Re: Yanmar 4JH4 rev limit

Our Yanmar 75 hp 4JH3-TE 2002 revs at 3600 rpm! We usually cruise at 2500-2600 rpm and occasionally push the engine at 3500-3600 for an hour or so, under load of course!
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Old 16-07-2017, 08:16   #13
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Re: Yanmar 4JH4 rev limit

You are correct but the newer 4JH series engines are designed to turn slower than the circa 2000 4JH engines. It leads to a lot of wrong answers on the internet. Each owner should review the manual for their own engine. The model number series is not enough to know the specs.
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Old 17-07-2017, 05:47   #14
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Re: Yanmar 4JH4 rev limit

Verify that your tachometer is working properly.
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Old 18-07-2017, 12:54   #15
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Re: Yanmar 4JH4 rev limit

Thanks for all your replies! Rick, how would I check the tacho?
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