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Old 25-09-2019, 19:38   #1
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Yanmar 39HP Diesel Low Revs Issue

I am looking at a boat to purchase:

on one engine the revs will not go past 2600RPM and the other 2800RPM in Neutral - Both engines are original 2005 with 3200 hours on each. Both start easily and there is no visible smoke from them.


As far as I am aware these engines should at least hit 3000RPM?


What sort of issue am I looking at here? Could it be a major problem?


Obviously I will get a mechanic to look at the engines before purchase, but just want to get an idea up front of the problem.


Thanks in Advance
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Old 25-09-2019, 20:07   #2
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Re: Yanmar 39HP Diesel Low Revs Issue

Good chance it's exhaust elbows partially blocked. Beware Yanmar parts prices are a kings ransom.
It could be air supply, fuel supply, injector or injector pump issues as well . What model Yanmars are they? That might help to get owners of that model to chime in.
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Old 25-09-2019, 20:13   #3
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Re: Yanmar 39HP Diesel Low Revs Issue

Thanks Models are 3JH4E
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Old 25-09-2019, 20:32   #4
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Re: Yanmar 39HP Diesel Low Revs Issue

FYI they are 29hp & should reach 3600 rpm according to google.Thats quite a bit below their spec. They definitely have a problem

https://www.yanmarmarine.com/Product...nes/3JH4E-404/
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Old 25-09-2019, 20:43   #5
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Re: Yanmar 39HP Diesel Low Revs Issue

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Originally Posted by Compass790 View Post
FYI they are 29hp & should reach 3600 rpm according to google.Thats quite a bit below their spec. They definitely have a problem

https://www.yanmarmarine.com/Product...nes/3JH4E-404/



OK Getting confused now.... The site says Rated output 29 at 3600 but the specifications in the manual state: 39 at 3000 - See image
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Old 25-09-2019, 21:47   #6
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Re: Yanmar 39HP Diesel Low Revs Issue

We have a 3GM, so I would suspect that the 3JH's are 29hp rather than 39hp.

Ours too is supposed to rev to about 3500 and though I've never tried it in neutral, it sounds and smokes like it's about to blow-up past about 28-900; I've never dared leave it even there for more than 10-20 seconds - burning off the carbon. That said, 2800 and particularly 2600 does sound very low, though it may easily be something really simple. I see a partially choked exhaust has already been mentioned, but simpler yet, it may just be that the throttle cables need adjusting?
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Old 25-09-2019, 21:49   #7
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Re: Yanmar 39HP Diesel Low Revs Issue

The throttle lever on the engine with the cable disconnected is the best way to check max rpm. Also make sure the stop control is fully in the run position and don't trust the accuracy of the tachometer, they often read hi or low.
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Old 25-09-2019, 21:51   #8
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Re: Yanmar 39HP Diesel Low Revs Issue

I have 3jh3e engines they do 3600 rpm. The 3jh4e is a very different engine, only does 3000 rpm.
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Old 25-09-2019, 21:52   #9
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Re: Yanmar 39HP Diesel Low Revs Issue

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I have 3jh3e engines they do 3600 rpm. The 3jh4e is a very different engine, only does 3000 rpm.

Do you know if the 3JH4E is 29HP or 39HP?
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Old 25-09-2019, 22:01   #10
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Re: Yanmar 39HP Diesel Low Revs Issue

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Do you know if the 3JH4E is 29HP or 39HP?
39hp3jh4e.pdf
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Old 25-09-2019, 23:59   #11
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Re: Yanmar 39HP Diesel Low Revs Issue

Quote:
Originally Posted by UFO View Post
OK Getting confused now.... The site says Rated output 29 at 3600 but the specifications in the manual state: 39 at 3000 - See image
Ok I would believe manual before site. I just googled it & that came up. If it's a turbo engine I suspect it's more likely the 3000rpm.
Sorry for confusing the issue
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Old 26-09-2019, 00:10   #12
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Re: Yanmar 39HP Diesel Low Revs Issue

Hunter 38 year 2005, max rpm under load clean hull 3300, lower when fouled. Max rpm in idle never done. These are Yanmar tach figures. When I checked rpm 3000 Yanmar RPM=2830 measured.
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Old 26-09-2019, 00:11   #13
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Re: Yanmar 39HP Diesel Low Revs Issue

Quote:
Originally Posted by Compass790 View Post
Ok I would believe manual before site. I just googled it & that came up. If it's a turbo engine I suspect it's more likely the 3000rpm.
Sorry for confusing the issue
Whoops I confused it some more! It's naturally aspirated. I'll get offthread now

However still stick to possible low rev problems but agree with take throttle cable off & try engine control first as skipper pete suggests
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Old 26-09-2019, 13:40   #14
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Re: Yanmar 39HP Diesel Low Revs Issue

Three major things to check as a last resort, two being the most likely. They are blocked exhaust, restricted air inlet, and the third is incorrect timing. The incorrect timing is the least likely because it can only happen after a major overhaul by careless people, but I have found it on at least two engines, one a heavy diesel the other petrol.

There are of course the simple ones to check first--governor springs, settings of the governors and fuel pump delivery often caused by partially blocked fuel filters.

If engine will not rev but runs OK under load where the revs are reduced, that sounds to me like the governor settings.
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Old 26-09-2019, 16:24   #15
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Re: Yanmar 39HP Diesel Low Revs Issue

Quote:
Originally Posted by UFO View Post
I am looking at a boat to purchase:

on one engine the revs will not go past 2600RPM and the other 2800RPM in Neutral - Both engines are original 2005 with 3200 hours on each. Both start easily and there is no visible smoke from them.


As far as I am aware these engines should at least hit 3000RPM?


What sort of issue am I looking at here? Could it be a major problem?


Obviously I will get a mechanic to look at the engines before purchase, but just want to get an idea up front of the problem.


Thanks in Advance
I have one in my 42 ft steel yacht,14 ton. with no adversities I get 6kts at 2- 2.2litres an hour with 650 hours on her. 3JH4E is an Excellent engine and is 39 hp. It also only revs to 2800 in neutral.

The rev counters would be the most likely problem. Yanmar dashboards are not the greatest. Your mechanic will most likely have an electronic rev counter.

Great little engines, Cheers.
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