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Old 17-08-2021, 19:58   #1
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Help w/ Yanmar 3JH3 Power Drop-Off Mystery

I need some forum help diagnosing a engine problem that's past my mechanical skills.

I have a Yanmar 3JH3E with about 2000 hours on it. Engine was built in 2002.

I noticed that the engine was not providing max RPM ( about 2900 was my prior max) and assumed I had a fuel issue.

I changed the primary and secondary filters. Minor improvement was noticed that quickly reverted to the same problem. So I changed them again. No improvement.

So I changed the mechanical lift pump on the engine. No improvement.

I checked all the fuel hose clamps. All OK.

I happened to have had the injectors serviced last winter. I removed and installed them myself but had them serviced by experts. So hopefully it's not the injectors. I was told the injectors had a clean nozzle spray pattern but they were popping off early by about 15% - which they adjusted.

My engine will only now do around 2300 rpm. I can put the throttle all the way forward and no change after 2300. There is no smoke from the engine at all. It runs smoothly, no knocking or stutter.

One weird item I should mention...., for the 1st 30 min of run time from cold, i can get up to 2800 RPM from the engine. After that, it drops down to max 2300 RPM. The only thing i can think of is that fuel is getting hot after 30min and it's impacting some other system?

Any thoughts? Bad governor? Bad high pressure fuel pump. Engine too hot? Engine starved of air?

Thanks in advance for any advice you may have.

Britishsea
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Old 17-08-2021, 21:40   #2
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

Clogged fuel tank vent ? Or some sort of blockage between fuel tank and lift pump, since it is reducing the maximum continuous fueling rate ? Sometimes fuel line itself will degrade through internal swelling, closing off the flow path. Sometimes it pays to replace fuel lines on a time basis or at least carry some spare material along in case needed.



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Originally Posted by Britishsea View Post
I need some forum help diagnosing a engine problem that's past my mechanical skills.

I have a Yanmar 3JH3E with about 2000 hours on it. Engine was built in 2002.

I noticed that the engine was not providing max RPM ( about 2900 was my prior max) and assumed I had a fuel issue.

I changed the primary and secondary filters. Minor improvement was noticed that quickly reverted to the same problem. So I changed them again. No improvement.

So I changed the mechanical lift pump on the engine. No improvement.

I checked all the fuel hose clamps. All OK.

I happened to have had the injectors serviced last winter. I removed and installed them myself but had them serviced by experts. So hopefully it's not the injectors. I was told the injectors had a clean nozzle spray pattern but they were popping off early by about 15% - which they adjusted.

My engine will only now do around 2300 rpm. I can put the throttle all the way forward and no change after 2300. There is no smoke from the engine at all. It runs smoothly, no knocking or stutter.

One weird item I should mention...., for the 1st 30 min of run time from cold, i can get up to 2800 RPM from the engine. After that, it drops down to max 2300 RPM. The only thing i can think of is that fuel is getting hot after 30min and it's impacting some other system?

Any thoughts? Bad governor? Bad high pressure fuel pump. Engine too hot? Engine starved of air?

Thanks in advance for any advice you may have.

Britishsea
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Old 17-08-2021, 22:40   #3
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

Check the exhaust elbow / water injection point for restrictions (both restriction to exhaust flow and restriction to raw water flow.
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Old 17-08-2021, 22:42   #4
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

Quote:
Originally Posted by Britishsea View Post
.............Engine starved of air?

Thanks in advance for any advice you may have.

Britishsea
Check the exhaust elbow / water injection point for restrictions (both restriction to exhaust flow and restriction to raw water flow).
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Old 17-08-2021, 23:00   #5
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

Assuming that the prop and hull are not fouled, the first quick check is to open the fuel deck fill. If the engine revs better, it was the tank vent.

If that doesn't do it, I break out my outboard fuel squeeze bulb with clear plastic tubing and put it in the system ahead of the first filter and ahead of a jerry jug. If it pumps easily with no signs of air in the tubing, I hook the output of the squeeze bulb to the input of the first filter, run the engine, and see if squeezing the bub makes the engine rev better. If no bubbles and no improvement in revs, I move the bulb between the fuel filters and repeat. I've never had to do anything more.
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Old 18-08-2021, 01:49   #6
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

Quote:
Originally Posted by Westcliffe01 View Post
Clogged fuel tank vent ? Or some sort of blockage between fuel tank and lift pump, since it is reducing the maximum continuous fueling rate ? Sometimes fuel line itself will degrade through internal swelling, closing off the flow path. Sometimes it pays to replace fuel lines on a time basis or at least carry some spare material along in case needed.
Thanks for reply. Very good point you are raising. I forgot to mention that i did open a vent on the tank to see if that was the issue. No change.

Could be a blocked fuel line. Good to thing to check for sure. Perhaps when the fuel gets hot, the blockage gets worse or the hose gets softer?
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Old 18-08-2021, 01:52   #7
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

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Originally Posted by Wotname View Post
Check the exhaust elbow / water injection point for restrictions (both restriction to exhaust flow and restriction to raw water flow.
I replaced the exhaust elbow last year with a stainless steel replacement of the yanmar part.

That could be the issue but seems unlikely.

I do have a pressure guage i can put on my exhaust manifold to check for exhaust back pressure.
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Old 18-08-2021, 02:02   #8
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

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Originally Posted by donradcliffe View Post
Assuming that the prop and hull are not fouled, the first quick check is to open the fuel deck fill. If the engine revs better, it was the tank vent.

If that doesn't do it, I break out my outboard fuel squeeze bulb with clear plastic tubing and put it in the system ahead of the first filter and ahead of a jerry jug. If it pumps easily with no signs of air in the tubing, I hook the output of the squeeze bulb to the input of the first filter, run the engine, and see if squeezing the bub makes the engine rev better. If no bubbles and no improvement in revs, I move the bulb between the fuel filters and repeat. I've never had to do anything more.
I did check the vent (forgot to mention in the OP). My hull and prop are spotless ,as the boat was hauled in June.

Thxs for your suggestion. A manual bulb ( with a clear hose to look for air bubbles) seems like a good idea to see if I find the issue on the negative pressure side of the fuel system.

BTW - I did also blow on the tank outlet to see if there was something blocking the tank outlet pipe ( made no change).
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Old 18-08-2021, 09:00   #9
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

Maybe your throttle cable needs adjustment

Is the throttle arm on the injection pump against the stop when you have max throttle applied at the pedestal?

cheers.
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Old 18-08-2021, 09:45   #10
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

If I were to put money on it, and you had not already checked it, I'd say it was a fouled prop.
A badly fouled prop will change my RPMs around 800 RPM on my Yanma.
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Old 18-08-2021, 09:51   #11
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

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Maybe your throttle cable needs adjustment

Is the throttle arm on the injection pump against the stop when you have max throttle applied at the pedestal?

cheers.
Thxs for your reply. I did observe the throttle lever while the engine was running under load to be sure it was free and moving forward correctly. It is and does not seem to be the problem.
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Old 18-08-2021, 09:55   #12
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

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If I were to put money on it, and you had not already checked it, I'd say it was a fouled prop.
A badly fouled prop will change my RPMs around 800 RPM on my Yanma.
I have a sail drive so it would be difficult to foul the prop. Possible with perhaps fishing line. I can check next time im in the water.

But as i mentioned in the OP - I get full power for the first 30 min of motoring, which makes me think this problem is somehow linked / exacerbated by engine heat.
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Old 18-08-2021, 13:50   #13
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

Blocked exhaust.
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Old 18-08-2021, 15:11   #14
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

So am I correct in thinking the injectors were serviced and the Rpm drop came AFTER that? Id be inclined to adjust the valve clearances and do the rpm check under full load with the cable disconnected from the throttle lever.
There are several variations of 3JH3 engines , one is an EPA model with a single bolt injector clamp and the others have 2 bolt clamps and there is some sort of variation with the sealing washer and nozzle protector.
On an older thread a guy discovered that he had 2 sealing washers Instead of one on each injector, one copper and the other was the usual Yanmar hard washer......it made a difference.
Are there any other loads on the engine such as a water maker pump,hydraulic pump or high output alternator?
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Old 19-08-2021, 00:17   #15
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Re: Help w/ Yanmar 3JH3 Power Drop-Off Mystery

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Originally Posted by skipperpete View Post
So am I correct in thinking the injectors were serviced and the Rpm drop came AFTER that? Id be inclined to adjust the valve clearances and do the rpm check under full load with the cable disconnected from the throttle lever.
There are several variations of 3JH3 engines , one is an EPA model with a single bolt injector clamp and the others have 2 bolt clamps and there is some sort of variation with the sealing washer and nozzle protector.
On an older thread a guy discovered that he had 2 sealing washers Instead of one on each injector, one copper and the other was the usual Yanmar hard washer......it made a difference.
Are there any other loads on the engine such as a water maker pump,hydraulic pump or high output alternator?
I do have the EPA (3JH3E) model with the single bolt injector clamp. Seems like a very insecure set up but I have not seen signs of injector blow-by.

I did make sure sure the sealing washers came out of the engine so there should not be double washers. It is possible the injector shop mixed up the specifications for 3JH3 vs 3JH3E version when they set the 'pop' pressure. I guess that would change the timing, but would it should do so if the engine is hot or cold.
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