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Old 01-04-2018, 08:46   #16
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Re: Yanmar Engine runs 4h and revs drop and then stops

I had a similar problem with my yanmar. It would run fine for 4-6 hours then start to idle down and die. Start up after 15 or so minutes.
It ended up being a clogged fuel line. Purchased the vessel in Mobile and had the same issue bringing boat to Florida. Found a brand new fuel line that the previous owner never installed.
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Old 01-04-2018, 08:59   #17
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Re: Yanmar Engine runs 4h and revs drop and then stops

I had a similar oddball problem and turned out to be a piece of gasket
caught on screen of the pickup tube in the tank. Would run for a few
hours, slow and stop. Good luck ! It drove me crazy for a while.
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Old 01-04-2018, 09:04   #18
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Re: Yanmar Engine runs 4h and revs drop and then stops

I agree with Cavalier. Return should run to the day tank. Make sure it enters the day tank well away from the suction line. The return fuel is usually aerated and hot. If the boat has been running this way for a while you might want to use a good fuel lubricity additive for a while. I use Stanadine but any good quality product would do. The additive reduces heat and wear in mechanical fuel injection systems.
Good luck
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Old 01-04-2018, 09:11   #19
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Re: Yanmar Engine runs 4h and revs drop and then stops

It sure sounds like vacuum building up in the tank. I guess for you it would be in the day tank. Is the day tank vented? If it is, check the vent fitting outside the hull, insects often plug those up.
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Old 01-04-2018, 09:13   #20
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Re: Yanmar Engine runs 4h and revs drop and then stops

I think you are on the right track to add a dedicated day tank vent and move the return to the day tank.
However, I would 2nd the call to double check the pickup tube for the day tank. I've had two yanmar experiences that were related to partial blockage in the pickup tube. Both resulted in the engine running fine for many hours and then cutting out.
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Old 01-04-2018, 09:44   #21
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Re: Yanmar Engine runs 4h and revs drop and then stops

I suspect that the draw tube in the fuel tank has a bit of brass filter material over its end, which is intended to keep junk from making it into your other filters. This piece of mesh collects flug from your tank and slowly because covered with the stuff, until it chokes off fuel to the engine and the engine quits. Allowing the fuel pump to stop sucking for a half hour allows the vacuum to subside, and the particles are released into the tank, only to begin their nefarious mischief when you restart the engine. Simply disconnecting the fuel connection at the top of the tank would likely speed this process, although it would allow air to enter the system.

How do I know this? I suffered through a similar problem with Charley, the Holland 67 owned by Nolan Bushnell in 1983, only it was every two hours. Once I discovered the problem, I removed the pre-pre-filter, and promptly clogged the electric fuel pump that was the next item upstream. I now realize that you want the fuel filter close to the fuel tank (in terms of fuel flow) so that the filter can protect everything that follows.

Cheers,

Chuck
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Old 01-04-2018, 09:56   #22
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Re: Yanmar Engine runs 4h and revs drop and then stops

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Originally Posted by Chuck Hawley View Post
I suspect that the draw tube in the fuel tank has a bit of brass filter material over its end, which is intended to keep junk from making it into your other filters. This piece of mesh collects flug from your tank and slowly because covered with the stuff, until it chokes off fuel to the engine and the engine quits. Allowing the fuel pump to stop sucking for a half hour allows the vacuum to subside, and the particles are released into the tank, only to begin their nefarious mischief when you restart the engine. Simply disconnecting the fuel connection at the top of the tank would likely speed this process, although it would allow air to enter the system.

How do I know this? I suffered through a similar problem with Charley, the Holland 67 owned by Nolan Bushnell in 1983, only it was every two hours. Once I discovered the problem, I removed the pre-pre-filter, and promptly clogged the electric fuel pump that was the next item upstream. I now realize that you want the fuel filter close to the fuel tank (in terms of fuel flow) so that the filter can protect everything that follows.

Cheers,

Chuck
Chuck, he has a gravity feed day tank.
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Old 01-04-2018, 11:06   #23
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Re: Yanmar Engine runs 4h and revs drop and then stops

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Originally Posted by paulajayne View Post
Check the fuel tank vent.
Is it clear?
If not tank is running into vacuum and stopping fuel getting to motor.
+1 for fuel tank vent. I think the tank is getting sucked into vacuum. After a half hour "rest" the pressure has equalized. Easy test, remove filler cap when revs are slipping. I had this on my boat (A4). Spiders had clogged the vent line. Stupid spiders.
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Old 01-04-2018, 12:12   #24
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Re: Yanmar Engine runs 4h and revs drop and then stops

I agree with other posters that your fuel is vapor locking. Rather than trying to fix this issue why not just get rid of the stupid, useless, day tank? Not that I have an opinion or anything lol.

Day tanks are/were a military creation for war ships. The idea is to have 24 hours of guaranteed clean fuel to plan with.

Small boats never have large enough day tanks and in my experience are prone to all kinds of failures. I delivered tons of boats for 15 years and some had day tanks and all were problematic at best.

With a small boat day tank you have introduced yet one more failure point as you are experiencing now in your fuel delivery system plus the fact that you need to refill it continuously. With modern filtration (Racors and the like), biocide in the fuel tank religiously, you should have no issues and therefore no need for the day tank.

Think about it. What benefit do you derive by having a day tank? Your fuel goes from the main tank to your secondary filtration system to the day tank to the primary filter and then engine. What is the benefit of storing it in a day tank? None.

I say KISS and take get rid of the day tank.

If you haven't polished your fuel, do so and you will be fine.
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Old 01-04-2018, 14:38   #25
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Re: Yanmar Engine runs 4h and revs drop and then stops

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Originally Posted by zeseb View Post
This is exactly what we were talking with the mechanic. The plan is to create a new hole in the tank for the breather line only and to separate from the fuel inlet.
And also, having the fuel return into the day-tank instead of the pre-filter.
I think you are on the right track and should go ahead with the above plan and test it before getting into anything more complicated.
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Old 01-04-2018, 16:38   #26
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Re: Yanmar Engine runs 4h and revs drop and then stops

What do you mean by a "fuel line"? You mean the pipe between the day-tank and the pre-filter?
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Old 01-04-2018, 17:58   #27
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Re: Yanmar Engine runs 4h and revs drop and then stops

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Originally Posted by mkrajcar View Post
I agree with other posters that your fuel is vapor locking. Rather than trying to fix this issue why not just get rid of the stupid, useless, day tank? Not that I have an opinion or anything lol.

Day tanks are/were a military creation for war ships. The idea is to have 24 hours of guaranteed clean fuel to plan with.

Small boats never have large enough day tanks and in my experience are prone to all kinds of failures. I delivered tons of boats for 15 years and some had day tanks and all were problematic at best.

With a small boat day tank you have introduced yet one more failure point as you are experiencing now in your fuel delivery system plus the fact that you need to refill it continuously. With modern filtration (Racors and the like), biocide in the fuel tank religiously, you should have no issues and therefore no need for the day tank.

Think about it. What benefit do you derive by having a day tank? Your fuel goes from the main tank to your secondary filtration system to the day tank to the primary filter and then engine. What is the benefit of storing it in a day tank? None.

I say KISS and take get rid of the day tank.

If you haven't polished your fuel, do so and you will be fine.
While I don't wish to derail the OP's thread with a discussion about day tanks, it might be pertinent to the OP's issue to raise some argument for a day tank (of which I'm a believer ).

I say KISS and keep a day tank (assumed to be above engine height).
  • A day tank will normally stay gunge free.

    It delivers fuel to the engine by using the most universal and constant method available - gravity - it never fails.

    Any air leak between the day tank and engine high pressure pump will show up as a weep / leak and thus much easier to detect.

    With correctly sized lines, there is no need for an engine driven lift pump.

    There are other arguments but that should wait for an more appropriate thread.
Back to the original issue, it seems to me to be a vent issue.
As for where the return goes, yes best practice would take it back to a tank but I have seen enough cases where it goes back to the top of a filter to say this usually works OK.
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Old 02-04-2018, 05:27   #28
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Re: Yanmar Engine runs 4h and revs drop and then stops

+1 for returning the fuel to the day tank, not the filter. The return fuel is hot and is effectively cooled by the tank's contents, unless of course it's going around and around getting hotter and hotter. Hot fuel is less dense and gives proportionally less power. Very hot fuel? Who knows but it's not good.

Good luck
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Old 02-04-2018, 11:24   #29
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Re: Yanmar Engine runs 4h and revs drop and then stops

check item #3. Remove the line, plag the fitting so that the day tank breather is unobstructed. I think that'll be it.
The excess fuel return line is questionable, but it's unlikely to cause the shut down.
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Old 02-04-2018, 19:35   #30
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Re: Yanmar Engine runs 4h and revs drop and then stops

I contacted the Yanmar engineer in Sydney, and he confirmed that the return line should go to the tank not the CAV filter. Hot fuel will simply not feed enough the chamber at some point,
He also confirmed the breather line clogged theory.
Well, at least I learned something from you guys. Thanks a lot.
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