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Old 21-05-2013, 05:01   #1
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Mystery diesel leak solved

Hello Cruisers,

I'm sure if I had looked properly before, or asked the right question, someone would have pointed me in this direction, but I have had the "joy" of working this one out without assistance.

ANYWAY, way back in December when we first bought the boat and subsequently sailed her home in January, I was troubled by a very slow diesel leak that appeared to be coming from somewhere higher than the main header tank. That should have been a clue in itself, in that clearly pressure was needed to get the diesel up there (ok, in some cases wave motion can also achieve strange things with fluids). Unfortunately my understanding of diesels was a bit poor.

Now, after spending nearly $3K on repairs I am richer in diesel-understanding, if somewhat poorer in cash.

What I learned in the process of repairs is that there is a return line for diesel directly from injectors on some engines, certainly in old mid 80's truck engines like mine. (Lots of discussion on CF about this line, unfortunately I did not connect this with my leak when I read about it) In my case the return line joined the air breather up in the guts of the boat, well out of sight. That connector was a bit ratty and that was the source of the leak.

So if anyone else is suffering a similar leak that only appears when the engine is running, be sure to check your injector return line. Then read lots of other, more learned discussion about how important that line is to your engine (or not), according to various opinions.

Matt

P.S. Also for what it is worth, the $3K repairs were caused by water in the fuel system, another lesson learned the hard way.
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Old 21-05-2013, 18:31   #2
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Re: Mystery diesel leak solved

Hi:

Thanks for the report. I am a newbie who is going to run into stuff like this.

Cheers,

Boulter
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Old 21-05-2013, 18:39   #3
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Re: Mystery diesel leak solved

Hmmm... so someone ran the return line to the vent line.. DUH!
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Old 21-05-2013, 18:40   #4
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Re: Mystery diesel leak solved

to the best of my knowledge, ALL diesel engines have a fuel return line. the reason is that the fuel injection pump overpressures the injector lines. the resulting excess fuel is simply returned to the diesel tank.

also to the best of my knowledge, i've never heard of a return line that connected to the diesel tank breather hose; it has it's own fitting on the tank.

all of the above notwithstanding, you never know what you're going to find until you find it.....
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Old 21-05-2013, 18:47   #5
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Re: Mystery diesel leak solved

Quote:
Originally Posted by onestepcsy37 View Post
also to the best of my knowledge, i've never heard of a return line that connected to the diesel tank breather hose; it has it's own fitting on the tank.
A lot of premade tanks (like the one I had to install when my in-hull model leaked) only have a fill hole, outlet (to engine), and vent. It's common practice to route the return line (from engine) into the vent hose via a T.

No problems in a few thousand miles.
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Old 21-05-2013, 19:13   #6
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Re: Mystery diesel leak solved

Quote:
Originally Posted by rebel heart View Post
A lot of premade tanks (like the one I had to install when my in-hull model leaked) only have a fill hole, outlet (to engine), and vent. It's common practice to route the return line (from engine) into the vent hose via a T.

No problems in a few thousand miles.
Yes, that describes it nicely as my day tank is certainly a pre-fab. Pretty nice Stainless Steel in fairness, and a very good fit, just a bit low on connection points.

Glad to hear it has cause you no headaches, I've replaced all the lines and hose clamps so maybe I can relax a bit.
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Old 21-05-2013, 19:19   #7
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Re: Mystery diesel leak solved

Quote:
Originally Posted by onestepcsy37 View Post
to the best of my knowledge, ALL diesel engines have a fuel return line. the reason is that the fuel injection pump overpressures the injector lines. the resulting excess fuel is simply returned to the diesel tank.
Oooh, this one might get some interesting responses. I am not sure where I ran into the thread, CF or another forum (WHAT!? There are OTHER forums!!?) but it got REALLY heated about the function of the return line.

Arguments ranged from it being critical to pointless, but I err on the side of caution and treat it as critical. Certainly my mate who is a diesel mechanic tells me it reduces injector wear by keeping the injectors lubricated and cool and should be kept clear at all times. He used to work on big ships and apparently the return line went to the fuel tank furthest up the chain from the engines whereapon it kept the fuel in those tanks at a good temperature in freezing climates (he is Scottish) and served a double function of polishing the fuel. Sadly, I am down to one tank so I could not go to that level of smarts and mine just slowly warms the day tank.
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Old 21-05-2013, 19:23   #8
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Re: Mystery diesel leak solved

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Originally Posted by GILow View Post
Yes, that describes it nicely as my day tank is certainly a pre-fab. Pretty nice Stainless Steel in fairness, and a very good fit, just a bit low on connection points.

Glad to hear it has cause you no headaches, I've replaced all the lines and hose clamps so maybe I can relax a bit.
I don't know what kind of engine you have but my Yanmar 4JH probably puts more down the return line than it does to the injectors. Whether or not it's a good idea or not I'll leave to the engine manufacturers. They build what they build, they give you instructions on how to handle it, and there we go.

But yeah, no problems with the routing the return that way.
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Old 21-05-2013, 19:25   #9
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Re: Mystery diesel leak solved

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Originally Posted by Boulter View Post
Hi:

Thanks for the report. I am a newbie who is going to run into stuff like this.

Cheers,

Boulter
Good luck Boulter, I am in the same basket. Every day is a new suprise, some good, some not so good, some downright horrid.

So far my safety buffer of $10K had been depleted by about $5K, so provided I stop "learning the hard way" soon, I will be ok. Otherwise it's going to be a matter of selling family members or pets to science to fund the further education.
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Old 21-05-2013, 20:04   #10
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Re: Mystery diesel leak solved

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Originally Posted by rebel heart View Post
I don't know what kind of engine you have but my Yanmar 4JH probably puts more down the return line than it does to the injectors.
I can't easily tell on my engine setup (an old ED33 Nissan truck engine), but what you say is consistent with what my diesel friend told me to expect. Actually, his point was that the rate of fuel flow through your input filters could look really frightening, until you realised that most of the fuel was going straight back to the tank. Another useful bit of info for those newbies like me who a struggling to come to terms with this stuff.

Matt
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Old 22-05-2013, 04:30   #11
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Re: Mystery diesel leak solved

I believe Yanmar and others use the extra fuel flow through the injector pump as a coolant. Certainly, our day tank gets quite warm after a few hours running.
Regards,
Richard
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Old 22-05-2013, 04:57   #12
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Re: Mystery diesel leak solved

Returning the fuel to a different tank from the one you're drawing fuel from has to be done with a bit of care.

Over time, you might overflow the tank to which the return fuel is run, if it started out full or very nearly so.

OTOH, small day tanks have been known to overheat on long hauls if they're hooked up to an engine with lots of return flow, and they're fairly empty: much of the fuel ends up being recycled multiple times.
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Old 22-05-2013, 12:42   #13
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Quote:
Originally Posted by Andrew Troup View Post
OTOH, small day tanks have been known to overheat on long hauls if they're hooked up to an engine with lots of return flow, and they're fairly empty: much of the fuel ends up being recycled multiple times.
I've noticed that too, the tank is probably 90 degrees Fahrenheit once it's running for over 12 hours. That's in the Baja heat though where everything is hot.
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Old 22-05-2013, 15:35   #14
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Re: Mystery diesel leak solved

Oh great... next thing you know you need to run a cooling loop through the fuel tank! A horrid thought.

It all just reinforces my preference for using sails whenever I can and turning of the engine as soon as practical.
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Old 22-05-2013, 16:32   #15
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Re: Mystery diesel leak solved

The return on my Perkins put very little fuel back... I didnt realize the Yanmar's flowed a lot back....
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