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Old 14-04-2016, 03:37   #16
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Re: Air in Fuel System -- Yanmar

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Originally Posted by leftbrainstuff View Post
. . .Always bleed before starting if you've introduced air on either the suction or pressure side. Rule number one of diesel fuel systems.. . .
Is that true? I thought Yanmars were self-bleeding. Also I just got advice that if the engine starts, then no bleeding is necessary in any case.

Which is correct?
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Old 14-04-2016, 03:50   #17
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Re: Air in Fuel System -- Yanmar

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Originally Posted by Stu Jackson View Post
You're right. We have an electric fuel pump that runs all the time. Our fuel tank is above the engine. I usually keep my tank half filled and can (and have) turned my ignition key off for one reason or the other and the engine keeps running. On our boat some skippers have reported that once the fuel tank is less than half full the engine may sputter, but I've never experienced that (in 18 years). So, between the "head" of the higher tank and the siphon effect, even with the fuel pump shut off the fuel flows. It's a Facet pump on a Universal M25 21 hp 3 cyclinder naturally aspirated engine. Good luck.

PS - A good friend once advised me: "Only do one thing at a time, so if it doesn't work, you'll know what it is."
OK, I'll keep that in mind.

But I'm going to concentrate on air leaks before messing with that.

At least if we are sure (see LeftBrain's comments above) that there are no bleeding issues. Engine starts instantly so bleeding the high pressure side would be awkward for me being single handed today, unless you can do it with the engine running.


I think I will:

1. Replace both Racor elements, replace seals and o-rings, carefully reassemble the Racors. This will almost surely not solve it, but just to get this out of the way. Start the engine and try.

2. Run the engine from a fuel can, bypassing Racors and sensors. To eliminate fine fuel filter and high pressure side. I presume the engine will run fine. If not, then then bleed high pressure side and take apart and reassemble fine filter.

3. Bypass the fuel flow sensors with a new piece of fuel hose run directly from Racors to fine filter. If the engine runs fine, then my suspicions will be confirmed and I will need to remake the connections to the fuel flow sensors. If after that the engine doesn't run well, then I ditch the fuel flow sensors or add a boost pump.



This problem was initially incredibly frustrating, because I had intended to spend one hour on installing these sensors, then move on to the other items on my list, but now I've blown two whole evenings on it and it's not over yet.

Cruisers have to be philosophers The bright side is that I've learned more about my fuel system than I ever knew, and that is one system you really, really want to know inside and out.
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Old 14-04-2016, 03:52   #18
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Re: Air in Fuel System -- Yanmar

No that's not correct.

You should PRIME the fuel system so the injection pump isn't dry but they will self bleed the air out thru the injectors.

People bleed a few cylinders to get it running in a timely manner. You don't want to crank until the starter or battery gives out.

Each stroke of the plunger moves a tiny bit of fuel closer to the nozzle. It just takes a long time to fill the line.

Once it's running, although rough, it will pump the air thru the rest of the lines and clear up.
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Old 14-04-2016, 03:56   #19
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Re: Air in Fuel System -- Yanmar

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Originally Posted by twinboat View Post
No that's not correct.

You should PRIME the fuel system so the injection pump isn't dry but they will self bleed the air out thru the injectors.

People bleed a few cylinders to get it running in a timely manner. You don't want to crank until the starter or battery gives out.

Each stroke of the plunger moves a tiny bit of fuel closer to the nozzle. It just takes a long time to fill the line.

Once it's running, although rough, it will pump the air thru the rest of the lines and clear up.
OK, well, what I did was, since only a small amount of fuel was lost when I cut the lines (tap was off and I plugged the lines), I didn't bother priming or bleeding. The engine started right up, then bogged and missed a little as the air passed through it, then ran again ok except that it would not run over 2500 RPM. The injection pump was certainly not dry; the fine filter was full of fuel and the lines were not empty.

I presume that's all ok, and I don't need to worry about bleeding anything now?
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Old 14-04-2016, 05:45   #20
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Re: Air in Fuel System -- Yanmar

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... I'm single handed so would be complicated to crank with stop button pushed while bleeding...
Remote Starter Switch ➥ Actron Remote Starter Switch CP7853: Advance Auto Parts
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Old 14-04-2016, 06:23   #21
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Re: Air in Fuel System -- Yanmar

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I'll need a remote stop button as well!
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Old 14-04-2016, 06:31   #22
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Re: Air in Fuel System -- Yanmar

"At least if we are sure (see LeftBrain's comments above) that there are no bleeding issues. Engine starts instantly so bleeding the high pressure side would be awkward for me being single handed today, unless you can do it with the engine running."
Bleeding the system while running is ok have done it many time just catch spilled fuel sometimes this is required to get that last tiny bit of air out of the high loop in an injector line, also this is the method I use for cylinder balance test it will help determine which cylinder is not firing or not as well as others if the need be
SELF bleeding is a loose term
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Old 14-04-2016, 07:22   #23
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Re: Air in Fuel System -- Yanmar

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Originally Posted by sartorst View Post
"At least if we are sure (see LeftBrain's comments above) that there are no bleeding issues. Engine starts instantly so bleeding the high pressure side would be awkward for me being single handed today, unless you can do it with the engine running."
Bleeding the system while running is ok have done it many time just catch spilled fuel sometimes this is required to get that last tiny bit of air out of the high loop in an injector line, also this is the method I use for cylinder balance test it will help determine which cylinder is not firing or not as well as others if the need be
SELF bleeding is a loose term
I thought that should be possible, as I know that people crack the injector nuts one at a time to check cylinder balance.

But is it needed? That's the $64 question.


Wouldn't air in the high pressure side have a great effect at lower RPM, than at higher RPM? My problems are not at all with low RPM running, or starting. It's all higher load/RPM.
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Old 14-04-2016, 07:44   #24
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Re: Air in Fuel System -- Yanmar

It's not the air in the lines limiting the rpm. It's the lack of fuel. Any air getting in, is secondary to the problem. If you look at the ID of the line, you would see that air does not just sit there.

Diesels RPM is controlled by fuel. More fuel more RPM.

The system is designed to supply enough fuel to reach full RPM, under full load.

You can only get enough fuel, if air is not being mixed in the fuel.

The other reasons for low RPM is a restriction keeping the fuel from moving towards the cylinders, a mechanical problem in the linkage that controls the fuel or an overload on the engine due to oversize prop or dirty hull.
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Old 14-04-2016, 07:47   #25
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Re: Air in Fuel System -- Yanmar

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Originally Posted by twinboat View Post
It's not the air in the lines limiting the rpm. It's the lack of fuel. Any air getting in, is secondary to the problem. If you look at the ID of the line, you would see that air does not just sit there.

Diesels RPM is controlled by fuel. More fuel more RPM.

The system is designed to supply enough fuel to reach full RPM, under full load.

You can only get enough fuel, if air is not being mixed in the fuel.

The other reasons for low RPM is a restriction keeping the fuel from moving towards the cylinders, a mechanical problem in the linkage that controls the fuel or an overload on the engine due to oversize prop or dirty hull.
That's what I thought and I am about 90% sure that I've got an air leak at one of my new connections at the fuel flow sensor.


Did you look at my proposed diagnostic procedure posted above?


Any comments or suggestions?
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Old 14-04-2016, 08:43   #26
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Re: Air in Fuel System -- Yanmar

restrictions in the fuel supply lines, etc. lead to higher suction pressure. this exacerbates air leak problems. if there's restriction, eventually you'll get air leaks. the half empty Racor is half-full of air, that came in somewhere nearby. I've wondered if the Racor doesn't act like an air separator, until it gets too much air in it.
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Old 14-04-2016, 08:43   #27
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Re: Air in Fuel System -- Yanmar

No I was not saying air in the injector line was the problem just that it could be done!The air is coming from the suction side
Your idea about the clamp size is a good place to start sometimes when air is introduced when changing filters other air leaks show up that were not obvious before hand making you think it was something you did. I changed my filter once and afterwards it would not rev over 1500 rpm after a lot of time spent stressing about what could have happened a crack in one of the plugs in the fuel filter was found to be the culprit you could tell by the staining in the crack it had been that way for a long time Wish I could be there to help but not much can be done over the net
My dock neighbor changes his filters and just grinds away on his starter until it starts and when I recommend bleeding he tells me "It's self bleeding " "Well you don,t have to but there are only so many starts in a starter and I will save mine for the day I am close to the rocks" Is my response
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Old 14-04-2016, 08:45   #28
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Re: Air in Fuel System -- Yanmar

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Originally Posted by briblack View Post
restrictions in the fuel supply lines, etc. lead to higher suction pressure. this exacerbates air leak problems. if there's restriction, eventually you'll get air leaks. the half empty Racor is half-full of air, that came in somewhere nearby. I've wondered if the Racor doesn't act like an air separator, until it gets too much air in it.
I'm guess that's what drained out when I cut the supply line.

I'm pretty sure there's no air leak because the engine ran perfectly just two days ago.

Would the residual air in the Racor cause any problems? I wouldn't think so -- and I tried switching to the other filter and it didn't make any difference.
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Old 14-04-2016, 08:47   #29
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Re: Air in Fuel System -- Yanmar

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Originally Posted by sartorst View Post
No I was not saying air in the injector line was the problem just that it could be done!The air is coming from the suction side
Your idea about the clamp size is a good place to start sometimes when air is introduced when changing filters other air leaks show up that were not obvious before hand making you think it was something you did. I changed my filter once and afterwards it would not rev over 1500 rpm after a lot of time spent stressing about what could have happened a crack in one of the plugs in the fuel filter was found to be the culprit you could tell by the staining in the crack it had been that way for a long time Wish I could be there to help but not much can be done over the net
My dock neighbor changes his filters and just grinds away on his starter until it starts and when I recommend bleeding he tells me "It's self bleeding " "Well you don,t have to but there are only so many starts in a starter and I will save mine for the day I am close to the rocks" Is my response
Thanks! And these comments, like all the others posted in this thread, are incredibly helpful!


This incident has been a PITA but the learning has been priceless.
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Old 14-04-2016, 08:51   #30
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Re: Air in Fuel System -- Yanmar

the engine might run fine, until enough air sucked into the Racor to start going on to the engine
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