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Old 01-20-2015, 07:41 AM   #1
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Yanmar - need to bleed air after two days nonuse

Looking for things to check. Bay Pelican has a 27 hp Yanmar as a backup engine. Worked wonderfully until last year when I installed a separate 12 US gallon diesel tank and new Racor filter so that the engine was totally isolated from the main engine and its fuel supply.

The tank I installed was a decent brand 12 gallon tank with a top feed, typical Racor 500 series, new 5/16" fuel lines.n While the fuel line approaches the engine from below the fuel pump and thus diesel could drain down this is the same arrangement as before.

Now every time the engine sits I have to bleed air from the engine to get it started. A slight amount of air collects after the fuel pump at the location of the second bleed screw right before the injection pump.

I have tightened all fittings. I have checked the seal on the Racor. Tightened the bleed screws on the Yanmar (there are two).

The 12 gallon tank while it is a top feed is above the level of the Yanmar's fuel pump, as is the Racor. The old tank feed was from the bottom of Bay Pelican's fuel tanks and thus there was pressure from the weight of the fuel.

My next approach is to connect the current Racor to one of Bay Pelican's bottom feed fuel tanks directly and not use the new tank and see if that changes anything. If that doesn't work I will bypass the Racor and see if it is the problem (anything is possible.)

Wondering if the Yanmar fuel system is such that there is a slow leak of air and the fuel is draining back to the tank or the Racor.

Another approach I am thinking of is raising the last 12 inches of the fuel hose above the fuel pump so that that fuel drains down and puts pressure on the fuel pump.

Any other things to check or suggestions would be appreciated.
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Old 01-20-2015, 10:39 AM   #2
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My next approach is to connect the current Racor to one of Bay Pelican's bottom feed fuel tanks directly and not use the new tank and see if that changes anything. If that doesn't work I will bypass the Racor and see if it is the problem (anything is possible.)

Wondering if the Yanmar fuel system is such that there is a slow leak of air and the fuel is draining back to the tank or the Racor.
Marty,

Obviously you have a small air leak somewhere and your approach to troubleshooting it sounds right. I have been around and owned several smaller Yanmar diesels (18-100 hp) with both top and bottom-fed tanks and Racor 500 filters. No configuration was any more troublesome than the other.

The factory installation manuals reference tanks located both above and below the level of the lift pump, the only requirement being that they be located within the suction head capacity of the pump if located below it.

The mechanical lift pumps are pretty durable, but I did have to replace one once that was exhibiting a problem like yours due to a ruptured diaphragm.

Sounds like you will find it pretty quickly using your troubleshooting process. Re-connecting to the original supply source is definitely where to start, particularly if nothing downstream of that connection has been changed or disturbed. Oh, a handful of new copper crush washers of the appropriate size for bleed screws and banjo fittings might be in order.

Good luck
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Old 01-20-2015, 10:58 AM   #3
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One trick is to temporarily replace your suction and return lines with clear vinyl hose. Prime up and run engine, then observe hoses and see where the air is coming from. Tracking down tiny air leaks in fuel systems is a PITA.
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Old 01-20-2015, 11:51 AM   #4
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One trick is to temporarily replace your suction and return lines with clear vinyl hose. Prime up and run engine, then observe hoses and see where the air is coming from. Tracking down tiny air leaks in fuel systems is a PITA.
That's a great tip. I once saw that method used on a old Volvo diesel that had defied everything else. In fact, it took a full day to actually see any bubbles accumulate. The wise old mechanic that showed me that trick told me to be sure to put a loop or hump in the line so that any smaller or emulsified bubbles could accumulate in the clear tube. I had completely forgotten that one.
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Old 01-20-2015, 01:34 PM   #5
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If the bleed screws have copper washers, either replace them or anneal the old ones. They get rock hard and it's very difficult to get a seal.
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Old 01-20-2015, 03:28 PM   #6
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Do you have an engine mounted filter? Last time I did my filters, I had a small air leak in one engine secondary...not enough to leak fuel, but enough to let air in when idle. Same symptoms. The seal was very slightly pinched in the seat.
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Old 01-20-2015, 05:34 PM   #7
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look for any fuel weeping with the system pressurized while running.
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Old 01-21-2015, 07:13 AM   #8
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I had a similar problem years ago. I taped paper towel around every junction in the fuel system and waited until one got wet. Also found a very small leak in my potable water system with same method.
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Old 01-21-2015, 10:23 AM   #9
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Many thanks. Working on several of the suggestions.
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Old 01-29-2015, 07:11 AM   #10
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Follow up. It appears I have solved what should have been a very easy problem to fix. The feed hose from the Racor to the engine is 6 feet long and is 5/16" - small. When I replaced the hose with a clear hose it was obvious that manually filling the hose left several air pockets in the feed line. I had been using a funnel. After seeing the air bubbles I was able to work them out of the clear hose and the engine worked very well. Since I couldn't see the air bubbles in the regular fuel hose and thus could not work them out manually, I attached the hose to my fuel polishing pump and pushed fuel through the hose to a bucket. Engine appears to be starting and running well.
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Old 03-07-2015, 10:01 AM   #11
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I have continued to work on this problem. The Yanmar will need to be bled if I let the engine set for two days. However if I close the ball valve between the fuel tank and the Racor after using the engine and then open the ball valve right before using the engine the engine will start and run.

Looking for an explanation and whether this means the leak is in the fuel tank and its fittings or is it just that with the ball valve open the suction pulls the fuel back into the tank because of an air link between the ball valve and the engine?

Any thoughts?
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Old 03-07-2015, 10:47 AM   #12
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Top or bottom feed?

Sounds like it must be top feed, then there is the problem.
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Old 03-07-2015, 12:06 PM   #13
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Perhaps you have a bad fuel check valve? Or need to add one now that you changed the style of tank feed.

I assume you have tightened all the fuel line connections and checked the seals on the Racor. Have you actually removed, cleaned and resealed the fuel connections as well at this point?
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Old 03-07-2015, 12:19 PM   #14
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The leak is behind the valve or the valve itself. I had a stopcock valve with a crack which which too me forever to diagnosis. Eliminate this valve in your set p and try.
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Old 03-07-2015, 06:31 PM   #15
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It is a mueller 12 gallon tank with a top feed. Put this in so it would be independent of the main fuel supply.

Yes everything has been tightened from the ball valve to the engine. Hose clamps replaced. Washers on Banjo bolts and bleed screws replaced.
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Old 03-07-2015, 09:50 PM   #16
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It could possibly be a slight leak at the connection between the plastic pickup tube and the top elbow on your top feed tank fitting. You'd have to remove the fitting from the tank to check it.
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Old 03-07-2015, 11:51 PM   #17
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Run a fuel line right from the first connection at the tank directly to the genset bypassing everything else that's new and see what happens.
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Old 03-08-2015, 12:16 AM   #18
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Run a fuel line right from the first connection at the tank directly to the genset bypassing everything else that's new and see what happens.
And make it clear tubing so you can spot any air in the system, and which direction it is migrating to/from.

If that is leak free, then re-plumb using the clear tubing in every segment of the system and observe again. Something is bound to become visible at some point.

If the Racor is new, it is possible, but not very likely that you could have a leak around the cover or one of the threaded fittings. There is also an internal ball check valve and seat that could be leaking.

Finally, here is some info from Yanmar and Racor on the fuel system and filter installation. I also included a breakdown that shows the internal check valve.

Hope that helps. Good luck.
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Old 03-08-2015, 03:55 AM   #19
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Many thank to all for the suggestions. It gives me something to work on over the next week.
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Old 03-08-2015, 07:13 AM   #20
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I was going to mention the checkvalve in the Racor that Larry talked about. If this is not a new Racor and has been rebuilt it is possible that the seat for the ball valve was installed upside down. Refer to the link from Compass on the rebuild of Racors. Rebuilding A Racor Turbine Filter Photo Gallery by Compass Marine How To at pbase.com
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