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Old 26-06-2013, 12:34   #1
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recurring air in fuel line

Dear all,
Yesterday for the fourth time we got air bubbles in the primary diesel filter. The fuel line was rebuilt three times in the last 2 months but the problem keeps coming back. I dont know what I can do next so any suggestions are highly appreciated!!

1. If we use a juice bottle instead of the tank there is no problem, indicating that the problem is in the tank or the nipple on top of the tank.
2. No air bubbles for hours in a row after each rebuild, they show up after a couple of hours for no apparent reason.
3. Fuel is reasonable clean, filter is clean and polishing the fuel by hand through a three stage filter does not show any polutants.
4. The tank does not build up a vacuum, bubbles continue coming after we open the tank.
5. Tank, fuel lines, filter housing are all firmly attached to bulkheads, no apparent vibrations. All connections are/remain tight.
6. According to the builder of the boat there is no filter at the bottom of the fuel intake, just a small pipe going to the bottom of the tank.

Sorry to bother you with this but I really ran out of ideas.

Cheers, Arjan

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Old 26-06-2013, 13:06   #2
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Re: recurring air in fuel line


I feel your pain. Went through this when I bought my boat a few years ago and it can make you crazy.

In my case I finally ended up buying a new Racor filter to solve my problem. The original was 25 years old and it just wasn't sealing well. Plus the design had changed slightly and I was having a problem locating the old parts and gaskets so finally gave up and replaced it.

I have a couple of ideas you might try in your case and a question or two.

1. You don't see bubbles when you use a bottle (yes I did that too) and conclude that the problem is in the line or tank. But you say the problem sometimes doesn't appear for day after a repair so are you sure the problem is just not happening when you switch from tank to bottle? Have you tried using a larger container, like a 5 gallon jerry jug and motoring for a few hours to see if the leak appears?

2. Is it possible there is a piece of trash of some kind in the tank that is sticking in the pickup? So when you repair the lines it may drop back down in the lines but suck back in after some time of use?

3. Other than polishing the fuel have you looked into the tank to see what's inside?

4. How is the nipple attached? Is it possible there's a tiny crack in a weld or leak in the fitting where the nipple attaches?

5. Have you pulled the pickup line/hose/tube from the tank and checked or cleaned that?

The water is always bluer on the other side of the ocean.
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Old 26-06-2013, 14:01   #3
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Re: recurring air in fuel line

What size are the air bubbles?

What find of fuel? Diesel/gas?

Does the filter actually fill with air or do you just see air bubbles?

If the air is filling the filters then might want to look at installing bigger double filters to trap the air, and/or a day tank?
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Old 26-06-2013, 14:08   #4
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Re: recurring air in fuel line

I've had a couple of similar issues from time to time. Suction side air leaks can be difficult as often there's no fuel drip.

One time on an old Volvo one cyclinder, the engine would run provided that we left the bleed screw in the top of the fuel filter cracked open and dripping a bit. If we closed the bleed screw the engine would stop. The air leak turned out to be a rubber fuel hose that had damaged internal wire reinforcing that had worked a pin hole in the line between the tank and fuel pump. No drip evident. By leaving the bleed screw open, the air could escape (with a bit of fuel) and the engine would run.

Another time we traced an air leak on our Universal to a microscopic crack in the brass elbow at the top of the tank where the fuel pick-up tube goes into the tank. No fuel leaking out just air getting sucked in.

I guess the lesson is to make sure everything is clean and tight, hoses are new, fittings replaced every few years.

On our Ericson with the Universal, the tank had several pick-ups installed (it had a diesel heater and a genset once upon a time). I led hoses from these extra pick-ups to a small electric fuel pump and a valve such that I could switch pick-ups if I thought one had become clogged. I also had two Racors mounted with a valve between so that I could switch filters. This system also allowed me to replace filter cartridges while the engine was running. I ran the 2 micron filters which given our sometimes dirty fuel only lasted maybe 50 hours before they clogged up. Sometimes not even that. Putting a small electric fuel pump in the line from the tank also allowed for pre-bleeding the system without turning the engine over. I had a switch rigged to shut off the electric pump but I almost always left in on. Probably made life easy on the engine's fuel pump. The other cool thing about the electric fuel pump was that as it became difficult for the little guy to pump fuel, it would start thumping louder and louder ... sort of a built in alarm that it was time to switch filters. Also as I would bleed air out of the system, the electric pump would change pitch, thus letting me know that I could close the bleed screw.

Nothing worse than an engine quitting just when you need it.
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Old 26-06-2013, 14:22   #5
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Re: recurring air in fuel line

After much agony, I once found a pin hole in the bottom of a copper fuel line which ran thru the bilge and sometimes was immersed in seawater. That was the cause of the air ingress into the fuel line and the occasional speed up/slow down of the engine (Perkins 4-108).

Replaced the fuel line; no problems since.

Be sure your fuel filter is properly seated. If the gasket isn't just right, it can develop leaks.

Look for the simple stuff first.

Good luck,

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Old 26-06-2013, 15:33   #6
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Re: recurring air in fuel line

Hi, Arjan,

If there are bubbles in the filter, the air is coming in with the fuel. Therefore, as others have suggested, air's getting in from somewhere, and after the engine is warm. Maybe what you have is something that expands when warm, allowing air in, and contracts, closing itself, when it is cool. So it's gotta be a tiny crack or pinhole, if all the connections are tight. If all the hose is new, then looking with a really good magnifying glass and with as much light as you can bring to bear, at the top fitting on the tank may show you the little devil! I hope so.

Who scorns the calm has forgotten the storm.
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Old 26-06-2013, 16:14   #7

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Re: recurring air in fuel line

Any crush washers involved in the plumbing? Before I got educated about them, we were haunted by one. They will deform on first use, and if reused they will cause either an air or fuel leak that is usually so small you can't ever quite find it. If there are any crush washers in your plumbing, replace each one after any one use. A whole box of them costs about the same thing as just buying one or two--after postage and handling.
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Old 26-06-2013, 16:39   #8
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Re: recurring air in fuel line

if your pickup is straight down without a curve up at the end, you will continue to run air inline. i did..guess what--isnt difficult to modify your pick up so it doesnt leave a bubble in the line. just curve it slightly up at the end so bubbles go to surface of tank. this is what i did and mine actually works it, dont laugh. is common sense info.
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Old 26-06-2013, 18:54   #9
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Re: recurring air in fuel line

Thank you thank you thank you!!! Today we rebuilt the fuel filter but after 55 min the bubbles were back.... (like a half inch in diameter coming from the round diesel distributor in the racor 500). Tomorrow I am going to bypass the filter and see what happens and tonight I will indeed crawl into the engine space and check for tiny cracks on top of the elbow on the tank. If one knocks hard on the filter bowl there are smaller bubbles showing in the bowl. I just ordered a new bowl seal, who knows..... The bottle is indeed good for slightly less than 1 hour and the hoses are too short to use a jerry jug. I am currently in the marquesas so no chandleries until papeete.... I can not look inside the tank and can not lift the intake pipe as the tank is underneath the cockpit floor. I can unscrew the elbow and nipple and lift it a couple of inches but that is it. Again, thank you!!!
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Old 27-06-2013, 09:20   #10

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Re: recurring air in fuel line

Arjand, if you can't see the parts clearly, you might try wrapping them in silicone or butyl tape, that would seal any hairline cracks or pinholes against sucking in air. Degreasing them and painting them over with epoxy would accomplish the same thing but taping would be a faster way to get a fairly good solution, I'd expect one that would last for years, certainly long enough to get new parts.

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