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  #1  
Old 07-07-2008
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Fuel system losing prime

I've just installed a new engine - everything is working smoothly, with one small exception. When we let it sit for a few days between running, the fuel system loses prime.

It's easy enough to bleed and get started, but I'm wondering if there is some method for tracking down the source of the problem?

We have 2 fuel tanks which run to a selector valve, then to 2 external filters, then to an electric fuel lift pump, and finally to the engine. Somewhere in the system there is air coming in, but very slowly. Anyone have experience tracking down something like this?

Thanks!
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The electric fuel pump should restore the prime effortlessly when it 'loses it'.

I'd look to your return lines as the source of your lost prime, boat design, gravity and liquid having a tendency to drain etc.. it's your most likely source.
On my my Westerbeake 30b I just hold the preheat button for 5-10 seconds and the pump takes care of it (self bleed, self prime).

Once your pump turns off gravity will do it's thing and empty the return fuel line into your tank. Suction created by the vacuum of the fuel going down will draw out more, ergo, lost prime.
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Yeah, this is most likely Chuck says, but if you insist to go for "gravity", change your actual hoses for a transparent one, temporarily. If air comes just from hose connection, you lost your time, but if comes from filters, joints, engine pump, etc ... then you might be able to see the air the bubble forming.
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Out of Cap'n Gary's bag of tricks:

Get between the Racors and the tank(s). Install one of those squeeze bulbs that outboards use. This does two things for you. It gives you a very easy and neat way to refill the Racor housings after a fuel filter change. (Beats the h*** out of a bucket or something.) The second thing it does is provide a check valve to prevent the fuel from finding its way back to the tank. Once the main low pressure fuel pump, be it electric or mechanical, starts drawing, fuel will run happily through the squeeze pump.

This squeeze pump is also much easy to use than those bloody little levers they put on most mechanical pumps. (You know! The one that takes about two hundred strokes with your thumb to pull four ounces of fuel from the tank!)

I'll take the credit (or blame) for this one. I got sick of trying to get my old Westerbeke/Perkins going again after I changed filters, and came up with this solution. The only fly (a small one) in the ointment is that you have to change the squeeze bulb every couple of years.

Hmmm. Maybe I'll patent it. Never mind. Don't do it! (Joking).
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Great Gary !! I used it also in an occasion where my pump failed at all, by just sending an "animal crew" to pump it mannually until we return to dock. I do the same on my AC plumbing system and work perfect as well.

Another positive point to your solution !!
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pump ball

The pump ball doesn't last forever but the advantages sure do. We have them on every boat except the Island Packet (it's electric). When you have to do lots of filter changes ,the only thing better is having someone else do it.
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I had on old racor that just one day started leaking in air somewhere. Took me weeks to figure it out. I replaced the o-rings and the filter but it would drain back to the tank every nite. I purchased a new racor and put in a check valve and no more problems
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We actually have an electric lift pump fitted behind the filters, this is even easier than the squeeze pump for bleeding the system, but maybe this seal is faulty.

It's also worth noting our fuel tanks are in the bilge, very far below the engine, so some kind of syphon effect from the fuel return line is a definite possibility. I've read the fuel return hose is supposed to loop down to the level of the bottom of the tanks, before heading up to the tank return. I didn't have quite enough hose to go all the way to the bottom, maybe I'll give this a try as a first shot.

Thanks.
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I had a problem with losing prime. I happened to mention it to a Yanmar guy who was working on a neighbor's boat. He said it is typcially the banjo bolt as attaching that to the engine is the responsiblity of the yard and they generally don't tighten enough for fear of collapsing it. He climbed in my boat, ask for a wrench, pointed it out to me, gave a tightening and I haven't had a problem since (it is the bolt that connect the fuel line to the engine. It is on the starboard side towards the aft end of the block on a 3GM30).
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Overtightening a banjo bolt will often lead to a fuel system air leak as well.
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