fuel line priming bulb can pump without opening downstream hole

[CaptainBob]

On a boat with a flawless beta engine which exhibited no problems whatsoever, it was always possible to make a few pumps of an inline pre primary filter bulb before resistance was detected.

Have the same thing now on current boat and people are saying that it is a problem.

Confused.

Any idea how it might be possible to add about 4 full pumps of a bulb into a flawless fuel system?

 

[oldbilbo]

What kind of 'inline pre primary filter bulb' do you have?

 

[Billjratt]

"People are saying it's a problem"...... the only person who knows if you've a problem is YOU.
The bulbs on my outboard and my Renault Meganne both take a few squeezes ( if I try ) and both work perfectly well, so what's to worry about?
If it's working please don't fix it. Let "the people" do the worrying.

 

[CaptainBob]

Hmm. Thanks for replies.

It's a standard fuel priming bulb I've installed in-line, before my primary fuel filter (which is a Racor Duplex).

Take your points that if it aint broke don't fix it, but it _is_ broke. I have air issues somewhere in the system. Engineers are pointing to the bulb pumping possibility as proof positive that there is a LARGE air leak somewhere downstream of it. But as I said in my OP I had the same thing on my previous engine which had zero air leaks.

And I'm intrigued as to how a closed system could possibly allow incompressible fluids to be pumped into them.

Is it possible the relief valve in the injector pump (which leads to the fuel return line) allows fuel to flow a little before closing? Or?

 

[alant]

Hmm. Thanks for replies.

It's a standard fuel priming bulb I've installed in-line, before my primary fuel filter (which is a Racor Duplex).

Take your points that if it aint broke don't fix it, but it _is_ broke. I have air issues somewhere in the system. Engineers are pointing to the bulb pumping possibility as proof positive that there is a LARGE air leak somewhere downstream of it. But as I said in my OP I had the same thing on my previous engine which had zero air leaks.

And I'm intrigued as to how a closed system could possibly allow incompressible fluids to be pumped into them.Is it possible the relief valve in the injector pump (which leads to the fuel return line) allows fuel to flow a little before closing? Or?

Maybe you are creating a 'back pressure', which is forcing the fuel through any non-return 'valve', back to tank/upstream.

 

[sarabande]

I suspect that the pumped fuel is taking up a bit of expansion potential in flexible tubing somewhere, and/or that the non-return valve is not 100% efficient and back-filling the bulb.

If there are no other signs of air in the downstream side, don't worry, but keep an eye open.


Edit. Have you tried to bleed theystem using the squeezy pump ? If so, did the process work properly ?

 

[davidpbo]

If you have a pump on the fuel line which pressurises it slightly then if there is a means of air getting in surely fuel will leak out if the pipe is pressurised and engine not running.

 

[puddock]

If you can't see a leak, then the first thing I would be doing is checking the NRV on the bulb setup

 

[CaptainBob]

Thanks for additional replies. In response...

Edit. Have you tried to bleed theystem using the squeezy pump ? If so, did the process work properly ?

Yes I have and it worked fine. I installed it for that purpose, and also so I can use it to fill the Racor filter chambers into which the drop-in elements go, without having to faff on with a can of diesel - which would be awkward in a bit of seaway.

Perhaps you're right that it's just pipe expansion. I have a considerable amount of flexi hose in the installation downstream of the bulb. They're good thick rated pipes tho so I didn't consider that option as I thought they'd take a heck of a lot of pressure to inflate by any kind of amount.

If you have a pump on the fuel line which pressurises it slightly then if there is a means of air getting in surely fuel will leak out if the pipe is pressurised and engine not running.

I looked but could not find any sign - not even a smear. Did wonder though that the situation is different when running. Some parts of the pipe will be in suction rather than compression. This will change the dynamics of the connections - especially those involving flexi hose and clips.

If you can't see a leak, then the first thing I would be doing is checking the NRV on the bulb setup

But even if there was no NRV in the bulb, with no downstream compromise in the system, fuel shouldn't be able to backflow.

 

[rob2]

A priming bulb, by definition, has two NRVs, one each end of the bulb. These are fairly inefficient, but more than adequate for their role, so pumping will slowly pressurise the bulb itself. It isn't always the case that an air leak also allows fuel to escape. At one time my engine kept stopping due to air and there was no visible symptom. Happily my mate removed the flexible sections and found that the hose ends (under pressure from the Jubilee clips) had softened and cutting a short length off cured the problem. Of course, by then I'd ordered a whole new set of hoses!

Rob.