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Question: small bubbles in the fuel line ❓❓❓

romanov

Well Known Member
Guys,

SLtq30r.jpg



I am testing the fuel line with transparent pipes,
just to see that there is no leakages in the system.
The result of the test is that there is no leakage on
the line but I do see very small bubbles running down
the stream once the boost pump is on. We already
done soap water test on the line and tied all the fittings.
So the first question is:
Where those bubbles are coming from ❓
the second:
Is it ok for Lycoming injector to get those bubbles,
is it going to be ok dealing with a bit of air ❓

Thanks a lot fellow builders.
Roman.
 
I think Dean Martin had a song about tiny bubbles.

There have been a lot of threads over the years. It is better to have no bubbles.

What temperature is it in your hangar? ARE YOU USING LL100?
What gascolator or filters are you using between the tanks and the pump?
What make of boost pump are you using?
How full are your fuel tanks?
If you restrict the output flow from the discharge line to full power full rich engine draw flow rate, do the bubbles go away?

Negative pressure is what allows the bubbles to form. Can be turbulence at elbows or other points of restricted flow.

I suspect (but do not have test comparisons to prove) that different makes, models and sizes of boost pumps cause their own turbulence and will have different results.

Try YOUR pump with a large diameter clear line to the input side with 2 or 3 feet of static pressure head . If no bubbles on the discharge when running then try the same at zero head. If still no bubbles, then go looking for small elbows or restricted flow areas in your fuel system.

Also, if you have a gascolator or any spring loaded quick drains in the fuel feed system, if the pump has sufficient draw to make enough negative pressure it can open a spring ball water drain(as in the bottom of some makes of gascolators ) and allow air to enter.
 
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Bubbles

Roman,
Not to worry. Since you have no pressure on the outlet there will always be bubbles (vapor).
I had the same issue when testing. I worried about an air leak.
If you were to install a restrictor or valve to increase the pressure on the line rather than letting it free flow you would see the vapor disappear.
 
If you have fuel injection, your boost pump has a short-loop re-circulation line, and your are restricting the flow to simulate real-world system pressure, then the bubbles may well be fuel vapor released in the re-circulation heated fuel. ALL such installations should have full-sized return lines to the fuel tank instead of short-loop recirculation through the fuel pump. -Otis
 
Roman,
Not to worry. Since you have no pressure on the outlet there will always be bubbles (vapor).
I had the same issue when testing. I worried about an air leak.
If you were to install a restrictor or valve to increase the pressure on the line rather than letting it free flow you would see the vapor disappear.


bRmvr8t.jpg


Looks like "no pressure down the stream"
was the issue. I have added some pressure
by shutting 70% of the pipe and all the bubbles
magically disappeared. Thanks for all the
masters.

:D:D:D

P.S: we are testing with 95 mogas but the system
will run eventually with 100LL - so even less
vapor in the final system
 
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