Frans, While the air bubble in the filter is not uncommon, you might
want to check the selector valve. Slight chance that you have a leaking
"O" ring. Other than that, get a hand operated suction pump and check
the entire suction side of the system.
Jeff - Baby Blue
Frans Veldman wrote:
>
> I guess somebody here can tell me whether this is normal or not.
> While testing my just finished fuel system on my 914, I noticed that quite
> some
air was passing through the filters. (I temporarily use the factory supplied
"glass" filters, to see what is going on and how much gunk will be collected
during the fuel flow tests). I of course expected some air in the system, and
also expected it to last some time before the last bubble would be gone.
>
> Today I decided to let the fuel pump run while I was doing some other things,
just to see if there were no small leaks and to "flush" the fuel system. After
some time I noticed that the "air" still wasn't gone, actually it got worse.
It looks like massive amounts of air are sucked in, also regulary I hear the
pumps swallowing some air.
>
> Although I have no fuel leaks if I leave the system alone, I suspected to have
an air leak upstream the suction side of the filters. Air can pass through
smaller
holes than fuel I think. It doesn't matter whether I'm on Main or Reserve,
so it had to be after the fuel selection valve and before the filters. Happy
with that, because that is just one hose, and a Tee to both filters.
>
> Now I'm not too happy anymore, because after spending a few hours chasing this
mystery air leak, I'm quite convinced there is no leak. Of course there could
be air leaks in both connections before the fuel selection valve, but this would
be a dual fault, and not too likely.
>
> If I close the fuel selection valve while the pumps are still running, the
> fuel
in the filters is instantly gone. In fact, this is how I empty the filters
before disconnecting them. After test #687 and disconnecting once again one of
the filters, a few minutes after I got the filters sucked empty, I noticed how
the air got sucked in again when I pulled the hose of it. It indicates that
the vacuum is pretty good preserved during these few minutes, and this is in
contradiction
with the assumption that I have an air leak. At the same time, the
amount of air I see through the fllters is increasing all the time, it looks
at the moment like the air/fuel ratio is 50%. Still though, the pumps humm happy
and don't seem to ingest much of this "air". Something is not right here.
As indicated by two fuel flow sensors, there is 130 liters per hour of flow, and
the fuel pressure is about 5 psi over the airbox pressure. (The engine is not
running, can't do that yet because I'm not finish
ed
> with the rest of the installation).
>
> Before I go crazy completely, let me ask a question. Is it possible, that
> while
the pumps are pumping 130 liters per hour through the system, that the
underpressure
on the suction side of the pumps just lowers the boiling point of the
fuel so much that the fuel starts "boiling"? Maybe it is just normal to see
all these bubbles dancing around in the fuel filters? If anyone has noticed this
as well, please let me know, it saves me a lot of time chasing "ghost" air
leaks. Temperature at the moment is a nice 20 Celcius and the airplane has been
standing in the sun for a while. (Yup, I know that sun isn't good for composite
airplanes, but messing with the fuel in a confined space isn't very good for
me).
>
> It would explain why today it is much worse than yesterday evening: the
> temperature
is much higher today, It also explains why the amount of "air" apparently
sucked in is much more than the occasional burp I hear through the fuel pumps.
>
> If this is not normal, I can only suspect to have air leaks on both tank
> connections.
Tiny enough not to let them drip overnight, but big enough to let massive
amounts of air in while the pumps start running...
>
> Frans
>
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>
> Visit - www.EuropaOwners.org
>
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> ------------------------------------------------------------------------
>
>
> Checked by AVG - http://www.avg.com
>
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