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Air in fuel lines.

 
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dean.psiropoulos(at)veriz
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PostPosted: Fri Nov 06, 2009 7:50 pm    Post subject: Air in fuel lines. Reply with quote

Hi all:

Been having a lot of trouble with a rough idle problem on my Fuel Injected
0-360 parallel valve engine. I've been going through several tests trying
to locate the source of the problem but so far no joy. One suggestion that
I have received concerns air getting into the fuel lines. It seems a little
far fetched that air could get in through a fitting that is not leaking fuel
but I'm running out of ideas so I'll go through and tighten all my fittings
to see if that helps.

But, that leaves one other possibility and that's air getting in through the
fuel selector valve. I'm using the Van's fuel selector that came with my
RV-6 kit. It sat in its little brown bag on the shelf in my shop for several
years before it got used but seems to working fine. It's not hard to move
the selector and its not leaking, at least externally.

Anyone experience leaking of this valve or any issue with that might let air
into the fuel lines through it? Any ideas on how I might test the valve to
see if it's leaking air into the system?

I'm using the Van's high pressure pump setup that puts the pump ahead of the
selector valve so there will be some suction at the valve. I'm running out
of ideas and thinking about switching to a carb. Hate to do that but can't
find anything wrong after working on it for the last year, maddening.
Thanks.

Dean Psiropoulos
RV-6A N197DM


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Wade



Joined: 16 May 2007
Posts: 16
Location: California

PostPosted: Sat Nov 07, 2009 6:48 am    Post subject: Air in fuel lines. Reply with quote

Dean:

Rough idle covers a fairly broad range. My variety of roughness was usually
associated with heat and probably best described as a series of 'pauses' in
an otherwise smooth idle. I worked with Don Rivera at Airflow Performance
who set me up with smaller injectors and a stiffer flow divider spring to
increase fuel pressure downstream of the fuel control. Don's theory was
simple: heat causes partial vaporization of the fuel... increased pressure
decreases vaporization. Worked for me.

Dennis
RV6/IO360/N54X


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GrummanDude



Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA

PostPosted: Sat Nov 07, 2009 11:11 am    Post subject: Air in fuel lines. Reply with quote

Interesting thing about that air in the fuel line.

In the early 90s there were a few Slingsby T-3 Firefly crashes. These planes were used by the U.S. Air Force Academy for flight training. The belief, at that time, was air in the fuel lines.


The Division in which I worked at that time was tasked with determining if there was air in the fuel lines. We built a mock-up of the fuel system and, sure enough, there was air in the fuel lines. Problem solved . . . or so we thought. As it turned out, the air we followed from the pump to the divider disappeared as the air/fuel mixture was sprayed through the injector nozzle. No big gaps of air sprayed out.


Any time a fuel pump pumps, air is separated from the fuel. The only way to keep the air out, is to raise the fuel line pressure above the pressure at which air separates out. This happens, partly, in the fuel divider. If your engine is carbureted, the float does that; it lets the air vent into the top of the carb.


Just for grins, hook up 20 feet or so of clear hose to the outlet of the fuel pump and pump fuel from one wing to the other. LOTs of air.


As for the crashes, we speculated the crashes were due to instructors telling the new pilots NOT to lean. Why? Jets don't have to be leaned so why teach leaning? Even at the academy's 7200 foot elevation one observer told us the planes would cough black smoke while taxiing.



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AuCountry Aviation
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matrix02(at)shaw.ca
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PostPosted: Sat Nov 07, 2009 4:19 pm    Post subject: Air in fuel lines. Reply with quote

Then reason that you get air in a fuel line is that the shaft of the fuel
selector valve is usually sealed with a single O ring. If the selector shaft
is on the suction side of the pump air will be drawn in past the O ring. An
O ring is designed to seal with pressure pushing it into a small crevass in
a U shaped groove, and it works quite well. The shaft seal will not leak
under pressure, which is the normal way to test for leaks, but the O ring
will be sucked out of position in a U groove under suction and allow air
draw in. The groove for an O ring for negative pressure service is
trapezoidal in shape. The Parker O ring catalogue shows this style. The
trapezoidal groove is difficult to machine in a narrow hole and probably not
considered. An old fashioned packing shaft seal with a compression nut is
better for negative pressure.

Jim Aitken, P.Eng.

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