|
|
Rick, Mike
I have looked at all common components and cannot find an air leak or loose
fitting anywhere. If it is intermittent I don't know how to troubleshoot,
but it has not happened again!
My son came up with a possibility that has a high probability of causing the
problem. The only hang-up is that it happened on both vent lines; that seems
to lower the probability.
Here is the theory. The 1/4" tube from the NACA scoop takes a 180 degree
1" uphill bend. The flex tube from the NACA scoop to the tank vent line then
goes uphill another 1" to the fuel tank vent line. The 1/4" vent line goes
downhill to the tank by the camber of the wing. The PROBLEM: a 1" uphill
metal tube, a 1/2" downhill flex tube then 1.5" uphill to the tank vent
line. Think of a "P" trap under your sink.
I took the wing tips off and sure enough that is exactly what I had. Because
I built everything symmetrical, both wing vents were the same. Not only
that, there was clear evidence that there had been something in the P trap
at some time because the flex tubing was discolored in the low spot. It was
very evident in the right wing and not so evident in the left wing. I
naturally fixed the problem by replacing the tubing with a length 1" longer
than 1/2 of the old length.
How did water get into the P trap? As I said, it is the only time I
squeeged the wings parallel to the cord. There was enough moisture that it
was running down the wings in places. (I need to do the experiment of
drizzling water down the edge of the wingtip to see if it collects in the
NACA scoop.) If there was water in the scoop, then it could have been sucked
into the trap from start-up prop wash, taxing, or on takeoff.
Why both tanks? My standard starting procedure is to prime and start on
the left tank and run on it until the Chelton system boots up. Then I switch
to the right tank for taxi, run-up and takeoff. No boost pump until takeoff.
Soooo--
It is possible for both the left and right traps to get water in them from
prop wash or taxi and maybe at takeoff. The boost pump would suck hard
enough to overcome the small amount of water in the trap, but it did not
suck it into the tank. Since there was a small vacuum in the tank, when I
shut off the boost pump the fuel flow and pressure would immediately go to
zero and the engine quit. This assumes that the mechanical fuel pump could
not overcome about 1/2" to 1" water pressure. Before I landed the water had
evaporated and on the ground and ever since I could not/cannot repeat the
problem.
The best reasonable explanation I have come up with!
Yes, I was running LOP as I always do. I have more time than money. I
usually run 12.5 with no wind and 13 to 13.5 against the wind. I tested the
vent lines and neither had a restriction except for the 1/4" diameter. No I
have not drained the gas filter because I hoped to repeat the problem. Now
that I have a "high probability" solution I will sump the filter.
Eugene Long
Lancair Super ES
glong2@netzero.net
-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of
rtitsworth
Sent: Thursday, May 10, 2007 1:12 PM
To: lml@lancaironline.net
Subject: [LML] Re: IO-550 fuel injection
Eugene,
More (silly) thoughts...
At first it seemed as if the symptoms could be related to a partially
plugged vent line (bugs, dirt, etc), which was perhaps aggravated by some
moisture. However, it seems unlikely that both lines would be plugged and
that the results would be so binary at the same time. Thus, I'd first focus
on the common components.
To test the vent line, open the filler cap and suck on the vent line (naca
end). DON'T BLOW ON THE VENT LINE, AS ANY FOD MAY BE FORCED INTO THE TANK.
Also naturally, caution against inhaling any fuel and/or vapor.
What about an intermittent suction leak between the boost pump and the mech
fuel pump? The mech pump would then just draw air (versus fuel). You would
think that a loose fitting (etc) would also leak fuel - but I've seen poor
fittings that would suck air and yet not drip/spray liquid.
The mech fuel pump has a case seal (and associated drain). If the case seal
fails, does/can the pump draw air??? If you run the boost pump with the
engine off and the mech pump case seal has failed, will fuel typically
run/drip out the drain? (that's my assumption as a test). Do your injector
drains and fuel pump case drain all run together (making individual ID
difficult)?
Still unknown as to why it (seemingly) fixed itself and/or whether the
humidity had any relevance.
At 12.5 GHP it sounds like you were LOP (guess). Were you at similar power
settings on the later return trip when things were fine?
Rick
--
For archives and unsub http://mail.lancaironline.net:81/lists/lml/List.html
|
|