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For a year I have been discussing with some fellow Lancair owners about
my engine fuel surge problem. N9070R IVP. Today it happened again and
for the first time I was able to look at the fuel pressure gage. The
pressure came down from 23.8 to 10psi... Your thoughts, Noel
Noel,
This sounds to me like fuel pump cavitation. It could be from a small leak
that allows air to enter the fuel system between the tank and the pump, or a
partial blockage in this section of the fuel system. Two other potential
causes are noted in the next paragraph. The easiest "blockage" to check for
is in the selector valve itself. Take the fittings out of your fuel selector
and confirm that the valve passageways line up well. Specifically check to
confirm that when the selector is in the detent for left wing, the holes
align. I've seen two selectors that partially block the passageways in the
left tank position due to manufacturing sloppiness. The next thing is to
check for kinks or flattened bends in the fuel line between the left tank
and the selector. This is easy to do during construction, and difficult to
detect when installed in some areas. Lastly, almost every leak large enough
to allow air to enter the fuel system will result in a small amount of fuel
leakage as well. Look for any wet or discolored areas around joints and
fittings--any at all is cause to fix.
Last area to check is the flapper valve in the left wing. Don Goetz used to
check them by bouncing the wing up and down. The valve will make a distinct
clicking sound as it opens and closes due to fuel sloshing. If the left side
valve sounds different when compared to the right side, I'd investigate
further. Specifically, it's easy for Hysol to drip onto the valve during
wing closing. This can cause the valve to stick open--allowing the left
pickup tube to "unport" and suck air during any uncoordinated
flight/maneuvering that moves the left wing fuel away from the pickup port,
or it can be stuck closed, causing the sump part of the left tank to be
exhausted during normal flight. The flapper valve has a small hole that
allows a little fuel to come through even when closed, and fuel can enter
this compartment from an opening near the top wing skin, so it's not as
simple to diagnose as you might first think. It seems to me that if the
first flapper valve situation is your problem, you'd notice it during
maneuvering/uncoordinated flight, and if the latter, during or just after
high power operation such as climb out.
You've probably thought of and tried these, but if not, worth considering.
Bob Pastusek
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