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Here are two stumpers I'd really
appreciate some advice on. I'm just wrapping up the annual and started the
engine up for a test flight.
1. Vacuum failure - after
startup the vacuum gage read zero and the artificial horizon did not
budge. In addition there was a new squealing sound when the rpm was 1,000
or less. Sounded terrible, like dry bearings. All other engine
readings were normal. The AP thinks it's the vacuum pump (Parker unit with
300 hours and 4 years on it). What's confusing him is that the pump shaft turns
with the prop, when every case he's seen, the shaft shears and the pump
internals don't turn with the prop. There are holes in the pump case allowing
you to see rotation. Since all lines are attached (I did not disconnect
anything in this circuit for the annual), I figure it has to be the pump or the
regulator. The AP thinks that if the regulator somehow stuck open, there
would still be enough vacuum drawn to show something. Anybody think it's
not the pump?
2. Fuel Boost pump not
pressurizing - My system has the normal header tank, an inline fuel filter
mounted horizontally right below the tank, then the boost pump below the filter
(all on the cabin side of the firewall), then to the gascolator. The Dukes
boost pump never worked well from day one. Most days, it would seem like
it was sucking air. Sometimes it would pressurize. I switched this
annual to another used pump that I was assured worked well. This pump
appears to be sucking air too. To check fuel flow, I opened the drain
valve on the gascolator and got a steady flow out. That means fuel is
definitely flowing past the boost pump. Leaving the drain open, when I
turn on the boost pump, the flow out the drain valve stops. Turn the pump
off and the flow starts back up again. It's as if the pump was working
backwards. I switched the power leads and no difference. My previous pump
would flow like gangbusters out the gascolator drain when I did this. The
new pump went in exactly the same way as the old pump, but it's 20 or so serial
units newer. When looking at the fuel dump overboard port, I have fuel
going into the right side and exiting the left side.
One mistake I made on the old pump
installation is that I assembled the fittings to the pump without o-rings and
used thread sealant (no idea what I must have been thinking). These
fittings are straight threads requiring o-rings. Don't know if that
explained the sucking air problem and the customary fuel smell for 30 seconds
immediately after takeoff.
Anyway, I'm stumped. Some of
you have mentioned putting your boost pump low under the seats, but that's been
guys running without header tanks. In my case, I can't see how that would
make a difference. Could the horizontally mounted inline fuel filter
(trapped air in that thing) cause a problem?
Regards,
Ed de
Chazal
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