Fuel servo output problem (long)

[Jim Burns[_2_]]

PA32R-300 Lance with factory fresh Lycoming OH IO540-K. After installing a
JPI 800, with fuel flow transducer, the factory Piper and the JPI fuel flow
only indicate 5-6gph fuel flow with the electric boost pump rather than the
normal 10gph during normal start procedures. The engine driven pump
produces the same results after starting the engine and boost pump off. If
you let the airplane sit for 20-30 minutes, an initial 12gph will indicate
on both gauges with the boost pump only, but it drifts down to 5-6gph within
a couple seconds. No greater flow indications are obtained while running
the engine, even up to 1500rpms, full rich mixture.

Idle is rough, engine wants to cut out unless you jockey the throttle.

So with throttle full open and mixture full rich we did the following:
Pulled the output line from the boost pump, volume and pressure like a
firehose. Same thing coming out of the JPI transducer, same going into the
Bendix/Precision RSA 10ED1 fuel servo. Pulled the servo's screen, found
only a couple small magnetic metal slivers. Flushed the screen cavity,
again huge flow and pressure. Reinstalled screen. Pulled the metered fuel
output hose feeding the fuel divider at the fuel divider... no pressure...
BARLEY any fuel! Completely removed the factory new metered output hose
from the servo, inspected, blew air through it with compressor, no foreign
debris or blockages.

Flowed fuel through servo, without the output hose on servo, no pressure and
barely any fuel moving through servo.

We've got huge flow and pressure entering the servo but next to nothing
coming out. Piper fuel flow gauge was just overhauled and calibrated, only
required changing a control arm in the gauge. JPI transducer was installed
according to all JPI documents including factory trouble shooting tips and
trick. Piper and JPI agree during every test that fuel flow is only 5-6gph.
We even removed the fuel line with the JPI transducer, installed the
original factory fuel line, same result, no pressure, not much flow, Piper
gauge indicates 5-6gph.

Fuel flow coming out the individual injector lines is anemic. No line shows
adequate pressure or volume.

The airplane has been sitting inop for 3-4 weeks while this and other work
was being done. Could the ball valve which meters the servo's output be
stuck limiting the output? Any other ideas? The servo should be covered by
warranty through Lycoming if approved by Precision.

Jim

[Robert M. Gary]

On May 7, 7:00 am, "Jim Burns" > wrote:
> PA32R-300 Lance with factory fresh Lycoming OH IO540-K. After installing a
> JPI 800, with fuel flow transducer, the factory Piper and the JPI fuel flow
> only indicate 5-6gph fuel flow with the electric boost pump rather than the
> normal 10gph during normal start procedures. The engine driven pump
> produces the same results after starting the engine and boost pump off. If
> you let the airplane sit for 20-30 minutes, an initial 12gph will indicate
> on both gauges with the boost pump only, but it drifts down to 5-6gph within
> a couple seconds. No greater flow indications are obtained while running
> the engine, even up to 1500rpms, full rich mixture.
>
> Idle is rough, engine wants to cut out unless you jockey the throttle.
>
> So with throttle full open and mixture full rich we did the following:
> Pulled the output line from the boost pump, volume and pressure like a
> firehose. Same thing coming out of the JPI transducer, same going into the
> Bendix/Precision RSA 10ED1 fuel servo. Pulled the servo's screen, found
> only a couple small magnetic metal slivers. Flushed the screen cavity,
> again huge flow and pressure. Reinstalled screen. Pulled the metered fuel
> output hose feeding the fuel divider at the fuel divider... no pressure...
> BARLEY any fuel! Completely removed the factory new metered output hose
> from the servo, inspected, blew air through it with compressor, no foreign
> debris or blockages.
>
> Flowed fuel through servo, without the output hose on servo, no pressure and
> barely any fuel moving through servo.
>
> We've got huge flow and pressure entering the servo but next to nothing
> coming out. Piper fuel flow gauge was just overhauled and calibrated, only
> required changing a control arm in the gauge. JPI transducer was installed
> according to all JPI documents including factory trouble shooting tips and
> trick. Piper and JPI agree during every test that fuel flow is only 5-6gph.
> We even removed the fuel line with the JPI transducer, installed the
> original factory fuel line, same result, no pressure, not much flow, Piper
> gauge indicates 5-6gph.
>
> Fuel flow coming out the individual injector lines is anemic. No line shows
> adequate pressure or volume.
>
> The airplane has been sitting inop for 3-4 weeks while this and other work
> was being done. Could the ball valve which meters the servo's output be
> stuck limiting the output? Any other ideas? The servo should be covered by
> warranty through Lycoming if approved by Precision.
>
> Jim

If anything is sticking in the fuel servo, that's bad. I had some
sticking in mine (900 SFNEW) , when I'd try to lean in flight the
engine would die before getting near peak, it was also hard to make
idle. There really is nothing field servicable in a fuel servo other
than the fuel screen. I sent mine to a fuel O/H facility who tried to
help me out by cleaning it out and asking me to try it. Of course it
didn't work and it ended up costing me an addition $500 or R&R time
"trying it". In the end I sent it to Aircraft Carb/fuel Service in
Dallas for a full overhaul. Since then I've not had any problems.
Figure about $2000.

-Robert

[Jim Burns[_2_]]

Thanks Robert,
This should be a warranty claim, but the simplest thing to do would be to
simply swap the servo with a rebuilt. My real interest is what is happening
and why. I've been through Precision's website and online documents. The
only thing that makes sense to me is a stuck ball valve limiting the metered
fuel output. It's just about the only moving part within the fuel flow.
Jim

[Robert M. Gary]

On May 7, 11:14 am, "Jim Burns" > wrote:
> Thanks Robert,
> This should be a warranty claim, but the simplest thing to do would be to
> simply swap the servo with a rebuilt. My real interest is what is happening
> and why. I've been through Precision's website and online documents. The
> only thing that makes sense to me is a stuck ball valve limiting the metered
> fuel output. It's just about the only moving part within the fuel flow.
> Jim

Ya. The fuel servo is a super simple system. As you mention, as long
as the diaphragm moves correctly, everything else is just physics.
When I sent mine in the fuel shop they seemed to have a low opinion of
the materials Precision has been using in their products since the
mid-90's. In any case, everyone seemed pretty surprised that a fuel
servo would go bad early. We never did find out what caused mine to
fail, but a full O/H fixed it. Hopefully your installation isn't as
tight as mine, I had to remove the exhaust tubes each time as well as
part of the lower cowl. Plus, after reintalling the servo, figure
several hours to get the idle settings correct.

BTW: When my servo would hang up I would get insane high fuel flows
from the JPI (like 13 gph at idle). In flight I would get high flows
too but when I tried to lean it out, it would eventually drop to zero
(making the anti-sweat fan stop). After the O/H I flew for 45 mintes
over the airport at various speeds and mixture settings, running LOP,
ROP, etc.


-robert

[[email protected]]

Jim Burns > wrote:
> PA32R-300 Lance with factory fresh Lycoming OH IO540-K.

Disclaimer: This is based on experience with fuel systems on ground
vehicles and equipment. I don't have an A&P; I don't even have a TG&Y.
Your mileage may vary.

Having said that, I like practical puzzles, so here goes...

Did this servo ever work right on this engine? I can't tell from your
post if you got the new engine, flew (and/or ran the engine on the
ground) for a while with no trouble, and then did the JPI install, or
if this all happened at once. If the servo never worked on this engine,
there is some fundamental problem - bad servo, bad engine. But if it
worked until the JPI install, then if you back out the JPI install far
enough, it should work again. It looks like you've taken some steps
towards backing out the install (like replacing the JPI transducer with
the original stock fuel line) and not gotten anywhere, so perhaps there
is a more fundamental problem.

This might sound silly, but have you checked the rigging of the throttle
and mixture controls? If the throttle lever on the servo was at idle
but the throttle control in the cockpit was set at 25% when the linkage
got reassembled, you'd never be able to set the servo to full throttle.

> Idle is rough, engine wants to cut out unless you jockey the throttle.

In my extremely limited understanding of how a fuel servo works, it is
looking at both the setting of the manual controls AND the volume of air
being sucked in by the engine when it decides how much fuel to send out.
If the engine is getting "extra" air - air that didn't come through the
fuel servo - then it will run lean, because the servo isn't adding fuel
to compensate for the "extra" air. "Extra" air can come from leaky
gaskets, seals, O-rings between the fuel servo and the intake ports on
the heads. Maybe a crack in the intake plumbing, including one or more
of the heads? Valves not adjusted correctly (if applicable) or valve
timing wrong? (Wrong valve timing would only really apply if this engine
has never run correctly for you... this would imply that the camshaft
was not installed correctly, or the pushrods or rocker arms are the
wrong ones - things that could only have happened at engine assembly
time.)

> Flowed fuel through servo, without the output hose on servo, no
> pressure and barely any fuel moving through servo.

If the engine wasn't turning, this might not be weird. If the servo
doesn't "see" lots of air flowing, it won't send out much fuel. (I
think.) The official way to test this is probably to put the servo on
a calibrated flow bench and suck a known amount of air through it,
while maintaining input fuel pressure/volume at a certain level, and
measure the output pressure/volume. The unofficial way might involve a
shop vac, some duct tape, and fervent prayers that the motor brushes in
the shop vac don't spark *too* much, what with all that fuel vapor
around...

> Could the ball valve which meters the servo's output be stuck limiting
> the output?

I would think that if it was stuck really good, you'd get nothing at
all from the output hose of the servo. If there was a piece of crud in
the ball valve, that might explain what you're seeing, but how'd the
crud get in there?

> Any other ideas?

Precision seems to have most of their service literature online

http://www.precisionairmotive.com/rsasupport.htm
http://www.precisionairmotive.com/rsatroubleshooting.htm
http://www.precisionairmotive.com/servpubs.htm

and their troubleshooting stuff seems to be very much geared to "tests
you can do at East Frog Toe Airport without disassembling the servo,
so you can figure out what's going on before you spend lots of money".
It might pay to take a look at it.

I hope this helps!

Matt Roberds

Disclaimer: This is based on experience with fuel systems on ground
vehicles and equipment. I don't have an A&P; I don't even have a TG&Y.
Your mileage may vary.

[Jim Burns[_2_]]

We're takeing a few steps back and looking at what's new vs what's old. We
checked the fuel pressure on the electric fuel pump, only 9 psi. Lycoming's
overhaul manual calls for the engine driven pump to push 18-26 psi. 9
doesn't sound like enough. We pulled the pump, opened it up and found
plenty of metal dust. It could be that the electric pump simply wasn't
pushing enough pressure against the diaphram and spring to open the ball
valve. Once we get a new electric pump installed and pressure checked we'll
know more.

Jim