PA28-181 Fuel pressure drop off - Inspired by the Gascolotor thread.

I have seen the pressure problem on two different models years of
Seminole as well.
Michelle

Kyle Boatright wrote:

"Roy Page" wrote in message
ink.net...


I bought our Piper Archer [PA28-181] just over a year ago from a
Californian owner and flew it home to it's new base in Ohio.
On the flight home, we noticed on long climbs that the fuel pressure fell
off from an indicated mid scale reading to about one third scale.
In level cruise the indicated pressure returned to mid scale.
Although we took the top cowl off at every stop to check carefully that we
still had all the important bits still hanging on, we failed to notice
that the gascolator had a slight fuel leak until a few days after we got
the bird home.
You need to remove the lower cowl to get access to the gascolator.
Our A&P put a new bowl seal on the gascolator which cured the leak and
appeared to cure the fuel pressure changes.
A few months passed and then occasionally I noticed a slight fall off in
fuel pressure when climbing.
I concluded that the mechanical engine driven fuel pump must be getting
tired and, to be safe, replaced it with a new pump three weeks ago.
No change, the fuel pressure still falls off when climbing and returns to
mid scale in level cruise.

I also belong to the Taylorcraft Flying Club
www.taylorcraftflyingclub.org.
The club, these days, flies a fleet of three PA28's.
Our PA28-180 has a similar fall off in fuel pressure when climbing.

Ideas please ?

Roy
N5804F



Here's one...

The pressure transducer in your airplane may not have a large enough vent
port, which causes a false low fuel pressure reading when you climb and a
false high pressure reading as you descend. The transducer vents to ambient
air, so it can compare the internal (fuel) pressure to the ambient
(reference) conditions and arrive at the difference. That difference is
"fuel pressure".

If the vent port on the transducer is undersized (and they ARE small), or if
it is partially clogged, its reference pressure is off. If you're climbing,
the transducer thinks the reference pressure is higher it than it actually
is, because the higher pressure air inside the transducer's reference side
hasn't equalized to ambient. Therefore, it compares the internal (fuel)
pressure with the reference pressure and sees a smaller difference than
expected. That plays out as a low fuel pressure reading. When you descend
after being at altitude, the reference pressure is low (because you were at
a higher altitude where the air pressure is lower), and the transducer
compares the actual pressure with a low reference number, and, volia - high
fuel pressure reading.

I saw the same thing in 300 hours of flying my Tomahawk, and see it again in
my RV-6.

KB

I watched this carefully in out PA 28-151 today...

In "enroute" climb, (83 Knts) , with the elect pump off, fuel
pressure would drop 30% down from the indication with elect fuel pump
on.

Seemed to be related to the pitch attitude,....... nose up = fuel
tanks lower related to pumps/carb.

Would make sense...

Was always in the green, engine ran fine...

Dave


On Wed, 28 Sep 2005 01:06:28 GMT, Michelle P
wrote:

I have seen the pressure problem on two different models years of
Seminole as well.
Michelle

Kyle Boatright wrote:

"Roy Page" wrote in message
link.net...


I bought our Piper Archer [PA28-181] just over a year ago from a
Californian owner and flew it home to it's new base in Ohio.
On the flight home, we noticed on long climbs that the fuel pressure fell
off from an indicated mid scale reading to about one third scale.
In level cruise the indicated pressure returned to mid scale.
Although we took the top cowl off at every stop to check carefully that we
still had all the important bits still hanging on, we failed to notice
that the gascolator had a slight fuel leak until a few days after we got
the bird home.
You need to remove the lower cowl to get access to the gascolator.
Our A&P put a new bowl seal on the gascolator which cured the leak and
appeared to cure the fuel pressure changes.
A few months passed and then occasionally I noticed a slight fall off in
fuel pressure when climbing.
I concluded that the mechanical engine driven fuel pump must be getting
tired and, to be safe, replaced it with a new pump three weeks ago.
No change, the fuel pressure still falls off when climbing and returns to
mid scale in level cruise.

I also belong to the Taylorcraft Flying Club
www.taylorcraftflyingclub.org.
The club, these days, flies a fleet of three PA28's.
Our PA28-180 has a similar fall off in fuel pressure when climbing.

Ideas please ?

Roy
N5804F



Here's one...

The pressure transducer in your airplane may not have a large enough vent
port, which causes a false low fuel pressure reading when you climb and a
false high pressure reading as you descend. The transducer vents to ambient
air, so it can compare the internal (fuel) pressure to the ambient
(reference) conditions and arrive at the difference. That difference is
"fuel pressure".

If the vent port on the transducer is undersized (and they ARE small), or if
it is partially clogged, its reference pressure is off. If you're climbing,
the transducer thinks the reference pressure is higher it than it actually
is, because the higher pressure air inside the transducer's reference side
hasn't equalized to ambient. Therefore, it compares the internal (fuel)
pressure with the reference pressure and sees a smaller difference than
expected. That plays out as a low fuel pressure reading. When you descend
after being at altitude, the reference pressure is low (because you were at
a higher altitude where the air pressure is lower), and the transducer
compares the actual pressure with a low reference number, and, volia - high
fuel pressure reading.

I saw the same thing in 300 hours of flying my Tomahawk, and see it again in
my RV-6.

KB

Dave,
Thanks for your input to this discussion.
Your findings are almost identical to mine in my PA28-181.
I think that your comments are valid but would indicate that the mechanical
fuel pump on all PA28's is marginal.
A previous explanation of the pressure drop centered on the pressure
transducer not venting sufficiently in climb.
A situation is emerging in this discussion which clearly shows that many
PA28's exhibit this fall in fuel pressure.
Apparently without causing any fuel starvation to the engine.
Whilst a number of people have identified causes, no one as yet as come up
with a fix.
It seems that I had better take Cory's advice and cover up the gauge :-)
Thanks again.

Roy
Archer N5804F



"Dave" wrote in message
...
I watched this carefully in out PA 28-151 today...

In "enroute" climb, (83 Knts) , with the elect pump off, fuel
pressure would drop 30% down from the indication with elect fuel pump
on.

Seemed to be related to the pitch attitude,....... nose up = fuel
tanks lower related to pumps/carb.

Would make sense...

Was always in the green, engine ran fine...

Dave


On Wed, 28 Sep 2005 01:06:28 GMT, Michelle P
wrote:

I have seen the pressure problem on two different models years of
Seminole as well.
Michelle

Kyle Boatright wrote:

"Roy Page" wrote in
message
hlink.net...


I bought our Piper Archer [PA28-181] just over a year ago from a
Californian owner and flew it home to it's new base in Ohio.
On the flight home, we noticed on long climbs that the fuel pressure
fell
off from an indicated mid scale reading to about one third scale.
In level cruise the indicated pressure returned to mid scale.
Although we took the top cowl off at every stop to check carefully that
we
still had all the important bits still hanging on, we failed to notice
that the gascolator had a slight fuel leak until a few days after we got
the bird home.
You need to remove the lower cowl to get access to the gascolator.
Our A&P put a new bowl seal on the gascolator which cured the leak and
appeared to cure the fuel pressure changes.
A few months passed and then occasionally I noticed a slight fall off in
fuel pressure when climbing.
I concluded that the mechanical engine driven fuel pump must be getting
tired and, to be safe, replaced it with a new pump three weeks ago.
No change, the fuel pressure still falls off when climbing and returns
to
mid scale in level cruise.

I also belong to the Taylorcraft Flying Club
www.taylorcraftflyingclub.org.
The club, these days, flies a fleet of three PA28's.
Our PA28-180 has a similar fall off in fuel pressure when climbing.

Ideas please ?

Roy
N5804F



Here's one...

The pressure transducer in your airplane may not have a large enough vent
port, which causes a false low fuel pressure reading when you climb and a
false high pressure reading as you descend. The transducer vents to
ambient
air, so it can compare the internal (fuel) pressure to the ambient
(reference) conditions and arrive at the difference. That difference is
"fuel pressure".

If the vent port on the transducer is undersized (and they ARE small), or
if
it is partially clogged, its reference pressure is off. If you're
climbing,
the transducer thinks the reference pressure is higher it than it
actually
is, because the higher pressure air inside the transducer's reference
side
hasn't equalized to ambient. Therefore, it compares the internal (fuel)
pressure with the reference pressure and sees a smaller difference than
expected. That plays out as a low fuel pressure reading. When you
descend
after being at altitude, the reference pressure is low (because you were
at
a higher altitude where the air pressure is lower), and the transducer
compares the actual pressure with a low reference number, and, volia -
high
fuel pressure reading.

I saw the same thing in 300 hours of flying my Tomahawk, and see it again
in
my RV-6.

KB

Roy Page wrote:
: Dave,
: Thanks for your input to this discussion.
: Your findings are almost identical to mine in my PA28-181.
: I think that your comments are valid but would indicate that the mechanical
: fuel pump on all PA28's is marginal.
Given the explanation I got from Petersen on the autogas STC, it would seem
the FAA agrees.

: A previous explanation of the pressure drop centered on the pressure
: transducer not venting sufficiently in climb.
Interesting, but with as crappily as a PA28 climbs, it would have to be damn
near closed off. A 1000' change in altitude is only 1/2 PSI change in cabin pressure
and takes between 1 and 3 minutes to do in a Cherokee. I'm not saying it's
impossible...just seems a more unlikely explanation. Of course, that won't stop me
from experimenting on mine and seeing if I can see that.

: A situation is emerging in this discussion which clearly shows that many
: PA28's exhibit this fall in fuel pressure.
: Apparently without causing any fuel starvation to the engine.
As long as there's enough pressure to fill the carb bowl, everything is fine.
Sure there's a *slight* change in float bowl level with a varying input pressure, but
it's probably not enough to measure, let alone care.

: Whilst a number of people have identified causes, no one as yet as come up
: with a fix.
: It seems that I had better take Cory's advice and cover up the gauge :-)
: Thanks again.

Now you're talking.

Just FYI... while my electric fuel pumps are from the autofuel STC and are
different from stock, when they're off the system is the same. In a full-power,
full-rich, power-on-stall attitude, the mechanical pump can only muster about 1 psi on
the gauge IIRC. Not exactly comforting. In a less aggressive attitude, it's more
like 2 psi. In cruise or any attitude with the electric pumps on, it's rock-solid at
5 psi. I *do* seem to recall that with the stock (non-autogas STC) electric pumps,
the pressure would still fluctuate some at high power, aggressive attitude settings.


-Cory


************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************