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

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

Roy Page wrote:

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.

snip

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.

snip

Our PA28-180 has a similar fall off in fuel pressure when climbing.

I don't know what causes it, but the PA28-180 I previously owned did this, too.
First time I saw it, I made a precautionary landing, no trouble found. Like you,
I went over everything and never found any fault. Eventually I concluded it was
just a peculiarity of the model and was benign. If you find a way to fix it, let
us all know.

Dave

In article et,
"Roy Page" wrote:
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.

Check the POH, I believe that Piper recommends that the fuel boost pump
remain "on" during climb.





JKG

: Check the POH, I believe that Piper recommends that the fuel boost pump
: remain "on" during climb.

The stock Piper fuel pump is marginal, especially on 180hp engines. I talked
with Petersen at length about this when I bought his autofuel STC for our PA28/180.
Bottom line is to get the STC certified, they had to replace the electric fuel pump
due to "low fuel flow." My thought is the Piper system was marginal, but acceptable
in the early 1960's when the TC was issued. Now, it's not considered enough, so the
autofuel STC was required to "fix" it.

FWIW, my plane drops some fuel pressure when the electric boost is off as
well. In a full-power-on-stall, it reads about 0.5-1 psi without the electric on.
With, it comes right back where it belongs.

I believe it's not unsafe, although it *is* a bit unnerving.... just an
idiosyncracy of the plane.

-Cory


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

Any chance you are looking at a partial vapor lock? Are the fuel lines
insulated? I heard from a couple of sources (though years ago) that
Piper Cherokees are more prone to these problems than most other
autofuel STC holders.

Well you could be correct about a possible vapor lock.
As far as I can see the fuel lines are not insulated.
The autofuel STC on an Archer calls for mods to the fuel system to prevent
vapor locks.
But I do run on 100LL which is less volatile that Regular Gas.
But if the PA28 aircraft are prone to vapor locks, I would have expected to
see an AD a long time ago ?

Thanks for the input.

Roy

"nrp" wrote in message
oups.com...
Any chance you are looking at a partial vapor lock? Are the fuel lines
insulated? I heard from a couple of sources (though years ago) that
Piper Cherokees are more prone to these problems than most other
autofuel STC holders.

Roy Page wrote:
: Well you could be correct about a possible vapor lock.
: As far as I can see the fuel lines are not insulated.
: The autofuel STC on an Archer calls for mods to the fuel system to prevent
: vapor locks.
: But I do run on 100LL which is less volatile that Regular Gas.
: But if the PA28 aircraft are prone to vapor locks, I would have expected to
: see an AD a long time ago ?

: Thanks for the input.

My cherokee could be vapor lock, but I've seen it on three different cherokees
that I've flown, with autofuel or on 100LL.

The flow of the pump is just barely adequate. Petersen described the "ditch
test" where they had to put the tail of the plane down in a "ditch" to get a high
nose-up attitude. In the worst conditions, the stock fuel system was unable to
deliver the flow rate with an acceptable safety margin. Thus the modifications to the
fuel system for autofuel STC.

What *really* bugs me is that a PA28-160 has to do the fuel mod for the STC,
but if it's got low-compression pistons (i.e. PA28-140/150), the fuel mod isn't
required. You tell me how changing the pistons (i.e. low compression vs. high
compression) changes fuel flow. Safety through liability and regulation...

-Cory


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

"Roy Page" wrote in message
nk.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

Well Kyle that is a pretty convincing explanation.
I will take a another look at the service manual and follow your arguments
for myself.
If you are correct, and I am not doubting you, I am on my way to solving the
problem.

Thanks again

Roy

"Kyle Boatright" wrote in message
...

"Roy Page" wrote in message
nk.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 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