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

[Roy Page]

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

[Dave Butler]

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

[Jonathan Goodish]

In article t>,
"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

[[email protected]]

: 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 *
************************************************** ***********************

[nrp]

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]

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.
>

[[email protected]]

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]

It seems, from the initial comments that a fuel pressure fall off during
climb in PA28 aircraft is seen more often than I thought.
As a mechanical engineer, I can concur that the engine driven fuel pump is
only just keeping up with the full fuel flow needs of a 0-360.
Which is the reasoning behind why I decided to fit a new fuel pump.
My thinking was that the internal diaphragm was tired and could fail.
Sound reasoning, but on the assumption that the pump was more than man
enough to hold good fuel pressure. Not so maybe.

Well where do I go from here .....

Roy



> wrote in message
...
> 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 *
> ************************************************** ***********************
>

[[email protected]]

: Well where do I go from here .....

Piece of black tape to cover the fuel pressure gauge when you don't like what
it reads?

.... couldn't resist. :)

-Cory


--

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

[Roy Page]

Well that is what I wrote Cory, but I deleted it before sending my post :-)

Roy

> wrote in message
...
>: Well where do I go from here .....
>
> Piece of black tape to cover the fuel pressure gauge when you don't like
> what
> it reads?
>
> ... couldn't resist. :)
>
> -Cory
>
>
> --
>
> ************************************************** ***********************
> * Cory Papenfuss *
> * Electrical Engineering candidate Ph.D. graduate student *
> * Virginia Polytechnic Institute and State University *
> ************************************************** ***********************
>

[[email protected]]

Roy Page > wrote:
: Well that is what I wrote Cory, but I deleted it before sending my post :-)

: Roy

Petition the FAA to put out an AD requiring fuel system upgrades to all
PA28's?

Oh wait... bad idea....


--

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

[Roy Page]

Well you gave me a smile with that one Cory
This weekend I am going to rip out the Piper pump and put a Chevy Corvette
one in.
Yes ? .... :-)

Roy

> wrote in message
...
> Roy Page > wrote:
> : Well that is what I wrote Cory, but I deleted it before sending my post
> :-)
>
> : Roy
>
> Petition the FAA to put out an AD requiring fuel system upgrades to all
> PA28's?
>
> Oh wait... bad idea....
>
>
> --
>
> ************************************************** ***********************
> * Cory Papenfuss *
> * Electrical Engineering candidate Ph.D. graduate student *
> * Virginia Polytechnic Institute and State University *
> ************************************************** ***********************
>

[Kyle Boatright]

"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

[Roy Page]

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
>
>
>

[Michelle P]

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

Kyle Boatright wrote:

>"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
>
>
>
>
>

[Montblack]

("Roy Page" wrote)
> 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 ?


Any way you can sneak ...a Taylorcraft(!!) into the rotation? :-)

Looked through your club's site - interesting history: Anyone propose as a
fun member's project - restoring a $12K Taylorcraft?

www.taylorcraftflyingclub.org


Montblack

[MC]

Roy Page wrote:

> 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.

> Ideas please ?

I had lowish and fluctuating fuel pressure in an Arrow.
Turning on the electic fuel-boost pump would improve the pressure
and reduce the fluctuations.
Resealing the gascolator improved it a bit, but I was still worried.
Was considering replacing the engine-driven fuel-pump when my LAME
discovered that the the fuel-line out of the gascolator to the
electric boost pump had a crack.
He replaced that section of line and fuel pressure is now much more
into the green range with less fluctuations.

[Roy Page]

It has been a long term goal to once again have a TCraft in our fleet.
We have looked at several, but most folks want a lot of cash for a machine
that needs a "body off" restoration.
Thanks for your interest.

Roy

"Montblack" > wrote in message
...
> ("Roy Page" wrote)
>> 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 ?
>
>
> Any way you can sneak ...a Taylorcraft(!!) into the rotation? :-)
>
> Looked through your club's site - interesting history: Anyone propose as a
> fun member's project - restoring a $12K Taylorcraft?
>
> www.taylorcraftflyingclub.org
>
>
> Montblack

[Dave]

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
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
>>
>>
>>
>>
>>

[Roy Page]

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
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
>>>
>>>
>>>
>>>
>>>
>

[[email protected]]

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 *
************************************************** ***********************

[Jay Honeck]

> It seems, from the initial comments that a fuel pressure fall off during
> climb in PA28 aircraft is seen more often than I thought.
> Well where do I go from here .....

You join the Cherokee Pilots Association, where if you had been a member
(and if you had read the organization's "bible", "Cherokee Tips and Hints")
you would have known that this fuel pressure thing is a common, benign trait
of all Cherokees.

My Warrior did it, and my Pathfinder, with the O-540 engine, does it, too.
It's weird, but (if you look closely at the fuel gauge) the "green" goes all
the way down to 0.5. "Normal" fuel pressure stretches across a very wide
range!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Mike Noel]

Picking up on Kyle's transducer pressure differential explanation, I suspect
the static air pressure between the engine and the firewall may increase as
the nose is raised from level flight. This could be caused by ram air
entering the bottom of the cowling just forward of the firewall. No actual
altitude change would be needed.


"Aaron Coolidge" > wrote in message
...
> Roy Page > wrote:
> : 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.
>
> This has always happened on my PA-28-180 as well. It is more noticable in
> hot weather. As long as there is positive pressure to the carb there
should
> be no problems.
>
> --
> Aaron C.