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