-------------- Original message from Gary Casey <glcasey@adelphia.net>: --------------

For years I flew a Cherokee 180 that had a similar problem.  When going high, above 8,000 feet and especially when the outside temperature was warm, the fuel pressure would gradually drop to about 2 psi.  At low altitude it would be in the high end of the green and at high altitude the low end of the green.  I could never come up with an explanation and the condition didn't change with a new fuel pump.  It would rise slightly when turning on the electric pump.  I doubt that it is a tank vent problem, but I have no likely explanations.  Never caused a problem, but always bothered me.  I always wondered if it was something to do with the way the gage read the pressure, but that didn't make sense either.

Gary Casey

I normally have had around 4-6 psi flying around my airport, however,
yesterday, I flew on my first cross country. Cruising at 220mph, I noticed my
fuel pressure slowly start to drop as I got faster and higher. It went down
to 2 psi. I climbed higher in case I needed time to find a place to land.
Luckily I didn't.

I turned on my fuel pump with only a slight increase of 1 psi as a result.
I switched tanks and same results.

I learned my engine requires .5 psi minimum so I have remained about that,
however I don't like it. I want 4-6 to keep my peace of mind. Top psi limit
is 8.

So, I asked around to a couple A&P's. They all think it could be my vent on
the tanks is not providing enough positive pressure in the tanks. I thought
of maybe increasing or decreasing the angle of the vent into the relative wind
to increase the fuel positive pressure in my tanks, however I know it is bad
to have too much pressure in the tanks as well.

So, I am seeking input on this problem or if you guys think I shouldn't even
worry about it.

Interesting that when I reduced power and got into my pattern, the psi went
up to 5 psi.

Matt Reeves