I was flying a friend to an airport some 200 miles away where he’d had
some work done on his Mooney. I have a 1966 PA-24-260 with two main tanks,
two aux tanks and two tip tanks. About 5-10 minutes after I had leveled
off at 5500 feet and switched to the left aux tank, the engine started
acting up. It ran unsmoothely and the power varied up and down. I tested
the magnetoes, put on alt air, varied the power setting and mixture (I was
flying at 75% power, 24”/2350, OAT was 14F at 5500 feet) but it wasn’t
until I switched to the main tank that the engine reverted to normal
operation.

Later on I switched to the other (right) aux tank. A few minutes later,
the same problem appeared. This time I didn’t touch anything. Instead I
observed the fuel flow and the EGT indicator. Fuel flow increased from
15.3gph to 21.7gph, where it stabilized, while the EGT increased by 50F.
Before the increase in EGT I was running at 100F ROP. I let it go on for
maybe a minute or two, then switched back to one of the main tanks and the
problem went away. EGT and fuel flow came back to normal.

So what happened? The change in EGT and fuel flow are inconsistent. The
EGT indicates that the mixture is getting leaner while the fuel flow
indicator says the mixture is getting richer.

My friend’s first hypothesis was that air was entering the system. This
would cause the turbine blades in the fuel flow transducer to turn faster
(thus showing an increase in fuel flow) and give a leaner mixture
(explaining the rise in EGT). Sound plausible, but I don’t see why the
problem happens on both aux tanks but not on any of the main tanks. I have
two fuel selectors, one for the three left tanks (main, aux and tip) and
one for the three right tanks.

Other ideas that came up were clogged fuel lines to the aux tanks or
clogged air vents in the aux tanks.

On the flight home I got up to a higher altitude (12500 feet) and flew at
55% power. OAT was -13F. I was able to reproduce the problem. This time it
occured within a minute of my shifting to the right aux tank. Fuel flow
went up from 10.0gph to 17.2gph. Then it varied erratically between
14.2gph and 17.2gph. Unfortunately I didn’t check the EGT this time. When
I shifted back to the main tank the problem went away again.

I’m going to take the plane up tomorrow for a test flight. There are two
things I’m planning on testing. First, what effect does applying the
electrical fuel pump when the problem appears have? Second, does the
problem appear at a lower altitude, say 2000 feet, where it’s less cold?

Any ideas on what’s causing this problem?

/Thomas