Have had the problem in a C-150 and it was bugger to figure out.

To start with I've had a lot of experience with Cessnas having had a full
line Cessna shop during the late 60's. I am an A&P with AI endorsement at
the time. I only state this to let you know that I've had some background
with the aircraft.

I bought a 75 C-150 in the early 80's in California. Nice clean aircraft,
ran and flew very nicely. I had flown the airplane maybe 20 hours before
one morning departed for Las Vegas, if you know Ca. you know that means
going high because of the mountains.

Well, during the climb at about 8000 ft the engine quit just as if the
mixture control had been pulled. Went through all the normal checks but
could not get a restart. Notified approach and set up a glide for the
nearest airport. At around 4-5K the engine restarted on it's own. I
completed a normal landing and thoroughly check the engine over. Everything
checked ok and after power checks took off completed the flight.

To try to make this short let me just say that in the next couple of months
I had 5 or 6 more engine outs all while in a climb at around 6-9 thousand
feet. It got so that it didn't even scare me anymore because it would
always restart after descending 2-3 thousand feet.

It was obvious that it was fuel starvation but why? I went through the
service manual and checked everything I could think of. Fuel cap vent, main
tank vent behind strut, fuel lines, carb and main fuel strainers, etc. I
did the vent checks call out in the manual plus a few more that Cessna
engineers wanted after consulting with them. Everything check ok, or so I
though.

I was at a loss. But one day while I was doing a preflight I happen to pull
a little harder than I normally do on the main vent behind the left strut
and the vent line popped down. This is the line that makes a 90 degree turn
forward to catch ram air to slightly pressurize the tank in flight. I then
notice that the paint line where it comes out of the wing was now even with
the bottom of the wing. This is where the line was when the aircraft was
originally painted. Sure enough the line had been pushed up in the wing,
only about a half inch from its original position. I had checked this
dimension before but it's very difficult to get an accurate measurement on
the centerline at the bend.

The moral to the story is that this dimension is critical. The vent is
placed behind the strut to minimize the chance of impact icing since it's
not heated.
Turns out that the strut is somewhat of a airfoil and when the vent is too
high above the strut it puts it on the low pressure side during a climb. As
I'm sure you know you have to keep increasing the angle of climb at higher
altitude to maintain a climb and this puts the vent more into that low
pressure region. You reach a point where this actually will start pulling
air from the tank and when it reaches a flow that the fuel cap vent cannot
match you get negative pressure and engine stoppage. You then drop the nose
during the descent putting the vent back in positive pressure and eventually
the engine restarts.

Sorry for so long but that's the story. Cessna and I both learn something
from this one. Bill...


wrote in message
om...
I have had that vapor lock (I think) happen in a 1967 172 leveling off
after a long climb to about 8500 ft. It was dead smooth air during
the climb. We were speculating that maybe the under-the-floor temps
are high and if the coordination happened to be such that the fuel
flow went to near zero for an extended time in one of the tank lines
to the fuel selector. We had forgotten to switch from both to one
tank above 5 thou.

I understand Cessna had a hard time duplicating the problem after from
the few field reports that came in before the AD was issued. They
never published a reason for it as far as I know.

There was a total power interruption for maybe 20 seconds while the
two of us (both engineers) franticly tried to figure out what was
happening while we were over northern Wisconsin near night. The last
thing was pulling the fuel quick drain although I don't know if that
was the fix or if the problem corrected itself via other means. It
definitely was a fuel starve out type failure and not a flooded out
type of failure. We most certainly were leaning for the climb at
that altitude. We had EGT and we always monitored it.

Has anyone else encountered this stumble? Could it really happen to
be the dead smooth air and the coordination?