C172 fuel cap

I have two different after-market fuel caps on my C172. The right tank
cap is vented; the left is not. Should both be vented? Is a properly
functioning vented cap one that only allows air to flow into the tank?

On Tue, 21 Sep 2004 16:13:55 -0700, wrote:

I have two different after-market fuel caps on my C172. The right tank
cap is vented; the left is not. Should both be vented? Is a properly
functioning vented cap one that only allows air to flow into the tank?

That's probably right. Check your ADs. I seem to recall there being
one from the 1970s that deals with fuel caps. Something to do with
vapor lock above 5000 feet.

Don

I have two different after-market fuel caps on my C172. The right tank
cap is vented; the left is not. Should both be vented? Is a properly
functioning vented cap one that only allows air to flow into the tank?

That's probably right. Check your ADs. I seem to recall there being
one from the 1970s that deals with fuel caps. Something to do with
vapor lock above 5000 feet.

Don

To No Name...

There was an AD in '79 which made it mandatory for all Cessnas from the 140A
and beyond to install a half-vented cap, at least one, on either tank, leaving
the other non-vented cap on unless you changed both.

The purpose was to allow inward venting in the event the single point vent was
plugged with bugs or ice. Some have the original single vent on top of the
wing and others behind the strut.

The tanks are interconnected via a tank to tank tube so the original single
vent worked for both, except when the original single vent got plugged.

The half-vented cap has a small "umbrella" silicone vent which will allow air
in whenever the vacuum in the tank exceeds its (never defined) opening
threshold. The cap cannot allow fuel or pressure out, that being the reason it
is called the half-vented cap.

Cessna will sell you replacement half-vented caps but will not sell you the
silicone umbrella-shaped valve in the event it sticks to the seat or if it goes
missing.

Don is mixing problems and solutions in his description of another problem
with the 172's, a venting problem related to using the Both position of the
selector but not related to the cap. Above an xxx altitude using the Both
position, the engine will quit. You are supposed to have a placard that says:
"do not use the Both position above xxxx altitude". The planes which are
subject to the fault are specified in another service note and...maybe..an AD
as well. Find the serial range of the affected planes for the Both position
problem and yours may or may not be in it.

Neal

wrote in message ...
I have two different after-market fuel caps on my C172. The right tank
cap is vented; the left is not. Should both be vented? Is a properly
functioning vented cap one that only allows air to flow into the tank?

You have to very carefully study this. The Cessna fuel systems changed over
the years and you can get into big trouble screwing with the wrong caps.

However, for most of the 172's, the primary vent is always the left tank vent behind
the strut. The right tank is normally vented through a cross feed line that connects
the tanks roughly behind the pilot's head. The right cap has a normally closed vent
that only opens when the right tank gets sufficiently negative pressure due to the crossfeed
being blocked.

That all being said, one of these "normally closed" caps should not cause a problem on
the left side as well, but that's not the way Cessna ships 'em.

"COUGARNFW" wrote in message news:20040922022729.27462.00000414@mb-
There was an AD in '79 which made it mandatory for all Cessnas from the 140A
and beyond to install a half-vented cap, at least one, on either tank, leaving
the other non-vented cap on unless you changed both.

The 140 is a completely different deal. Some of the 140 the caps actually provide venting
in normal operation

"Don Tuite" wrote in message
That's probably right. Check your ADs. I seem to recall there being
one from the 1970s that deals with fuel caps. Something to do with
vapor lock above 5000 feet.

How would a vent affect vapor lock. The only vapor lock "fix" is the placard
to run either left or right (rather than both) above 5000 feet.

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?

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?

Thanks for the note. I've checked my vent tube and think it's in the
right place. My fuel caps are not original and I get a small amount of
blue streaking on the right wing aft of the cap. We've diligently
checked out the gaskets and the metal ring around the top of the tank
for leaks. Everything seems to be in place. So that got me wondering
if the vented cap might be allowing air in both directions, and I
wondered it that was the way the cap should function. Maybe it
requires a half-vented cap. I have a Cessna parts book but couldn't
tell.

Dale

wrote in message ...
Thanks for the note. I've checked my vent tube and think it's in the
right place. My fuel caps are not original and I get a small amount of
blue streaking on the right wing aft of the cap.

The right cap on a 172 should be vented so that it only vents inward.