fuel leak or auxiliary fuel pump malfunction?

[[email protected]]

>From time to time pilots have observed fuel stains on the nosewheel
wheelpant of our club's 172SP. The only time I have actually seen fuel
dripping from the engine was when I watched a pilot turn the auxiliary
fuel pump on for what seemed to me to be an excessively long period of
time. The fuel appeared to be dripping out of one of the two plastic
hoses that come down through the cowling just ahead of the firewall. I
have assumed that the cure is to prime the engine according to the POH:


5. Auxiliary Fuel Pump Switch -- ON.

6. Misture -- ADVANCE until fuel flow just starts to rise, then return
to IDLE CUT OFF position.

7. Auxiliary Fuel Pump -- OFF.

When I follow this procedure, I never see fuel stains on the nosewheel
wheelpant.

One of our pilots has suggested that fuel should not drain from the
engine when the auxiliary fuel pump is left on too long. He thinks
there is a leak in the fuel system.

Does anyone know the answer to this question?

Thanks

Stephen Ashley

[Peter R.]

> wrote:

> One of our pilots has suggested that fuel should not drain from the
> engine when the auxiliary fuel pump is left on too long. He thinks
> there is a leak in the fuel system.
>
> Does anyone know the answer to this question?

I see this all the time on all of my local flight school C172 SPs and it is
always because someone primes the engine too long. The tube is an overflow
and it is working as designed when it is dumping fuel on the ground because
some inexperienced pilot is not following the POH's engine start method.

--
Peter


















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[[email protected]]

On 16 Apr 2005 18:15:40 -0700, wrote:

snip

>Does anyone know the answer to this question?
>
>Thanks
>
>Stephen Ashley

If the pump is on, and the mixture control is NOT in ICO, there is
fuel being supplied to the engine, whether the engine is running or
not.

Eventually, it will run down the induction system and out of the
drain/drains that are in place to port it outside of the cowling (and
onto the wheel fairing, apparently)

TC

[Kyle Boatright]

> wrote in message
...
> On 16 Apr 2005 18:15:40 -0700, wrote:
>
> snip
>
>>Does anyone know the answer to this question?
>>
>>Thanks
>>
>>Stephen Ashley
>
> If the pump is on, and the mixture control is NOT in ICO, there is
> fuel being supplied to the engine, whether the engine is running or
> not.
>
> Eventually, it will run down the induction system and out of the
> drain/drains that are in place to port it outside of the cowling (and
> onto the wheel fairing, apparently)
>
> TC

Once the carb bowl fills, the carb float closes the valve that allows more
fuel in to the carb. This prevents excess fuel from being poured into the
carb and eventually leaking out. The original poster needs to check a
little deeper and figure out exactly where the fuel is coming from. If it
is excess fuel coming from the carb bowl, there is a problem with the carb
float or the valve it controls. Perhaps the carb float has sunk, stuck, or
is improperly adjusted. It is also possible that the valve no longer seats
properly due to wear or some sort of obstruction.

Another possiblilty is a leak in the fuel strainer valve that only manifests
itself with the aux pump on...

Again, there is more investigation to be done.

KB

[[email protected]]

On Sun, 17 Apr 2005 21:05:08 -0400, "Kyle Boatright"
> wrote:


>Once the carb bowl fills, the carb float closes the valve that allows more
>fuel in to the carb. This prevents excess fuel from being poured into the
>carb and eventually leaking out. The original poster needs to check a
>little deeper and figure out exactly where the fuel is coming from. If it
>is excess fuel coming from the carb bowl, there is a problem with the carb
>float or the valve it controls. Perhaps the carb float has sunk, stuck, or
>is improperly adjusted. It is also possible that the valve no longer seats
>properly due to wear or some sort of obstruction.
>
>Another possiblilty is a leak in the fuel strainer valve that only manifests
>itself with the aux pump on...

Could be wrong, but am thinking the SP is fuel injected.

Apologies in advance if it is not.

TC

[Kyle Boatright]

> wrote in message
...
> On Sun, 17 Apr 2005 21:05:08 -0400, "Kyle Boatright"
> > wrote:
>
>
>>Once the carb bowl fills, the carb float closes the valve that allows more
>>fuel in to the carb. This prevents excess fuel from being poured into the
>>carb and eventually leaking out. The original poster needs to check a
>>little deeper and figure out exactly where the fuel is coming from. If it
>>is excess fuel coming from the carb bowl, there is a problem with the carb
>>float or the valve it controls. Perhaps the carb float has sunk, stuck, or
>>is improperly adjusted. It is also possible that the valve no longer seats
>>properly due to wear or some sort of obstruction.
>>
>>Another possiblilty is a leak in the fuel strainer valve that only
>>manifests
>>itself with the aux pump on...
>
> Could be wrong, but am thinking the SP is fuel injected.
>
> Apologies in advance if it is not.
>
> TC

You may just be right...

KB

[J. Severyn]

"Kyle Boatright" > wrote in message
...
>
>> If the pump is on, and the mixture control is NOT in ICO, there is
>> fuel being supplied to the engine, whether the engine is running or
>> not.
>>
>> Eventually, it will run down the induction system and out of the
>> drain/drains that are in place to port it outside of the cowling (and
>> onto the wheel fairing, apparently)
>>
>> TC
>
> Once the carb bowl fills, the carb float closes the valve that allows more
> fuel in to the carb. This prevents excess fuel from being poured into the
> carb and eventually leaking out. The original poster needs to check a
> little deeper and figure out exactly where the fuel is coming from. If it
> is excess fuel coming from the carb bowl, there is a problem with the carb
> float or the valve it controls. Perhaps the carb float has sunk, stuck, or
> is improperly adjusted. It is also possible that the valve no longer seats
> properly due to wear or some sort of obstruction.
>
> Another possiblilty is a leak in the fuel strainer valve that only
> manifests itself with the aux pump on...
>
> Again, there is more investigation to be done.
>
> KB
>

The 172SP uses the IO-360-L2A which is fuel injected....i.e no carb bowl.

TC is correct. If the aux pump is "on" and the mixture is not in ICO then
fuel will eventually run out if the prop is stopped.

J. Severyn
KLVK

[Randolph Sumner]

From time to time pilots have observed fuel stains on the nosewheel
wheelpant of our club's 172SP. The only time I have actually seen fuel
dripping from the engine was when I watched a pilot turn the auxiliary
fuel pump on for what seemed to me to be an excessively long period of
time. The fuel appeared to be dripping out of one of the two plastic
hoses that come down through the cowling just ahead of the firewall. I
have assumed that the cure is to prime the engine according to the POH:


5. Auxiliary Fuel Pump Switch -- ON.

6. Misture -- ADVANCE until fuel flow just starts to rise, then return
to IDLE CUT OFF position.

7. Auxiliary Fuel Pump -- OFF.

When I follow this procedure, I never see fuel stains on the nosewheel
wheelpant.

One of our pilots has suggested that fuel should not drain from the
engine when the auxiliary fuel pump is left on too long. He thinks
there is a leak in the fuel system.

Does anyone know the answer to this question?

Thanks

Stephen Ashley
I own a 2001 Cessna 172SP with 350 hours on it. I just had the fuel servo overhauled after the plane exhibited similar fuel dump on the fairing after shutdown. You can get fuel dump if the engine floods due to overpriming at start, but fuel dump at shutdown may indicate a gummed up servo. Have an A&P remove the bottom cowling and look around the control arm connectors to the servo for leaks and look into the throttle body for gum up. The performance issues are subtle in this problem and you may not notice the reduced take off performance or loss of power at altitude due to overrich mixture.By the way, Lycoming blames this servo gum up problem on overpriming but I prime by the POH. The issue needs an AD for inspection of the servos in my opinion.