2003 Boxster howling from fuel filler cap area

Only problem I had was a random howling sound coming from the gas filler door area, so this weekend I installed a new vent valve on my 2003 Boxster. Didn't solve the problem.
What does stop the sound is partially plugging the hole located in front of gas filler doorsee picture. I took a piece of one of my kids nerf gun bullets which is made from sponge with same OD as the hole in the filler door and a 1/8" ID hole through the middle of it. I pushed a piece of piano wire through it to hold it in place see 2nd picture. With this in place the howling completely goes away.
Since this solves the problem would you have any idea what the problem could be? Gas cap appears to be OK. Thanks



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[Casey at Pelican Parts]

Does your kid know?! Haha! That hole is for the charcoal canister vent, and leaving it plugged will eventually cause a check engine light because if the canister vent is plugged the DME will not see a change in oxygen sensor activity when purging. There are two other valves behind the passenger headlight which can cause this issue, however the purge valve is usually the culprit. This is a tough one to offer advice on without seeing the car, and hearing the howl.

- Casey

[FPB111]

Post #30
Boxster hard to start after filling with gas - Page 2 - 986 Series (Boxster, Boxster S) - RennTech.org Community


"SOLVED!

This was a bear of a problem.

The whistle was caused by too much vacuum in the lines -- if you tapped the valve housing (#8 in previous pdf picture)
just right -- you could get it to resonate and whistle.

There were times it did not resonate at all -- at these times the vacuum was not as strong.

The key was determining this relationship.

There is a valve that controls the Evaporative shut-off valve. It was malfunctioning.
This valve is located under the intake plenum back in the engine. However the noise
emanates from the fuel door! Disconnecting the connector mimics the failure mode,
so in a sense the valve failed stuck open.

We pulled the hose that comes from this valve to the intake manifold right at the Y of the
hose that comes from the AOS. If we taped that hose off -- the engine ran fine.
If we left it open -- it mimiced the hard start after fueling to a T -- including having
to give it gas to keep it from stalling.

Once we replaced the evaporator shut off valve the vacuum reduced and was consistent.
The engine ran really well, and we have since filled it with gas with no issues.

Sadly I replaced the purge valve, and the charcoal canister up front to the tune
of $200 -- and a ton of wasted time. But now we know."


Also http://rennlist.com/forums/996-forum/575816-questioning-the-reliability-of-the-996-starting-problems-fuel-vent-system-pics-12.html

post 167 for answer (Evap rejuvenation valve replacement)

post 101 on shows pics tracing the whole system on a 996.