Porsche Repair: Moaning/whistling sound from gas filler compartment, vent tube, vent hole

Question
RE:  2000 BoxsterS

There's a train-whistle type sound coming from the compartment where the gas filler cap is.  The sound is independent of speed and and whether or not the transmission is engaged.  The sound is coming from a vent hole and can be started/stopped by placing a finger over the vent.

The local mechanic says it's caused by, perhaps, a bug crawling into a vent tube/breather/relief valve having something to do with the emission/evaporative control system.  Says it can happen on bigger Porsches, as well, and might cost $2-300 to blow out the system (if it can be unplugged in this manner at all) or up to $7000 if the line has to be replaced (since it is one of the first lines installed in the car's manufacture).

Any thoughts about what causes this and what to do about it?  I can't find any references on what this does or whether the line is under pressure or vacuum.

Would really appreciate your help.  The sound seems to be getting louder and happening more frequently.

Jim Tucker

Answer
Jim, since I received your question earlier today I've also been trying to locate a reference to this problem.  No luck so far.  I do have to give you credit for the new record for the widest divergence from the low to high estimate ($2 to $7,000).

As for the bug in the line, I'll have to leave that explanation to your mechanic.  Apparently he's seen this before.

I'll be the first to admit the Boxster's emission controls are a wee bit more sophisticated than my 356's...but conceptually at least, we should all be able to understand the basics.  

One problem the auto manufacturers are trying to overcome is the harmful effects of gas fumes entering the atmosphere.  So, much effort is made to both re-ignite partially combusted gasoline so it doesn't go out the exhaust pipe...and keeping gas fumes inside the emission system's closed loop.  Ideally, once the gasoline is in your gas tank (having entered via a special nozzle that contains spills and fumes) any fumes that try to escape are neutralized in a charcoal cannister apparatus.

The problem arises when the engineers realize that to move the gasoline from the tank to the engine and send the unused fuel back to the tank requires fresh air.  If there were no fresh air entering the system the fuel pump would suck the gasoline out of the tank only up to a point.  The fuel pump sucks and sucks until the void where the gasoline was becomes a vacuum...and at some point the sucking isn't strong enough to pull more gas out (and create additional vacuum).  At this point fuel delivery stops and the car acts like it's out of gas.  

I assume the vent you're referring to allows fresh air into the system.  What's not clear is how gas fumes are prevented from escaping from that same point.  Perhaps there is a one way valve of some sort???  So I assume the noise is caused by the rush of air sucked into the system as the fuel pump moves the fuel around at 40-60 psi.  This doesn't change with the speed of the engine or whether the transmission is engaged.

So now that I've covered that, it's still hard to visualize what that bug would be doing to create such a sound.  But if you'd like, I could work on the problem, and I'll do it for only $6,000.

It would be pure speculation to suggest some other valve/gizmo elsewhere in the system isn't modulating the air-flow.  In my more simplified world, I'd look first at the charcoal cannister...let's assume the cannister is on the opposite end of this system from your offending vent.

I know that the cannisters can eventually age and become ineffective in their fume neutralizing role.  Perhaps there's some connection.  Sorry I can't suggest anything else.

One last idea.  Since your mechanic has suggested this occurs periodically,  I'll bet some knowledge exists with owner groups, like the local Porsche Club of America, or perhaps a Boxster owners group.  Folks like these communicate continuously about common problems.  Search the web for Boxster clubs, registrys, forums, etc.

Here's wishing the only whistling you hear is the wind as you put this problem behind you and get that quiet beauty back out on the open road where she belongs.

Dave