Closing socket connection with XHR-polling

Does anyone know why my socket connection does not close when using xhr-polling? This is the server code I would like to execute, but does not fire when using xhr-polling. The socket connection works fine ( I am running behind varnish with stunnel, since this a secure socket). I tried manually emitting a disconnect event on the browser unload, but this isnt an optimal solution for me.

client.on('disconnect', function () {

    delete connected_clients[id];
    console.log('   debug - ' + 'client ' + client.id + 'is disconnected');
    console.log('   debug - ' + 'total number of connected clients is ' + Object.keys(connected_clients).length);               

});

In my client side code, I am connecting the socket like this:

socket = io.connect(BASE_URL, {
    "sync disconnect on unload":true
});

and my server side settings are this:

    io.enable('browser client minification');  // send minified client
    io.enable('browser client etag');          // apply etag caching logic based on version number
    io.enable('browser client gzip');          // gzip the file
    io.set('log level', 3);                    // reduce logging
    io.set('match origin protocol', true);     // ssl support
    io.set('sync disconnect on unload', true);
    io.set('transports', [                     // enable all transports (optional if you want flashsocket)
        'xhr-polling',
        'websocket',
        'flashsocket',
        'htmlfile',
        'jsonp-polling'
    ]);  

This is the debug message I get when the client exits the browser

debug: xhr-polling closed due to exceeded duration

I am using socket.io version 0.9.10 with node 0.8.8.