As mentioned all air must pass through your filter and to do this with an open window you'd need to create negative pressure. If your intake is too big then you can't achieve this and you will have a leak...may not necessarily be your filter although I'd definitely consider it to be the culprit. You can shake the filter to fill any voids created in the carbon over time and if that doesn't work..drill the filter so you can refill it or replace it altogether. You can also double up on a filter if you really want to