Looking for specific (mailbag?) episode ("capacitor in a box")

[exit_failure]

I've been searching for a couple of days now but I just can't find a specific episode. I think it was a mailbag episode where Dave opened some (more or less) fake and hilariously shitily built car filter thing which he called "basically a capacitor in a box" (which it was :-D ), if I remember correctly.
I would be very thankful if anyone could point me in the right direction.