I have a 1990 Toyota Corolla Hatchback (AE92). I first noticed a problem with car's performance when I bought the car; it was extremely sluggish. After a week or so with putting up with this I popped the bonnet and checked the air filter (photo below) and noticed it was totally black. I installed a new air filter and it was like a totally new car! Anyway, maybe after 4 weeks I inspected the condition of the new air filter and whilst it wasn't in the condition of the original filter, it was quite dirty for a 4 week old filter, again - sluggish performance with a few misses whilst accelerating. Any ideas what could be causing the air filter to become black really quickly? It's not "dirt", it's more of an oil colour on the filter.