Normally most backfire problems come from some problem with the ingion system l would check the cap roter wires and plugs. check your coil and pick up. Very rarely but possible a intake valve could cause this. would have to be on the intake because you said it blows the air filter cap off. if it were backfiring from the muffler l would say it is a exhaust valve. with that said I myself have never seen that on a late model Toyota.