My KF coil has been going for a long long time. It has turned that dull crackled finish light tan color. It probably has 150+ ml through it. I light it up screeching hot before I rewick it and douse it directly under a thin bead of running water. I do this a few times and then rewick and it tastes fine for another 3 tanks or so until it looks like a BBQ grill again. (This juice kills coils/wicks) But despite how the coil looks it still delivers with no taste issues whatsoever until the juice gunks up again.
I wonder if your cap, and body could also use a flush. I carefully run the deck of the KF under the water and blot it - blow it out every once and again as well so really old juice doesn't sit in it.
Every atomizer I leave sit with 'some' old juice in it - gets a staleness to it after a while if you were to just light it up and vape. I guess I should just pull the wicks and coat the coil in plain VG. But sometimes I forget.