When my oven racks get dirty or covered with spill-overs, this is what I do. Get a very sturdy thirteen gallon garbage bag, pour enough sudsy ammonia in it to cover the oven rack, lay on a flat surface (preferably outside for ventilation purposes), such as a half sheet of plywood or the picnic table, put the rack in the garbage bag, let sit overnight, remove the next morning.
You should be able to remove the grease and grime with a scrubber and very little effort. You can dilute the ammonia if you wish, but I would not dilute it any more than half water and half ammonia or the results won't be what you are needing.
If you feel that there is not enough of the ammonia covering the rack, you can very carefully turn the bag and rack over, but beware of poking holes in the bag with the oven rack.
By missysmom from Nokomis, AL
I used the ammonia method, but it took two whole bottles to cover the racks. I was unable to get back to them right away and on the third day I took them out of the trash bag, carried them to my bathtub and soaked them in hot soapy water. The burned on cooking spills came off really easily with a steel wool scrubbie, but - yikes! Now my racks are dull. Is there a prescribed amount of time to leave them in the ammonia? I'm really disappointed.