I found this by googling--it worked for me. Worth a try since it's easy and cheap...

This is an overnight process. In the evening just put the dirty oven racks in the bathtub and fill the tub with enough hot water so the racks are completely submerged. For this to work, the water needs to be very, very hot. Using water that is warm, not hot, does not give the same results. I add regular laundry detergent to the water. You can also use the granulated-type dishwashing detergent, but I think the laundry detergent does a better job. Be sure to put down an old towel or something soft to protect the bottom of the tub from scratches before you put in the racks. Slosh the detergent around in the water, check to be sure that the racks are completely submerged, shut off the water, and go to bed.

In the morning you will find that the baked-on grime that was on the oven racks has already dissolved overnight in the water and detergent and can be easily rinsed away. All thatÂs left to do is to rinse the racks and dry them with a soft cloth.