The fan blades / fins on the Jenn-Air downdraft counter-top stove that was in our house when we moved in several years ago was so caked with grease and unbalanced because of it, that the counter shook when we turned on the fan. Well, that would never do. We removed the fan from its housing and left the intact fan overnight in a bucket of water and dissolved trisodium phosphate. Next day, the grease was dissolved and the fan was fine. Washed it and re-installed it.
WARNING when using TSP: be sure to wear good quality rubber gloves (not the ones that they use at the deli counter or doctor's office) that extend above your wrist. TSP does no favors to your skin. Also, best if no children or pets are around when using TSP. Try a test area if you're going to use it to clean a painted surface as it probably will dull the finish. TSP is a heavy-duty product that works great to remove grease. Just use common sense and you will be fine.