Have my 500EFIs apart for a top end refresh and wanted to backflush and clean coolers while the motors were out. I pulled the inlet hose off of one of the oil coolers and a few chunks of impeller fell out, and I can feel some others in there, but they are wedged pretty solid. Anyway to clean these coolers without damage? Before I go in there with a pick, etc I want to make sure there are no fins/tubes, etc that are easy to puncture near the inlet.

aluminum welding rod works great , or use a straightened coat hanger carefully ....

Compressed air with a tapered nozle works good too.

Did you remove the end caps? If not, do so and you will have a straight shot through the inner tubes.
Eddie