You will get false knock detects and the crank sensor might lose sync at odd times. It's more likely to be noticeable with the crank sensor during fast accelerations where decoding is more critical.
The foil shield should be tied to the drain conductor (braided wire). Odd yours were not grounded, it could just have been age taking its toll. I have a few harnesses here that I can check and see if they are the same way.