What kind of injs are you referring to? Once the plastic cap is cracked, it may no longer grip the inj well enuf to stay..
Trying to remove the caps usually results in them being broken, or pulled out of shape.
The cheap way is to replace them.
I always change them when cleaning and flow testing a customers injs..
I keep most all types of caps, filters, and o'rings in stock. Let me know what you are dealing with.