Is there a good way to prevent bad breath from capped teeth?

It depends: Breath odor seldom originates from capped teeth. It is likely from the tongue surface, or beneath the gums between the teeth. Sometimes though, a cavity may exist beneath a crown (or cap) and you simply cannot see it. In this situation, it could be the source of an odor. This can be challenging to figure out. Good luck!

Many Possibilities: If cause of odor is from your oral cavity brushing the tongue and proper flossing may help. You may also have sinus problems, diabetes, "dry mouth"(xerostomia), periodontal disease, or digestive issues. All of these can cause odor issues. A dental exam first with a medical follow-up would be advised if not resolved. The treatment recommended will be based on the cause of your bad breath.

Material and method: A crown will have two issues that causes bad breath. Firstly, the gap at the margin should be as close as it can be (25 micron-50 micron). A crown with open margin will lead to food trap, bacterial colonization, bleeding gum and bad breath. Secondly, it should be an all ceramic crown to have better tissue compatibility and better bonding to further seal the margin gap. Good occlusion will help.

Bad crown: If the bad smell and taste is from the crown and nothing else , there is something wrong with the crown and needs to be replaced. A crown done by a general dentist typically lasts 5 to 10 years. Crowns done by specialists ( prosthodontist ) last longer10 to 20.