A unit of Johnson & Johnson has voluntarily begun a recall of certain children’s over-the-counter liquid medicines because of manufacturing deficiencies, the Food and Drug Administration said on Saturday. The deficiencies may affect the potency, purity or quality of the products, the agency said in a statement. It said it was investigating the plant where the products were made to make sure there were no other problems. Consumers should stop using certain lots of infants’ and children’s Tylenol, Motrin, Zyrtec and Benadryl products because some of them may contain more of the active drug ingredient than specified, the Johnson & Johnson unit, McNeil Consumer Healthcare, said in a statement late Friday. Other products involved in the recall may contain foreign particles or inactive ingredients that may not meet testing requirements, the company said.