i have a 89 MJ with 130k on a 1997 4.0, i had problems with my stock injectors spraying one for the engine, two for the engine compartment. so i put in ford 19# injectors, i barely noticed a difference... maybe like 5hp but my idle seems fine. and not much smoke compared to other 4.0s
have you checked the vacuum system for leaks, that affects idle... i had the crankcase vapor tube disconnect one time adn my idle jumped to damn 1700 steady.