I have an 04 elantra that just started stumbling once every minute or so. if the air is on it will die, but it wont if the air is off. do the injectors fire approx at that time or do they fire randomly. trying to figure it out so it dont cost me an arm and a leg. any help would be apprec. dont have many diag tools sooo.. it also does not pop any codes when doin this.