ok so my new 99 xj 4.0 has a rough idle that feels like a misfire almost every 3 or 4 seconds. so i replaced cap and rotor, plugs (champions) and plug wires and threw in some fuel additive in the tank. rough idle still continues every 3 or 4 seconds. while driving it feels fine its just at idle wether its in park or in gear. motor mounts look good too. should i look into fuel injectors or o2 sensors? what do you guys think?

 

id get codes checked firsr

 

Harmonic balancer?
Bad injectors?

 

no check engine light on and the bulb does work. i also did the gauge check. th eone where you hold down the trip button and turn the key. did the parameters fine. no codes.

 

that only check dash cluster functions ,get a bulb in there ecm will store codes

 

I'd clean the IAC and check the TPS.. as above, i'd replace that old O2 sensor (pre cat) too. Other than the 'miss', does it idle ok? I mean not too high or low?

 

Clean the throttle body with TB cleaner, scrub the IAC with some carb cleaner and an old toothbrush, test the TPS to make sure it is functioning properly, and consider replacing the upstream o2 sensor.

What kind of oil filter are you running? I'd change the oil and oil filter (if you have not done so recently) Do some research on filters to find a good one.

Check your exhaust manifold for cracks. That could cause a rough idle too.

Use some Seafoam in the brake booster vacuum line going to the intake manifold (search YouTube for this, there's a lot of good videos showing you exactly where this line is and how to do it)