01 4.0 bank 1 missfires

[TJS1011]

My 01 4X4 ranger 4.0l is misfiring on 1,2, and 3 (bank1). I've changed plugs and coil pack but it's still misfiring, any ideas what's next?

 

[SenorNoob]

If it were an earlier model with batch fire, I'd suggest bad wiring on injectors for that side. Since it's not it'd have to be 3 separate failures which seems... unlikely.

Any chance the upstream O2 is giving wrong enough readings that the PCM is adjusting fuel till it misfires?

I've had lots of bad new spark plugs lately. (Granted all NGK on powersports applications.) IDK if I'm just lucky like that or if manufacturing has gone that far downhill.

Did you set gap before installing? That can make a huge difference. Especially if you set it to the wrong spec. (Like say the 3.0 or OHV specs if you have a SOHC.)