3.4L with a weird misfire issue

OK, had a 303 problem on a 02 with 300+K; new NGK wires (and coil packs as they were pushing 17 years old), problem solved!

 

Same problem on 97 it's vacuum leaky in valves. Need to have both heads rebuilt.

 

you are wasting your time proceeding until you verify the compression.

rant on/

since the adoption of the OBDII standards, techs have seem to have lost the ability to diagnose... the computer says P030X, it MUST be right... they load the parts cannon and pull the trigger... all of this when a compression test and a vacuum gauge could have figured out if the motor was mechanically sound first.

rant/off

do a compression test on all of the cylinders with the throttle wide open. look for all cylinders within 10% of each other. I see you have swapped coils/plug wires etc with no change so we can eliminate that, for now. Next look at fuel trims when it is warm. anything greater that +5% indicates a vacuum leak. This could mean an actual vacuum leak like a gasket up to an intake valve not sealing 100% due to valves being too tight. There are too many variables to look at to say this or that is THE problem. Start with the basics. If you don't have the tools, time or experience this is just going to be a frustrating expensive exercise.

 

Valve problem. Leaking vaccum past valves. Need heads worked

 

I've got the random multiple misfire problem my ECM is bad may not be your problem but it was mine