Unhappy Ignition

[bigdaddy]

Jeep newbie here. Looked everywhere but no luck finding answers.

Story goes:
Bought my 2001 TJ about a month ago. Figured I should chip away at maintenance since I have no service records. Started with changing spark plugs. Purchased Autolite APP-985 as recommended on these forums. I drove the TJ to work the next morning and started getting misfires. On the way home the misfires were worse and CEL came on. P0302, which is #2 cylinder.

I figured maybe the coil pack did seat so pulled the coil, reseated and reset code. Still misfires. To check for faulty plug I swapped spark plug from cylinder 1 with plug in cylinder 2 and the Jeep drove amazing. Drove for a few miles and it ran better than ever. Sweet right?

Wrong. The next morning more misfires and CEL code P0306, which is cylinder 6. Tried to reseat coil pack but same issue. Gave up and put previous plugs back in. Jeep runs fine. No misfires.

Why would new plugs cause this? Coil pack? The plugs I replaced were Bosch Double Iridiums. They probably didn’t need to be changed but since I had the new Autolite plugs I figured what hell.

 

[Chris]

Were the plugs gapped properly?

You weren't having these issues before the spark plug change, is that correct?

 

[bigdaddy]

Were the plugs gapped properly?

You weren't having these issues before the spark plug change, is that correct?

Plugs were gapped at .035 out of the box. No problems before. Didn’t have as much power as it did for the short time it ran without misfiring.

 

[bigdaddy]

Plugs were gapped at .035 out of the box. No problems before. Didn’t have as much power as it did for the short time it ran without misfiring.

I know that in 2001 they just started using coil packs so I’m wondering if the older coil pack is not generating enough voltage for the newer type of plugs. I’ve read that Bosch plugs don’t fire as hot so perhaps they don’t need as much voltage. Just a running theory.

 

[Chris]

I know that in 2001 they just started using coil packs so I’m wondering if the older coil pack is not generating enough voltage for the newer type of plugs. I’ve read that Bosch plugs don’t fire as hot so perhaps they don’t need as much voltage. Just a running theory.

Have you actually tried testing the coil packs just to rule them out? That is where I would start. It would be awfully coincidental if they just decided to go bad right after you changed the spark plugs, but Ockham’s Razor would at least have you rule them out first.

 

[mots]

Definitely keep troubleshooting simple. Swap the plugs back out and run the old ones, then see if the problem goes away?

If it does, the new plugs are causing the issue. I’d double check the part number on the new plugs and make sure they aren’t damaged or improperly gapped somehow.