My 04 with 410,000km seems to not idle as well as it should be. It seems to misfire a little bit, and I noticed if I bring rpms up to 2-3000 the misfires and some small pops/burbles are there too, but the car runs fine when driving, and fuel economy is still pretty good. I put in a full set of the proper NGK iridiums at 370,000. Ive done the typical egr/throttle body cleaning recently. The air filter has been changed recently too. I did an oil change last week and inspected my vacuum lines and pcv valve, which both checked out fine. I used my Bluedriver scanner to check live data, and my fuel trims/o2 voltages on both banks seem to be ok. The only mods I have done are a 3rd cat delete, and a stage 2 exhaust with Magnaflow 11365 rear mufflers. Only thing I can think of is that the o2s are a bit worn out and making the car run rich. Any suggestions?

 

If they are the originals than it might be prudent.
I recently changed the original 17+ year old 54K mile O2's on my CL-S project car, just because, and it helped lessen my infrequent misfires. YMMV?

 

Ever did valve adjustment?

I believe the idea is that valve seats wear out, making exhaust valves too tight, and if left alone can lead to exhaust valves not closing all the way and misfires at idle, and if left alone again can lead to burned valves and loss of compression.

I've seen only one J series engine with burned valves and that thing had >200k miles, but you are in the ball park.

 

I adjusted valves at 180k, they were all in spec with some slight deviation, so my adjustments were more to make them all the same. Some witness marks from when I replaced upper & lower intake gaskets made me wonder if a cylinder head was replaced at one time (probably from broken timing belt). Maybe o2 sensors were a bit worn out. I sold the car last weekend, so I'll never know 🤷‍♂️