Last week out of nowhere I get to a stop light and the car starts idling rough and low. About 600rpms. Feels like misfiring. I get home let it cool off and the see that the last plug on the passenger side is loose. Tighten 'er up and check the others ones as well. I don't drive it for a week. Friday I go for a drive and everything is fine until about 3 miles down the road. It starts happening again. Rough low idle and misfiring. So I turn around and drive home. Within a mile the car is fine again. So I go home and check all the wires again. Start it up and now it's backfiring, misfiring, and idling at 600rpms in my garage.

I did a LT header install over a month ago and haven't touched anything else since. Does this sound like a dying Opti or something else? What codes trip the SES light for a bad opti?

 

After the LT's did the car drive like normal? Did the car behave screwy afterward?
Is there anyway you burnt a plug wire?
Just trying to rule out other outcomes before I would deem it an opti issue.
I would re check the plugs wires and the routing again. A number of other things could cause this.

 

The car ran totally fine for 3-4 weeks with the LTs without a single problem. Up until a week ago. But the thing backfires and pops in idle. And for a little while it was intermittent.

Also, I forgot to mention that crusing around 70mph there is a slight stumble. Like I let off the gas but I didn't. It was doing that for about a year or so. Nothing major, I just went a little faster and the problem went away.

 

Make sure the plug wires are not melting on the LTs

 

sounds like a plug wire to me.

I would pull all your plugs and check those too, common mistake is people crack the insulator installing the headers. For like $12, wont hurt to change to some tr55's anyway