Brand new here but let me throw my .02 in. I just went thru the timing belt change and a tooth off will make it run like crud and mine backfired when cold. You said this started when you did the tbelt w-pump etc awhile back right. Mine came from the shop a tooth off and it took this place and a good pic on a post for me to get it running right (40 teeth from cam to cam). One tooth will make it run just enough off that it seems like something else. I was convinced the guys had forgot to hook up some sensor or I had a connection loose. I didn't run cold though so...