2012 and it's sputtering, backfiring, stalling out at idle, but seems to run ok under load. I just had it in for a complete go through at the dealership. The did the routine warranty check up, replaced the diff fluids, oil, filter, and spark plug change. The last time I drove it, it idled fine. Started it up yesterday, and it's a mess. I pulled the plugs, left bank was black, the right bank was half black, and tan, both are getting spark with the key. Ideas?