I have a stock 04 gt, 60k miles,put gas in hte other day and parked the car -when I restarted check engine light came on. Car began a tiny roughness at Idle. Idle rpm rought at 700, smooth at 1000. But varies idle speed from smooth t rough. I suspected fuel cont or grade, add 6 gallons premium and stp fuel treatment-
drove about 20 miles varing speed and rpm high to low, still sough idle persists and comes and goes.I'm in tennessee and the weather is wet and cold-have had ice on car at night a few times-could it be ice cracked a vaccum line or gas cap problem-
Car runs perfectly over idle-cruise-full throttle-etc. So far. Or should I add more fuel -top tank for possible bad fuel octane ?