My '97 Deville has developed a rough idle recently. RPM varies between 625 - 700. DIC shows no codes. Seems to run fine other than idle. Gas mileage is about the same as always 25 - 27 mpg on highway. It has about 185,000 miles on the clock. Might use about a quart of oil per 5000 miles. Does anyone have ideas what might be causing the idle problem.