[corning_d3]

I also had an 88 Ranger (GT) with the same problem. I don't suspect a vacuum leak, because if there was a leak, the problem wouldn't disappear. I would suspect a bad idle control motor, throttle position sensor, or coolant temp. sensor. When you say idle control sensor, you mean idle control motor i hope. The first thing the check is the throttle valve bore. A lot of times they can get a piece of carbon or other foreign material caught in between the butterfly and the bore. Pull the air intake tube off the engine, and visually inspect the inside of the throttle body to make sure there isn't any carbon buildup(It will be nasty, a cleaning with carb cleaner and a rag should help it, anyway..). If this doesn't improve, inspect the idle control motor air passages for carbon buildup. Now would be a good time to test your throttle position sensor, coolant temp sensor, MAP sensor and EGR valve and sensor. Refer to a good Haynes or Chilton manual for testing procedures and sensor specifications.