Thanks for the advise. I figured to start with the most obvious and cheapest first: inspecting the throttle valvle bore. After taking it apart and cleaning it out throuroughly, the high idle problem is still there. Actually, the bore was pretty clean. I'll check the idle control motor next.
As for the idle control sensor, I wasn't referring to the motor but the black sensor box that is mounted on the rear of the engine wall to the right of the throttle body. I thought that was the name of it but escapes me right now. The truck idle was spiking before and replacing that sensor corrected the spiking.
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Originally Posted by 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.
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