As long as I’ve owned my 87 Coupe GT, I’ve been trying to track down the source of the rough idle when it’s warm. It runs perfectly smooth when cold but gets worse as it warms up. I’ve tested all of the temp sensors, idle valve, etc., and all test as good. The one part I’ve overlooked is the idle switch. I finally ran a factory test on it following the steps in the Bentley manual. It gets 12volts like it’s supposed to, but the ohms test failed. It’s supposed to go from zero ohms when the throttle is closed to infinite ohms when opened past a certain point. It reads 26k ohms closed and 35k ohms open. Okay honestly my measurements might have actually been 2.6k ohms and 3.5k ohms, but either way it failed the test. To test the theory I clipped the terminal 1 wire (ground) and it made it idle where it’s supposed to, around 800rpm, which smooths out the idle a bit. With the wire connected it idles between 720 and 750rpm. It doesn’t seem like much but that extra 50rpm makes a big difference. I’m thinking I have to do two things: 1) somehow take the idle switch apart and clean the terminals inside, and 2) trace that ground wire from the switch and check for a bad ground connection. My guess is the switch isn’t telling the computer that the throttle is closed and causing the rough idle. It’s just a guess, but it’s the most concrete theory I have right now. Thoughts?