A month or so ago, I noticed my idle was starting to waver and was on the high side, maybe 1400 cold or warm. Put a new ISC on order from DMCISC. While waiting, I cleaned the throttle body with a spray throttle body cleaner (after plugging the TB ports). Other than a helava lot cleaner, didn't notice much of difference, maybe idling a little higher. Installed the new ISC. Waver gone, and for awhile the idle lowered to maybe 1200. A few days later, the idle is 1800-2000 no difference cold or warm. Like that for a week. Replaced the engine coolant temp sensor Saturday. No difference.

I don't think I have a vacuum leak, as my vacuum gauge (tee'd off one of the plenum hoses) is a normal, solid 15psi when idling.

Today it was 1800 when starting and 1800 arriving at work 20 minutes later, and 1800 leaving work but down to 1200 arriving home 20 minutes later. Probably an anomaly.

I'm thinking the fast idle air valve in the throttle body is stuck open and the ISC isn't able to compensate.

Time to replace the TB?