How to Fix Surging Idle on 1991 Toyota Pickup [Video]
The fix is pretty simple and involves cleaning up a small pipe in the engine bay. You will need a ratchet with extender, 12mm plug and something thin like a screwdriver to clean the pipe.
It appears that there is a vacuum-powered line that runs coolant off from where the thermostat is located, up to the throttle body.
It’s hard to explain which line it is, so you’d better see it in the video bellow. After that, you’ll only need to take it out and clean the socket in which it was plugged.