i have a 91 wagon with a 4 banger in it. Its a manual.

the problem: when at idle the engine revs to 3k rpm and then goes back down and repeats. This is at a steady rate.

what i tried: I tried to eliminate sensors, although no CEL. When i unpluged the big sensor on the front of the throttle body the engine went to the idle it was supposed to go to and jumped up and town to 1k. When given more gas it didn't run right so obviously the sensor is important. I changed it out with a junkyard part and the same thing happens.

What could be the problem and what should i do to fix it?