Ford Repair: 88 Bronco (Full Size) Idle, vacuum leaks, vent valve

Question
The vehicle idles varies from 500 rpm to 1000
The odb1 codes i get are 23, 31 and 84

Answer
Code 23= TPS sensor out of range. Either the base idle speed has been turned up or the sensor is faulty.

Code 31= EVP out of range, not uncommon on this vehicle, usually carbon in the EGR valve holding the valve open. I always replace the sensor and EGR valve together......however:

Code 84= circuit problem in EGR vent valve, possibly a bad vent valve which could hold the EGR valve open enough to cause the idle problem.

Any or all of these could cause idle problems. Since the 23 is first, I would fix that one first and see what the results are. Remember that you will have to disconnect the battery and relearn the idle speed after repairs.

Another thing to look at is the intake plenum gasket and the throttle body gasket for vacuum leaks, which will also affect the idle. However these two will not give the codes you have.