Nissan Repair: Erratic idle 1986 Nissan D21 Pickup, nissan d21, vacuum leaks

Question
QUESTION: Greetings:  While attempting my smog check, the test had to be discontinued due to erratic idle between 900 and 1300RPM while the  engine was warm, in park.  There are no vacuum leaks and no way to manually adjust the idle.  The dealer declined to fix it so I must try myself. Any advice on what part(s) to replace is appreciated.  Thank you

ANSWER: Art,

The idle air control valve or the manifold absolute pressure sensor are the most probable issues.  Any codes?  If so, email them to me.

---------- FOLLOW-UP ----------

QUESTION: Thanks.  I replaced the FICD Solenoid Valve, no change.  Today finally took the seat out and got Codes 23 and 24. I got an Idle Up Solenoid Valve from Nissan, and it shows it in my Chiltons, but I don't seem to have one!?  I had another guy with me and we can't find it even though the diagram shows it.  I'm thinking that Code 24 (Park/Neutral Switch malfunction) could be it.  Maybe it doesn't know it's in Park and is trying to compensate, hense the up/down idle?

Answer
Art,

The manifold absolute pressure sensor (MAP) provides information to the computer that it uses to adjust the idle via the idle air control valve.  If the FICD did not correct the problem then the MAP is the culpret; most of the time.  Otherwise, the throttle position switch is out of range and the computer does not think it is at idle.  The park switch should not cause the problem.