High Idle Problem

[techies]

Hi all, I've been having some high idle problems with my 91 sedan. When in drive (at a stop) it idles at about 1300 rpm and when in park or neutral, it idles at 2200+. Also, I notice when I take my foot off the gas, the idle will drop about 700 rpms for a second or two, then jump back up. Any ideas? I looked for vacuum leaks and I couldn't find any, can it be something else causing this issue?

Thanks in advance!

[techies]

anyone?

[tashirosgt]

I have the service manual for the '90. Under "Symptom - High Engine Idle Speed (No drop) are
Check Accelerator Linkage
Check Auxiliary Air Valve
Check ISC system (ISC = Idle speed control)
Check Air Conditioner Idle-up Circuit
Check Diagnosis System (for diagnostic code)
Check Throttle Position Sensor
Check Fuel Pressure
Check Cold Start Injector
Check Injectors
Check EFI Electronic Circut using Volt/Ohmmeter

Following a checklist is usually a sign that you don't know what you are doing - and I haven't encountered the high idle problem before. If we being with "Check Auxiliary Air Valve", it shows:
"On Vehicle Inspection
Check the engine rpm by closing the air valve port on the throttle body
At low temp ( Coolant temp. below 80 deg C (176 F)
* When port is closed, the engine rpm should drop
After warm-up
* When the port is closed, check that the engine rpm does not drop more than 100 rpm.
If operation is not as specified replace the air valve.

The cartoon for "closing the air valve port on the throttle body seems to show the throttle body with the big black plastic air intake channel removed. A hand appears to be pressing a plastic cover over the metal channel where the air enters the throttle body. Anyone know if this is what is meant?