Idle Air Adjusting (IAA) Unit Replacement:
Background:
The dealer had already isolated my erratic idling (750-1800rpm whenwarm), so instead of paying the two hours labor for the dealer to replacemy IAA unit, I decided to try my luck and found out it wasn't as hard asI thought. The IAA is made up of the IACV-AAC (Idle Air Control Valve -Auxiliary Air Control) valve, the idle adjust screw, and the IACV-FICDsolenoid. Fortunately you don't have to remember all this because it comesin one unit. The IAA controls idle and compensates for cold engine startsand pressure changes. Work was performed on (& corresponding part numbersare for) a 1993 Twin Turbo.

Timing: 
As required. When idling becomes erratic and can no longer be adjustedusing the idle adjusting screw.

Parts: 
AAC Valve Assembly. P/N: 23781-40P10 (~$215 from Courtesy)
AAC Gasket. P/N: 23785-40F00 (~$1 from Courtesy)
Blanket or towel to protect the front quarter-panel from scratches
Zip ties

Installation: 

1. Look at the new AAC valve assembly (1.). There are 9 featuresyou need to familiarize yourself with. The two hose attachments, the twoelectrical connections, the four bolt through holes, and idle adjustingscrew.

2. Open the hood and drape a blanket or towel over the driver's sidequarter-panel to prevent scratches to the paint. The Idle Air Adjusting(IAA) Unit is located on the driver's side just behind the manifold andjust inboard of the master brake cylinder (2.). The idle adjustscrew is most easily visible.

3. Start removing air hoses one at a time to gain access to the IAAunit. You may need to unplug the plug wire going to the back cylinder andthere is a ground bolt that may need to be undone to ease access. Onceyou get down the unit, remove the short hose between the IAA unit and thetube between the master brake cylinder and the manifold (3.). Zipties may be useful to hold hoses and wires back out of the way.

4. Reach down and unplug the two electrical connections attached tothe IACV-AAC valve (black male connector, yellow female connector) andthe solenoid (light blue connectors). These connectors have wire retentionclips which do not need to be removed. A little bit and patience and gentleprying with a screwdriver will get these out eventually.

5. Once the electrical connections are removed, all that's left is onehose and 4 bolts. The red arrows (4.) show the two top bolts thatare easy to get to with a 3/8" socket wrench with a 10mm socket. Thereare two other bolts below that require a little more coercing to get out.You may need to change to a smaller 1/4" socket wrench also with a 10mmsocket.

6. The blue arrow (4.) is a tube that needs be ben back towardsthe driver's side once the top bolts are removed. I had no issues bendingthis, just be gentle.

7. Once all bolts are removed, pull the unit out and the last hose shouldwork itself free.

8. Take the old part and compare it against the new part (5.).There should be no physical difference.

9. Pull out the new gasket and apply a light coat of grease to bothsides. This will allow it to adhere to the IAA unit during installation(6.). Put the gasket on the IAA unit and attach the short straighthose to the unit.

10. Before reinstallation, take an opportunity to clean the solenoidand the IACV-AAC connectors (7.). I use WD40 and a small paintbrush.Make sure the connectors are completely dry before reattached.

11. Reassemble in reverse order. Put all four bolts in fingertight beforeusing a socket wrench to tighten them. Make sure that the gasket is stillaligned during installation. 

12. Once all hoses, wires, and bolts are back in place, remove all ragsand tools from the engine bay. Start the car and let it get warm (a shortdrive perhaps). Upon return, open the hood and turn the idle adjustingscrew counterclockwise to increase idle, clockwise to reduce idle. Z'swith manual transmissions (TT or NA) should idle at 700 +/- 50 rpm. Automaticsshould idle 50-70 rpm higher when put in neutral. The engine will tendto idle no lower that 700 rpm, so I would suggest turning the screw counterclockwiseuntil the rpms just slight start to rise. 

Steven(Phx)

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