9/13/2021
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Connector suits R33 RB25DET and S14 and S15 SR20DET’s. The idle control system on mid-1990’s Nissan RB and SR engines (among others) consists of an Idle Air Control Valve (IACV) unit, which consists of two devices – the auxiliary air controller (AAC) and the fast idle control.

Disclaimer:This information is provided for your information only, anything you choose to do with this information is completely up to you and in no way will I, Skylines Australia or any affiliates be held accountable for following said actions, causing damage or harm to you or your car.Introduction:There have been numerous references to cleaning your AAC (Auxiliary Air Control) valve on your skyline on these forums, however I have noticed that many of these descriptions are fairly basic. As I was going to clean mine anyway, I thought I’d photograph and document the process so as to provide a tutorial for those attempting this themselves for the first time.
This isn’t an overly hard thing to do, any many people with a basic knowledge of mechanics (Ie. Me) would be able to wing in without a manual, but nevertheless, if this DIY was available for me before I started, I would have used it.

Why clean your AAC Valve/ What does it do?

When your RB series motor is warmed up and idling it is your AAC valve which controls the amount of air entering your engine in order to control your idle. The best description I can think of for one of the things that your AAC valve does is if you were to sit in your car, stationary at idle and turn the steering wheel. The throttle blips in order to stop the power drain taken from the power steering stall the car. That blip was the AAC valve letting more air into the engine, bypassing the closed throttle body.

There has been much said about dirty/faulty AAC valves causing or attributing to idle hunting issues and this was my reasoning for cleaning my valve. This isn’t the only cause of poor idle issues, but it can definitely attribute if not cause poor idle/ hunting idle issues.
Basically over years of use, carbon deposits rising from the engine, minute grit etc from the intake builds up around the valve and causes it to work in less than ideal conditions.
(Note, this is not the valve which controlls cold start revs on idle, that valve is connected inline on the same intake pipe as the AAC valve but is situated under the intake plenum on RB25’s)

Car in Question:

1993 Model Nissan Skyline R33 GTS-T (Manual)

Difficulty Level of DIY:

Low-Med

Time Needed:

Budget at least one and a half hours, longer if its your first time.

Materials/Equipment Needed:

-Torch (A must for finding those stupid bolts)
-Ratchet Set
-Philips Head Screwdriver
-Flat Head Screwdriver
-A Tray to Hold the Bolts
-Carby Cleaner! (Don’t even bother if you don’t have this)
-CRC Cleaner (Not a must, but I like to spray the electrical plugs with this while they’re off)
-Gasket kit or Gasket Goop (Again, you may not need this providing your gasket holds together, mine did, but it will vary obviously from car to car)
Thanks to Justin911 for the gasket number from nissan (if you need it): 23785-57Y00
-Cotton Wool Buds
-Air Compressor/Air Gun (Not 100% nessecary, I just used the cotton wool buds instead, this would just make the job faster)
-A pair of pliers (For removing hose clamps)

Procedure:1) Pop the Bonnet….duh2) Locate the AAC Valve & Assembly at the rear of the plenum (near firewall), see below picture its circled in red.

3) Before you remove the bolts that hold the AAC valve assembly to the side of the intake plenum, there are two electrical plugs and two intake (ones actually an output that goes to the cold start valve) that need to be removed, the plugs are red(browny) and purple you can’t miss them (see above pic).

4) Before you start to remove the pipes from the AAC it’s a good idea to remove the two other pipes infront of the AAC can get in the way, ones a Vac line and the both come out pretty easy, it just make life easier when getting to the bolts and pipes on the AAC (circled in yellow in pic).

5) Remove the pipes going to the AAC Valve, circled in blue. They’re a little finicky.

I found it was easier to remove the pipe that goes back to the intake at the place marked with the blue circle, then once the AAC assembly was out, to remove the hose at the base of the AAC itself for cleaning.

6) Now using your ratchet, you need to remove the three bolts holding the AAC assembly on to the side of the plenum.
One bolt is on top, the other two are underneath, you can partially see one but you’ll really have to find them by feel. See some of my later pictures of the AAC assembly removed if you want to see exactly where the bolts are in relation to the assembly.

7) Once the three bolts, 2 hoses and two plugs are removed you are ready to pull the AAC Assembly off of the side of plenum. Be careful if you want to re-use your gasket. (see pic)

8) Hooray its off! Take a look inside the valve, mine was pretty filthy (see pic) its no wonder after 13 years of driving! This could definitely do with a good clean, I could already see it was far from optimum.
(Note in this picture I’ve already removed the AAC valve from the AAC Assembly as described in the next step)
Snapshot for mac.

9) (I presume you’ve already removed the other half of hose as described in step 5, clean this pipe with cleaner too).
Its time to familiarise yourself with the operation of the AAC valve and Assembly, basically there are 3 parts. Held on by two screws is the AAC valve itself, you can remove this now, but be careful as there is an o-ring between the assembly and solenoid (DON’T LOOSE THIS). There is the Assembly itself and you can see there is a spring inside there with a diaphragm above it that when the solenoid activates it, pushes against the spring and opens the diaphragm allowing air in the intake pipe through the assembly and into the intake plenum.
On the bottom of the AAC assembly there is a screw, this can be removed for cleaning and is generally the dirtiest part as this is the idle control screw. At all times when idling the tiny gap that the screw when in allows air to pass through keeps the car at its steady idle. It’s no wonder there is so much buildup.
(See pics, AAC solenoid by itself removed and circled in red the location of spring, diaphragm and idle spring, note you cant actually see the idle screw in this picture)

10) Its time to clean the assembly, carby cleaner time!
Be liberal, it might take a while for the crudd to break down as its really baked on there, but eventually it all comes off, as I said before, I used cotton wool buds to get to the harder spots but an Air compressor would most likely work much better. Make sure you manipulate the spring and diaphram, and make sure all those areas are clean as well as the pipe intake and out pipes. Also clean the idle valve screw (you’ll see its hollow down its centre)
Wait till the whole assembly is dry and make sure again its all clean. Check the smoothness and operation of the spring and diaphram.
Clean the machined faces that face both the intake plenum and AAC solenoid to leave a smooth surface.
Check out my pics of before and after of the dirty valve and then cleaned, its a big difference, trust me! Especially in regards to the operation of the spring and diaphram.

Before

Rb25 Idle Air Control Valve Control Valve

After:

11) Now its basically the reverse to put is all back together, ensuring all faces on the AAC Assembly are clean, re-attach the AAC solenoid ensuring you have the o-ring in place. Re-insert the idle screw, be sure not to overtighten this, but it in to an amount where you can just see the end of it protuding into the plenum intake hole, we’re going to adjust the idle properly later.

12) Make sure AAC Valve and Assembly is all dry and free of loose debris one last time, re-attach unit back into car in reverse order as above, ie: Pipes first, Install new gasket if needed, install bolts (DO not overtighten the bolts!).
Before re-attaching the electrical plugs give them a quick spray with CRC if you have it, and let it dry.
Also make sure you re-attach the two pipes we removed earlier in order to make getting to the AAC easier.
And we’re done! Now its time to set the idle!

13) Now there are two ways to set the idle, the long proper way and the short easier way. I did the long proper way, but it’s really up to you which you’d like to do. (NOTE: I HAVEN’T ACTUALLY TALKED TO A SINGLE PERSON WHO SAID THEY NEEDED TO DO IT THE LONG WAY, ITS PROBABLY NOT WORTH DOING IT STRAIGHT UP, JUST GO BACK AND DO IT IF NEED BE)
Basically in the long way you have to remove the kickpanel in the passanger footwell, remove the computer (while still connected) and locate the electronic idle tuning screw (see pic) DO NOT UNDER ANY CIRCUMSTANCES APPLY TO MUCH FORCE TO THIS SCREW, IT TURNS VERY EASILY!

14) Once you’ve located the screw, you’ll see it can move through about 70 degrees, it should be pointing towards the bottom left corner, do not turn it past 12 o’clock. If it is already facing as far left to bottom as it can, turn it ever so slightly clockwise, just a smidge. Now turn the car on and warm it up, your idle may be fairly high depending on how far your idle screw is in, don’t worry about this too much as we just want to warm the car up for now so we can work just on the AAC valve without the coldstart valve interfering.
BTW this is a picture of the Idle screw if you haven’t figured out where/what it is yet (you should have if you cleaned it!)

15) Once the car is warmed up disconnect the TPS (Throttle Position Sensor) See pic below circled in red, it is on the opposite side to where the throttle wire rotates the throttle butterfly. (Note if you have an R32, the connection you have to disconnect when adjusting the idle is actually the brown electrical connector on the AAC assembly, not the TPS)


See where the car idles. Now depending on where it’s idling depends how you turn the screw (AAC one, not the computer one), turning it clockwise lowers the idle, anti-clockwise increases idle. You want to aim for 900rpm. Once you have this by adjusting the AAC idle screw, reconnect the TPS connection with the car running, see if the revs change. If they do, you’re now going to have to adjust them on the computer (see this is why we got it out).
It’s the OPPOSITE on the computer to the AAC valve screw: Clockwise=increase idle, Anti-Clockwise=decrease idle.
You can move the idle in 50rpm increments so I’m told and at total variation of 250rpm, so DO NOT OVERTURN THIS COMPUTER CONTROLLED IDLE. This is only a fine tuning mechanism.

Rb25det Idle Air Control Valve Delete

I didn’t actually need to move my computer controlled rev, as my revs didn’t change once I put the TPS electrical connector back in.

Rb25 Idle Air Control Valves

16) All sitting smooth? Turn the car off, put the computer back in its kickpanel, re-check all connections in the engine bay to make sure they’re all tight etc.
Start the car up again and check idle.
Mine was sitting steadily on around 800rpm, perfect for me.

Symptoms

17) Obviously the easier way of setting your idle would be to do all of the above, minus the computer step. If it needs it, you could go back and do it properly.

18) GO FOR A DRIVE!
I don’t know about you, but this has actually fixed my idle hunting issue, not to mention my car doesn’t shudder or hesitate when coming to stop and pushing the clutch in anymore. Feel free to give me some feedback about your experiences with this DIY and how it has affected your car.

I hope this DIY has helped, please ignore my bad spelling and grammar!

ORIGINAL POST BY USER INSU. This only serves as a backup and for my own future referance

6817.info – 2018