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So I posted this on one of the other sites I am on; this is from last Saturday when I did some DIY work. I've got more plans to do work as the weather gets better so I'll keep posting. I'm new to this site so please let me know if this is not the right place to post.


I've always had problems with my idle on my r33 GTR running a power FC; it runs 264 cams and felt quite lumpy and often hunted for the right idle speed and AFR. I do the tuning myself but could never get the idle just right.. until today! This took me 4 hours to do but I am quite slow and I have quite a busy engine bay so I had to remove some things to get to what I was after.

From what I have learned, there are two air control parts on a typical GTR, the IACV and the AAC. I'll show a picture and brief explanation of both: -


This is relatively easy to get to on the RB26; it is just underneath the intake plenum and has 4 bolts that are roughly 2 inches in length and are a 10 socket. It is comprised of two major parts; the electrical component which shows the AAC label and the controller which has the spring and screw inside which allows the fluctuation of air.

As I understand, the AAC is designed to increase air flow when electrical load occurs, such as a power assisted steering wheel being turned.

When this gets dirty, it can cause high idle because the spring holds the gap that lets the air in open causing a leak in the system which will also cause lean behaviour. Alternatively, if it's filled with gunk, it could cause hunting due to bad air fuel ratios from going too rich.

See below for some pictures of me holding the AAC after I gave it a quick clean. (The AAC below is not my image)

The AAC has a screw that you can turn; it is facing the front of your car or at the right of the right component on the above diagram. Turning the screw clockwise will lower idle and turning it anti-clockwise will increase idle. The basic functionality of this is that it overrides the system and leaves a small gap that air can go through. There is some conflicting instructions that to change the idle you have to unplug the throttle position sensor and/or disconnect the brown connection but you'll see what I did later on.

IACV (Idle control valve)

I'm not overly knowledegable about this component as I don't actually have this on my car. A lot of people remove this and/or block the piping and rely on tuning to do its job. It is located very much under the air plenum above the oil filter.

Basically, this component will allow more air into the system depending upon how cold the water temperature is. As the water temperature rises, the valve will gradually shut until this device is no longer used.

A couple of problems that I can think of is that if this is faulty and it doesn't close, there would be an air leak and if it doesn't open it would mean there would be some lean behaviour on startup if not corrected by the ECU.]

The nissan skyline manual shows how to diagnose this if it's faulty but it is a real pain to get to.

My experience

I am fairly novice with cars but I'm learning more and more as I tinker around with my overly specced car that I bought on a whim one day because I felt my impreza wasn't powerful enough!

So, I've had major problems with my idle; either hot or cold, it would start to drop below 800rpm and I'd have to apply the throttle to get it to stop it from stalling. I tried modifying the fuel ratios, the ignition timing and the idle settings on the powerfc using datalogit to no avail. After doing some research, I decided that I needed to take off the AAC and give it a clean to see if that would work.

Required: -
  1. Size 10 socket wrench will do, it's not that tricky to get to like some people say; I had an ordinary sized socket wrench and managed to get to all 4 bolts easily
  2. AAC gasket or you can make one yourself using a stencil and some gasket material (You could re-use but I decided against it)
  3. RTV sealant
  4. Carb cleaner
  5. Phillips screw driver
  6. Toothbrush/pipe cleaner or some way of cleaning
  7. For a picture of the gasket, see below; it's quite tricky to find it and I had to get it from America and nearly ordered the RB25 one; I live in the UK.

Instructions for removal: -
  1. Firstly, remove anything on the left side of the engine bay that will restrict you from getting just underneath the intake plenum; I had to remove my catch tank and my fuel pressure regulator to get to this but this took a few minutes as I'm always removing these to clean up on my overly obsessive cleaning schedule of my car.
  2. Remove the brown wire socket which is clipped on fairly trivially
  3. Use the socket wrench to undo each of the bolts; mine were so easy to remove that once I'd loosened each bolt, I could use my finger to unscrew which was a lot easier than worrying about dropping the bolts down into the endless pit inside the engine bay.
  4. Simply pull at the AAC gently, trying to rescue the gasket if you want to reuse; I found that it was just stuck to the AAC and was easy to peal off.
  5. Remove the air tube that connects to the left hand side of the AAC
  6. You should now be able to take the AAC out of the engine bay and take it to your working area ready to be cleaned.

Half way through cleaning, I decided I was going to take some pictures but unfortunately I left my iPad at home so I had to use my laptop to take the photos. It was also very cold today and my fingers didn't like the combination of carb cleaner and freezing weather. Hard to believe they're actually normal looking when I'm in the nice warm area typing on my computer!

The AAC is comprised of two parts; I call them the electrical component. They are joined together with two small phillips head screws and come apart quite easily. Be careful as there is a little rubber seal that sits between them: -

and the air control component.. see below

Mine was fairly dirty and had some black grimey gunk all around the spring and screw. I immediately smiled and was quite excited at the prospect that my lovely car was potentially going to stop idling badly! I got a spare toothbrush and pipe cleaner and used carb cleaner spray to basically rinse the whole thing out and clean the spring and screw out.

This is a lot cleaner than it was but what you can see is two holes in the center of the air component. The hole on the left is the spring that is controlled by the electrical component and the hole on the right is the screw that you can manually screw with a phillips head screw driver. Make sure these are both clean before putting back together. I actually adjusted the manual screw so that there was a tiny gap as I've read that it needs to be open a slight bit. This could be wrong so this may need to be adjusted to meet your idle speed requirement.

I also gave the electrical component a cleaning but only on the side where it connects to the air component. : -

Once I had cleaned the air component, I scrubbed the port underneath the intake plenum with carb cleaner as there was a tiny bit of grime and old sealant.

I then screwed the components back together to form the AAC again and then took my nice new gasket out of the packaging: -

Applying the sealant
  1. Apply a very thin layer of RTV sealant on the AAC firstly by adding a small line, roughly 2mm thick in small sections and using my thumb to spread it out until the flat edge is completely covered.
  2. Place the gasket onto the AAC; it will only go one way so that the layout matches. If it doesn't match, you've probably bought the wrong gasket. It's quite easy to do as there is a lot of posts containing the RB25 part number and so on.
  3. Repeat step 1 but this time on the gasket on the side that will be attached back to the port
  4. The tricky part is now getting the AAC back on without fudging the sealant and getting into a right mess. I did this the first time but make sure that if you mess it up, you ensure the sealant is properly applied as if it is not, there could be an air leak.
I found that with the sealant I used, the AAC actually stuck to the port without the bolts which was nice as it gave me the opportunity to let go of it to push each of the bolts in and hand tighten them before I used the wrench to tighten them.

Remember to re-attach the pipe and brown connection which, for me, was quite easy. I thought it was going to be an annoyance trying to get them back on afterwards but they went on surprisingly easy.

PowerFC idle learning (only for power fc)

I did not adjust the screw on the ECU as I feel this is something that shouldn't really be messed with and I didn't change the throttle position screw. The only screw I set up was the one on the AAC which is shown above.

  1. I use datalogit but there are posts that let you know how to do this with the hand controller.
  2. Switch the ignition on and turn off any of the blowers/air con/lights etc.
  3. Reset the powerfc back to defaults but ensure that you've saved your map and can reload it with your preferred settings.
  4. Switch the ignition on and leave the car to idle for 10 minutes to learn idle at no load
  5. After those 10 minutes turn on the lights and the air blower for 10 minutes to learn idle at electrical load
  6. After those 10 minutes turn on the air conditioning for 10 minutes to learn idle at air con load
  7. The 30 minute period is required by power fc and should be performed, just for peace of mind after doing this as it can change quite a lot of things if the AAC was dirty.

Hopefully this fixes your hunting idle; it fixed mine. Sorry for the lack of pictures and quality, as well as the inexperience that is obvious in this post. Just figured it would be nice to try and help people who have this issue as it can really be annoying stalling at junctions through no fault of your own!

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Not sure this is the right part of the forum, but would make a very handy sticky elsewhere I think - its always good when people do write-ups on the less glamorous DIY jobs!
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