R33 GTR Idle Issue after cleaning up the AACV

[judasentinel]

I just finished cleaning up the AAC Valve under the intake plenum. However, now the car idles erratically. I adjusted the idle screw to bring the revs down, but it starts hunting. If I keep the idle at around 1200 rpm, it stays there, but after I warm the car up, the idle jumps to around 1400 rpm.

Do I need to readjust the TPS now after cleaning up the AACV? The car runs fine, but after driving around for a few minutes, it tries to stall on every stop sign. The revs go up and down a few times, and sometimes, it would stall, and at others, the revs would creep up to 1200 rpm. I am wondering what could be causing this issue.

Any help is appreciated.

BTW, is the thing under the intake manifold the AACV or the IACV?

Thank you,

Munib

[Skym]

Most likely a intake manifold airleak. Replace the paper gasket on the AACV.

In some cases the AACV sticks and can't be cleaned (needs to be replaced).

[judasentinel]

The gasket is intact. The hunting only happens when I come to a complete stop or put the car in neutral....the revs jump up and down and then settle down at around 1100-1200 rpm. If the car stalls and I restart it, it idles fine.

Could the solenoid be stuck?

Do I need to reset the TPS, now that the AACV is cleaner?

And one more thing. I also have this little black box on the passenger side inside the car, which says VACS - Variable Airflow Control System, for racing purposes only. It is somehow connected to the ECU, and has two knobs on it. One is the Control Point and the other is the Master Volume. Skym, you've been in the Skyline scene down under for a while now. Any ideas as to what this VACS is and what it controls? Both knobs can be turned from 1 to 10, and while nothing changes with the control point, I can feel the revs go up and down with the Master Volume. Does this thing control the airflow volume by tricking the ECU into thinking that there is more air coming in, or does it change the injector flow directly? Any clues as to how to tune this thing and if it is safer to just remove it?

I am also hearing BOV-like sound from the turbo side, which I am thinking cannot be BOVs, as they are usually tucked inside the front bumper on the driver's side. And coming off boost, there are always two BOV sounds, one after another. Its got me worried, that maybe the the turbos are making that sound when I let go of the gas. The car makes good boost to around 0.7 bar, and then flattens out in terms of boost.

Thanks for your inputs.

[Skym]

Ok, that's vital info to know.

A GTR has 2x BOV's at factory in the drivers side guard. Most likely they are aftermarket BOVs that are vented to atmosphere and would explain the erratic return to idle.

About the device, most likely it's a idle stablising device (similar to HKS EIDS) that adjust's the MAF signals. It's usually only needed with atmosphere vented BOV's and MAF's, as the MAF's get affected by reversion (turbo's slow down too much which creates the air reversing direction, that upsets the MAF signals and makes engine run rich and stall or close to).

From my limited knowledge on the device, I think one knob adjust's the point / cell the ECU looks at on fuel map and the other knob for how long (wouldn't know if each selection is 1 or 2 or 3 sec long, as only the manual could tell you). Looking at HKS EIDS manual should help, as it's similar in the way it works with both knobs.

Also the stock ECU has a fuel cut on deceleration which might affect the A/F ratio, hence the rev bounce (rich = revs drop and lean = revs rise). Backing off throttle from higher revs should eliminate this feature as a cause.

Other option is refit stock BOV's with new gaskets and remove the idle stabilising device.