SR20DE High Idle, Zombie AAC?

Engine, Transmission and related drivetrain.
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okayfine
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SR20DE High Idle, Zombie AAC?

Post by okayfine » 08 Oct 2014 12:45

I am working out the little issues on my SR swap, and one of them is this high idle. I am running a pared-down KA24DE harness with Sentra MAF and ECU, wired with a CAN/AM box.

After the engine is warm, the idle remains at ~1500. These are the things I have checked:

-Checked the SB wire from the ECU is undamaged from the ECU plug to the AAC plug
-Checked the BR wire from the AAC plug is connected to key ON power (CAN/AM box) and that the CAN/AM box status LEDs are lit
-Checked AAC valve resistance, FSM spec of ~10ohm, I measured 10.4ohm
-Turned clockwise the idle adjust screw to its clockwise stop. If I turn the idle adjust screw counterclockwise, the idle DOES rise
-Checked for tension on the throttle cable, there is slight slack
-Checked the adjustable TB stop screw, verified there is a gap between it and the TB tang
-Set timing to ~15°BTDC, TPS plug removed
-Capped the three vac nipples on the SR TB

For grins, I also verified fuel pressure to FSM specs and performed the "Check Intake Air Leak" test by pinching the hose from the valve cover (rpm drops slightly, which it should).


Since the engine does idle, the AAC is working to some extent. If I set the timing to less than 15°, the idle drops. I can get a <1000rpm idle by setting the timing to 0°. CAS was reinstalled with the engine at TDC.

Any other direction? A vac leak on EFI should have the opposite effect, but I don't have a vac leak big enough to surge the idle 600rpm+, and there aren't any typical driveability issues related to vac leaks this big. There are no open ports I can see; the OE brake booster nipple is feeding my FPR.

If the AAC is stuck open, would I get the symptoms above? Shouldn't it fail closed?

Now...here's something interesting. I just went and disconnected the AAC. Engine needed throttle to start, but the engine idled at 900rpm. :shock:
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Re: SR20DE High Idle, Zombie AAC?

Post by icehouse » 09 Oct 2014 08:04

Why not just turn in the screw on the IACV in?

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Re: SR20DE High Idle, Zombie AAC?

Post by okayfine » 09 Oct 2014 08:24

Done, all the way in. Idle didn't drop...or I should say 1500 was as low as it would go. If I screwed it out a few turns, the idle started to climb.

My AAC valve looks different than that. I can't find a good picture on Google, but I don't have FICD, the AAC doesn't have a plug on the body but two black wires coming out to a plug, and my adjusting screw is on top.
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Re: SR20DE High Idle, Zombie AAC?

Post by finn » 09 Oct 2014 09:05

my KA had the same issue. its documented in the build. i removed the valve and replaced the soleniod but found that the spring was worn as well. the idle dropped to 1000rpm when its 180* or less.

pull the valve and inspect it. chances are its crap. is the SR on the back next to the firewall as well?.. pain in the ass :(
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Re: SR20DE High Idle, Zombie AAC?

Post by okayfine » 09 Oct 2014 09:34

Perhaps worse than that...it's inside the big bend of the intake manifold. That and the coolant hoses have to come off before I can get to the AAC.
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Re: SR20DE High Idle, Zombie AAC?

Post by finn » 09 Oct 2014 09:54

shadetree mech would suggest removing the idle adjust screw and spraying carb cleaner in there to break loose any gunk thats holding up the spring travel....

does it lower rpm even slightly when it reaches proper operating temp?
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Re: SR20DE High Idle, Zombie AAC?

Post by okayfine » 09 Oct 2014 10:02

Nope. It actually crept up a bit after driving for a while.

What I can't figure out is that when I disconnect the AAC plug, my engine still idles, but now at 1000rpm, about where it should. This would seem to indicate a stuck AAC plunger...but something obviously moves in the AAC when it is plugged in, so...?

Car otherwise runs great. Sitting at a stop light at 1500rpm+ is a drag.
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Re: SR20DE High Idle, Zombie AAC?

Post by okayfine » 09 Oct 2014 13:37

Disconnected AAC, idle adjust screw turned fully clockwise (closed/lowest idle) gets me ~1000rpm idle. Looked over the engine for a half hour, looking for where that extra air is coming from. Blocked off the crankcase vent to the intake pipe, no change.

Reset timing with new idle. Was 5°, reset to 15°. Drove it, put a light on it again, ~16.5° or so, but idle was ~1100 or so. I don't know where timing starts to get added by the ECU, but certainly somewhere between 1000-1500.

Engine start needs throttle. So engine start needs air, but once it has started the idle will bounce for a second, then stabilize at ~1000.
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Re: SR20DE High Idle, Zombie AAC?

Post by okayfine » 11 Oct 2014 09:17

It would appear that the Nissan AAC valves have an internal bypass for idle air, and that the AAC only boosts idle speed from there for cold-start or high-load (read: A/C) conditions. So my AAC valve, while not truly dead, is malfunctioning. The 1000rpm idle I have with the AAC disconnected is still higher than it should be (since factory spec is 850rpm or so), but it would see a replacement AAC is in my future. Just posting up so future people can at least have the idle bypass dealio confirmed.

The Tacoma in my fleet had its idle valve go bad, and that caused the engine to die unless you kept throttle to keep it running. So I hadn't figured on Nissan's failsafe method.
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Re: SR20DE High Idle, Zombie AAC?

Post by finn » 11 Oct 2014 10:51

Did you feel around it while its running to see if air is leaking from the gasket or o ring?
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Re: SR20DE High Idle, Zombie AAC?

Post by okayfine » 11 Oct 2014 11:41

Impossible to get to in a meaningful way; I could never be sure I'd confirmed it one way or another.
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Re: SR20DE High Idle, Zombie AAC?

Post by Three B's Racing » 13 Oct 2014 06:00

My S14 SR20DE idled high when I first got it running. Taking the IACV off and apart showed the internal moving parts were sticky from sitting for however long it sat. Your S13 and my S14 have very different IACV's. Check the throttle body's butterfly adjustment screw. It is only there to keep the butterfly just off the ID of the throttle body to keep it from sticking. While there you could also check the TPS because if the previous owner F'K with the butterfly adjustment screw it will put TPS out of spec.
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Re: SR20DE High Idle, Zombie AAC?

Post by Three B's Racing » 16 Oct 2014 07:33

Any progress on you high idle issues Jules?
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Re: SR20DE High Idle, Zombie AAC?

Post by okayfine » 16 Oct 2014 07:50

Used/replacement AAC is incoming. Leaving the 510 running until I get it and the new intake manifold gasket so I can do the R&R in a day instead of having the car nonop for a week.

Drives great, other than the cold-start issue, and inability to set idle below 1000rpm. We'll see if a non-NOS AAC valve will do the trick.
Because when you spend a silly amount of money on a silly, trivial thing that will help you not one jot, you are demonstrating that you have a soul and a heart and that you are the sort of person who has no time for Which? magazine. – Jeremy Clarkson

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Re: SR20DE High Idle, Zombie AAC?

Post by Three B's Racing » 17 Oct 2014 05:03

okayfine wrote:Used/replacement AAC is incoming. Leaving the 510 running until I get it and the new intake manifold gasket so I can do the R&R in a day instead of having the car nonop for a week.

Drives great, other than the cold-start issue, and inability to set idle below 1000rpm. We'll see if a non-NOS AAC valve will do the trick.
Sweet!!! if the used AAC works then used or not it's a good day.

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