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View Full Version : Building custom inlet manifold. Do i need the IAC valve?



sultennok
03-03-2008, 04:15 PM
Hi..

My engine tuner is building a custom inlet manifold for my new High Comp. Sr20DET engine. And he asked me today if we NEEDED to run the IAC/idle valve, because he would need to know to make room for it, but he would rather dich it because of the manifold design. I only use the car for drifting and some running around the streets every 2. weekend or so. Never run in the winter so should never start in the cold. below 10 degress celcious. The AC is no longer on the car either.
So my question is... does the IAC / idle valve control any timing and stuff like that under NORMAL engine operation or engine warmup, or does is it "only" bypass air to the plenum during warmup with input from coolant temp?

-Do any of you run without the IAC valve, and does it produce any running issues under normal operating temps? ( i know it might be obvious that idle during warmup of the car will suffer, but wont i be able to maintain a decent idle on the throttlestop?)

SoSideways
03-03-2008, 04:57 PM
You don't need it.

IACV is there for cold start warm up and to let in additional air to achieve a managable idle when the A/C clutch fan kicks in and drags the entire rotating assembly down.

Like you said, all you have to do is open the TB a little bit to achieve an idle, and use your EMS to achieve an idle below a certain throttle positions.

sultennok
03-03-2008, 05:21 PM
thanks.. :)

So it only controlles air and not any timing or funny stuff like that.?

SoSideways
03-03-2008, 05:38 PM
thanks.. :)

So it only controlles air and not any timing or funny stuff like that.?

No.

The plug that goes into the IACV only really controls the high idle when the A/C is on, and then there's a ground and power. That's it really.

The ECU does not look at what the IACV is doing in idle.

Nikeboy355
03-03-2008, 06:27 PM
Don't worry about it... Especially if you have a track car...

The Idle Air Control valve helps keep your idle stable but at the track you shouldn't care about that at all...

Epstein
03-03-2008, 09:54 PM
No.

The plug that goes into the IACV only really controls the high idle when the A/C is on, and then there's a ground and power. That's it really.

The ECU does not look at what the IACV is doing in idle.

Actually, none of that is true if you're on a stock (or retuned) ECU.

The idle up (high idle) valve is a different valve than the one that regulates the idle speed. That one is a simple solenoid that opens when the AC compressor is active (simple power and ground, like you said). The IACV is actually 2-3 different controls, but usually refers to the PWM ECU controlled one.

The ECU does look at what the IACV is doing in idle. If your car is idling faster than the target idle speed, then it will start pulling out duty cycle on the IACV. Once it reaches it's minimum value (around 4-8%) it will start pulling timing to try and slow the motor down. The timing pull will only affect idle, but it'll idle rougher than normal. Your motor does not like idling at 5* timing! haha.

For the record, I've had no issues running on the throttle stop for idle. That was back when I had a Haltech, though, and could control my idle timing. My idle was also 1200rpms so I could return to idle without stalling (even with speed density).

Greaser_
03-03-2008, 10:42 PM
even my car idles like a dream.

Drag_S13
03-04-2008, 01:42 AM
Anyone have a pinout for the S13 SR20DET IACV? I lost my diagram for my wiring harness :(

Basically I just need to know which pins are for the PWM controlled IACV, not just the on/off A/C kickup...

I know I could get away without it, but I'd like to have it running an auto and all. I don't want to have to worry about the thing stalling every time I click it into 1st...

sultennok
03-04-2008, 04:38 AM
Thanks Chris for giving a reply.

I did began to wonder when i was told that the ECU didnt even look at it.
Because i can see it in my "sensors check menu" on the powerFC.- So the pfc looks at it. Better install it if possible, i guess ill have lots of other problems to fight when i get the new motor ´back in..

Thanks the reply´s all!

SoSideways
03-04-2008, 11:38 AM
Ah, so that's what the light blue wire is for (I think it's light blue, it's been a long time since I've looked at and worked on an SR harness)!

I knew it was like the signal wire from the ECU to the IACV, but didn't know the ECU actually looks at what the IACV is doing. I thought it just read from the MAF and the TPS, then using the values from those 2 sensors to send a signal out to the IACV to achieve an idle via that light blue wire.

Cool cool. Thanks for the info :D

RiverEd
03-04-2008, 11:40 AM
Just for the record, my car run idle like a nightmare without that valve (i'm looking for a new one). Yesterday I borrow my brother's valve and I saw a huge diference on cold start and idle.

SoSideways
03-04-2008, 11:47 AM
That's because most stock SRs aren't set up to idle with the throttle stopped cracking the throttle body open.

You have to do that manually.

I just recently went back to having my IACV controlling idle from using the throttle stop method. The idle is definitely not as rock solid as when I had the throttle plate cracked open a little bit.

Also, the first time I take the car up to drive in any gear when cold, and I put it into neutral to coast to a stop, the idle will sit at like 500rpm until I come to a complete stop, then it'll shoot back up to like 750rpm. It'll do it just that once, then it'll idle fine after that. I'm guessing that's the ECU trying to pull timing?

Drag_S13
03-04-2008, 12:59 PM
Sounds like you just need to adjust up the base idle screw on the IACV.

SoSideways
03-04-2008, 01:06 PM
Sounds like you just need to adjust up the base idle screw on the IACV.

Nah... that's the only time it will dip that low. Any other time it just settles to 750-800rpm right away.

Drag_S13
03-04-2008, 01:10 PM
Is this TPS closing all the way so that it will activate the IACV? I don't know if the stock ECU does anything like that, I don't really have any experience with stock Nissan computers. I still think your car is relying too much on the IACV to idle. It should only be there to stabilize idle and cold start, not flow a majority of the air required to idle the car.

SoSideways
03-04-2008, 01:21 PM
My TPS value at full closed is 0.45v. At WOT it is at 4.10v I think.

IIRC as long as the TPS voltage is below 0.6v the ECU will try to achieve an idle. Epstein will know more about this than I do.

In any case, I had the throttle cracked open for an idle of 800rpm before, but I barely had any cold start high idle from the IACV, thus I backed the throttle stop out so that the throttle plate would close more, letting the IACV do its thing.

If anything, I'll just crack the throttle plate a little, to a TPS value of about 0.5v, that way it'll still idle.

Epstein, got any input on this?

BoostedVR4
03-07-2008, 09:59 PM
This is very interesting. I had nothing but problems with my cold start and idle with my EMS.. I couldn't get control of the the IAC to save myself. With the stock ECU, the car started and idled perfect. But with the EMS, I never got it quite right. This sounds like a neat option, when I get the car running again, going to have to try it...

Epstein
03-08-2008, 07:41 AM
I had nothing but problems getting my Haltech to control idle properly. That was my biggest complaint seeing as it was on a street car.

The IACV adjustment procedure that I use is: go into Consult and turn on idle setting mode. This locks timing at 15* and min IACV duty. Then adjust the IACV set screw to about 100rpms below what my target idle is. On my ECU, I programmed it for 1000rpms, so I set it at 900ish. If you don't have Consult, you can simply unplug the IACV-AAC valve (4-pin job bolted to the 3rd runner on SR). Do this when the car is warm, and it more or less guarantees that the engine has enough air to idle even if the IACV is off in lala land.

About the throttle closed stuff, I always set my TPS to read 0.45V when closed. Even if I tweak the throttle stop, you can reset the TPS voltage by clocking the sensor independantly. The ECU has a flag called "closed throttle" that it sets everytime the car is turned on. It's about 0.05-0.07V higher than what closed throttle is. You can have the TPS set to 0.6V and it'll still set itself correctly. You'll need that flag triggered for decel fuel cut to work properly. The IACV still starts moving at around 0.6-0.7V or so (with 0.45V as min).

The programming in the stock ECU does a ton of work to catch/maintain idle and to transition between that and low speed driving.

SoSideways
03-14-2008, 12:38 PM
^ Thanks for the input, I will go and try this stuff out tonight or tomorrow.

Tyler_240
03-14-2008, 01:24 PM
That's because most stock SRs aren't set up to idle with the throttle stopped cracking the throttle body open.

You have to do that manually.

I just recently went back to having my IACV controlling idle from using the throttle stop method. The idle is definitely not as rock solid as when I had the throttle plate cracked open a little bit.

Also, the first time I take the car up to drive in any gear when cold, and I put it into neutral to coast to a stop, the idle will sit at like 500rpm until I come to a complete stop, then it'll shoot back up to like 750rpm. It'll do it just that once, then it'll idle fine after that. I'm guessing that's the ECU trying to pull timing?

This is the exact way my SR runs right now, is it supposed to do that? Low idle while cold and when coming to a stop. Because as soon as its warm, it runs like a champ and idles fine.

Also, if I have to pump the throttle a bit when cold starting my SR, and everything fuel related is fine (O-Rings and Injectors) and No Boost Leaks, would my IACV be at vault? My TPS is set to .45v closed and I have verified my timing is correct. Thanks for any advice.

SoSideways
03-14-2008, 02:24 PM
Dude if I were you I'd just follow Epstein's way of adjusting the idle.

Haywood_Jablowme
03-16-2008, 11:53 AM
Will this work for rb20 w/ aftermarket intake mani? I would like to eliminate my iacv when i switch manifolds ill be running a re-tuned stock ecu though not a standalone.

jubee
03-23-2008, 05:33 PM
I aksed this in another thread but now that i found this one i will ask here also.

Me and a buddie of mine are gonna also be doing a custom intake mani, i am gonna be using the q45 tb. could i just use the fast idle function thats on the q45 TB. or would this just be a big pain

vanquish2
04-22-2008, 09:33 PM
This is the exact way my SR runs right now, is it supposed to do that? Low idle while cold and when coming to a stop. Because as soon as its warm, it runs like a champ and idles fine.

Also, if I have to pump the throttle a bit when cold starting my SR, and everything fuel related is fine (O-Rings and Injectors) and No Boost Leaks, would my IACV be at vault? My TPS is set to .45v closed and I have verified my timing is correct. Thanks for any advice.

I thought the same thing so I replaced my iacv tonight and I still have the same prob!!!!!!!!!!!!! I've had to pump the throttle since day 1 about 5 years ago.....now I can't get it to idle at all!:caged:

I am at a loss!

_Def_
04-23-2008, 02:39 PM
The programming in the stock ECU does a ton of work to catch/maintain idle and to transition between that and low speed driving.

Yep, it does do a lot to keep the car idling well, which works great on a stock engine, but isn't that great for a cam'd car. It seems the engineers used some hardcoded values rather than calculating duty cycles and gains on the fly, which makes sense given the processing power available at the time. Works great on a stockish engine, but start changing enough variables and it doesn't do well.

Particularly I've noticed an issue when in gear and decellerating at a low RPM and the ECU will try to stablize the RPM which works great on stock cams, but with larger cams it'll typically misfire enough to get into a feedback loop of surging, then back to decel, then surging, back to decel mode, etc.

sultennok
04-30-2008, 11:01 PM
Just to let people know, im now running the manifold WITHOUT the IACV and adjust the idle on the throttlestop. I´ve have absolutely no problems with doing so. Dont know if its because im runnin the Power FC instead of stock ECU.?
The car does have a rought idle for the first couple of minutes but then it idles fine at around 950rpm.

Just to let people know.:)