LT1 IAC (Idle Air Circuit)

[Bartman]

I've been doing a lot of research on the IAC and here's the way I think it works:

 

First Here's a diagram of an IAC circuit on shbox.com

http://shbox.com/1/iac2.jpg

 

If this is correct, this may confirm how I think the IAC works. Somewhere in the throttle body a small amount of airflow is diverted to the IAC. The amount of air that makes it through the IAC is determine by how much the pintel is open. The air that flows through the hole between the throttle blades is not restricted by the throttle body IAC circuit(the same amount of air will flow through regardless of the pintel setting in the IAC controller); but it will use the special IAC passages in the intake manifold.

 

Now, I still haven't found where the air actually enters and leaves the IAC circuit in the throttle body and I've had it apart a couple of times. Maybe someone can tell me where these passages are?

 

I have a pretty healthy cam in my engine and the idle is a little on the low side. If I crack the throttle blades a little, the idle goes up but I don't think this is the right way to raise the idle. If the engine needs more air at idle I think it should be via the IAC passages in the intake manifold and not the main plenum (better distribution to all cylinders). If the IAC counts are high at idle, I think the best way to compensate is to increase the size of the hole between the throttle blades, which delivers the air using the IAC intake manifold passages.

 

Please let me know if I'm correct in my theory or not.

[wheelman]

Bart,

I might be wrong in this so take it with a grain of salt.

 

Looking at how TunerCat is used to set the idle I'd say the IAC is used by the PCM to maintain the requested RPM. The IAC is also used as a throttle follower to keep the RPMs up a bit when the car is moving for emissions purposes.

 

Taking these things into account it shouldn't be necessary to increase the size of those passages to increase your idle. You should be able to use a tuner program to set the desired idle and throttle follower behavior and then let the PCM control the IAC to meet the requested settings.

 

Wheelman

[Bartman]

Bart' date='

I might be wrong in this so take it with a grain of salt.

 

Looking at how TunerCat is used to set the idle I'd say the IAC is used by the PCM to maintain the requested RPM. The IAC is also used as a throttle follower to keep the RPMs up a bit when the car is moving for emissions purposes.

 

Taking these things into account it shouldn't be necessary to increase the size of those passages to increase your idle. You should be able to use a tuner program to set the desired idle and throttle follower behavior and then let the PCM control the IAC to meet the requested settings.

 

Wheelman[/quote']

I found the "IAC Target RPM Vs. Coolant Temp, (In Park/Neut.) Table in TunerCat, and mine is set to 900 RPM once the collant is 44 Deg C or above. Are you aware of any other settings in TunerCat that affect the idle?

 

I agree with most of what you're saying, but I don't think the IAC controls all the air used at idle. There is a fixed amount of air that goes through the bleed hole between the throttle blades that is not restricted whatsoever. The IAC only controls the amount of air needed above and beyond what can be obtained via this passage. Here's a link that talks about the "bleed hole". http://members.cox.net/chipsbyal/page/idle/#stocktbi

 

If your engine is upgraded and requires more air at idle the IAC can only compensate so much to allow additional air into the system. You can see how much air the IAC is trying to supply to the engine by looking at the 'IAC Pos' values in DataMaster. If the values are too high, then increasing the size of the bleed hole will bring them down and allow the IAC circuit to operate within the proper range. I think an IAC count of around 35 - 40 at idle would be optimal.

[mark]

If the IAC is open (mine failed open) the car will idel close to 2000 rpms. The opening for the IAC is on the throttle body of the lt 1. On the underside at the front of the throttle body.

 

THere is an adjustment screw that can be used to opent he throttle blades a bit to compensate for idle. The pcm has a range of movement that is normal for the iac.

 

Even with a large cam and ported heads (425+ rwhp) my idle circut works fine. Have you tried to reset the IAC? Are you sure the IAC motor is working? I forget the procedure for reseting the IAC but it involved jumping some wires at the ald plug. Hope that helps.

 

If you can't find the reset procedure for the IAC, let me know and I'll dig thru my old papers.

[Bartman]

If the IAC is open (mine failed open) the car will idel close to 2000 rpms. The opening for the IAC is on the throttle body of the lt 1. On the underside at the front of the throttle body.

 

THere is an adjustment screw that can be used to opent he throttle blades a bit to compensate for idle. The pcm has a range of movement that is normal for the iac.

 

Even with a large cam and ported heads (425+ rwhp) my idle circut works fine. Have you tried to reset the IAC? Are you sure the IAC motor is working? I forget the procedure for reseting the IAC but it involved jumping some wires at the ald plug. Hope that helps.

 

If you can't find the reset procedure for the IAC' date=' let me know and I'll dig thru my old papers.[/quote']Do you have your throttle blades open a bit to get your car to idle correctly? I would rather have the throttle blades closed and increase the bleed hole to bet enough air for idle. My old throttle body was worn and air was able to get around the blades even when they were completely closed. This caused the idle to be a little uneven, and often too high. As soon as I switched throttle bodies and kept the blades closed at idle, the idle went down and stayed steady.

 

Now my engine is upgraded and the idle is a little low. If I wasn't running air conditioning I wouldn't need to make any adjustment, but the load when the A/C is on bogs down the engine too much. It seems like the PCM should be able to adjust to the extra load and keep the same RPMs, but it's not so this is the problem I'm trying to solve. I was able to raise the idle to an acceptable level, but this is by cracking open the throttle blades. Now I want to be able to keep the throttle blades closed at idle and still have the idle raised.

[mark]

I do not have the throttle cracked open at all. Specs for the IAC are with in normal limits for the stock pcm. If your idle drops when the ac is on, you have a problem with the iac motor. Pull the iac off the throttle body and have someone cycle the ignition key. You should see the plunger move. If it doesn't move, replace it.

 

Good luck.

[Bartman]

I do not have the throttle cracked open at all. Specs for the IAC are with in normal limits for the stock pcm. If your idle drops when the ac is on' date=' you have a problem with the iac motor. Pull the iac off the throttle body and have someone cycle the ignition key. You should see the plunger move. If it doesn't move, replace it.

 

Good luck.[/quote']Thanks, I'll do the test you suggested this weekend.

[Bartman]

Well I removed the IAC valve from the throttle body and it would not move one iota while cycling the ignition.

 

I'll have to buy another valve and see if that solves my problem. Rock Auto has AC Delco units for $59.79 and an aftermarket unit for $52.79...not sure if the AC Delco unit is any better, but it's not too much price difference.

[love-my-V8-280Z]

Bart,

I'm sure you know this, but as I was thinking about how to use the PCM to manage my A/C I found that it has the ability to bump up the RPM when the A/C is on. It would seem that you could figure out how to let the PCM know you A/C is on and to turn the RPM some.

[Bartman]

Bart' date='

I'm sure you know this, but as I was thinking about how to use the PCM to manage my A/C I found that it has the ability to bump up the RPM when the A/C is on. It would seem that you could figure out how to let the PCM know you A/C is on and to turn the RPM some.[/quote']Yes I'm aware of this ability, but I haven't been able to figure out how to make use of this with my Vintage Air system.

 

In order for the PCM to turn up the idle the AC pressure and evap temperature need to be within a certain range. Since the Vintage Air system is keeping track of this internally, I purchased a couple of potentiometers and wired them in to the PCM to fake it into thinking the values are within range. I haven't gotten it to work yet, but I haven't completely given up either. I'll continue to play with this and let you know if I get it to work.

[Bartman]

Received the new IAC valve yesterday from Rock Auto. Got it installed, cranked it up and everything seemed fine. Turned on the AC and a momentary drop in idle before the PCM is able to compensate with the now working Idle Air Control Valve.

 

Thanks Mark!

 

I've got to make a couple of test runs, but it looks like everything is working properly...now I can focus on the body and paint.

[mark]

Glad it worked out. Now drive it and enjoy!!!

[love-my-V8-280Z]

Bart,

Let me know if you figure out the pcm. Did you look to see if you can change these AC parameters in the tunercat program?

You might be able to have them set so low that the PCM always thinks they are at the correct level.

[Bartman]

Bart' date='

Let me know if you figure out the pcm. Did you look to see if you can change these AC parameters in the tunercat program?

You might be able to have them set so low that the PCM always thinks they are at the correct level.[/quote']Now that the PCM can keep the idle RPM fairly constant - even with the extra load of having the AC on - I'm not too worried about having the PCM think that the AC is on.

 

I may investigate this again in the future or if someone else figures it out I would definitely be interested.

[strotter]

Bart, what ECU are you using? Most of the GM units have a "A/C Request Signal" input line which goes high when you close the a/c switch. At that point, there'll be something like "RPM Added, A/C on" in your mask (which may or may not be available in TunerCat, depending on your ECU). Factory numbers are typically small, <50 rpm. At the same time, the ECU will trigger your secondary fan ON.

[Bartman]

Bart, what ECU are you using? Most of the GM units have a "A/C Request Signal" input line which goes high when you close the a/c switch. At that point, there'll be something like "RPM Added, A/C on" in your mask (which may or may not be available in TunerCat, depending on your ECU). Factory numbers are typically small, <50 rpm. At the same time, the ECU will trigger your secondary fan ON.

I'm using the PCM that came with the 94 LT1. I'm aware of the A/C request signal, and I even attempted to wire it in so that it gets signal when I turn on my A/C. In addition I added a couple of potentiometers and adjusted them to ensure that the PCM would think the evaporator temperature and A/C pressure were within range. One problem is that the PCM expects the pressure to rise once the A/C is on, so I would have to turn the potentiometer every time I turned on the A/C. I would also need to hook up the A/C clutch relay control and A/C clutch status in such a way that the PCM doesn't throw any codes.

[strotter]

Bart, the only thing I saw for your ECM (the 16181333) was "IAC Offset for A/C Anticipate Vs. A/C Pressure". Seemed like an odd name, so I checked into it a little. Seems the idea is that the ECM maintains the "base" idle with or without the A/C on - it uses the IAC to do this, of course. However, there's a finite period of time required for the motor itself to respond to IAC changes, so even under perfect conditions there'll be a momentary stumble each time the A/C cycles on and off (and I'm supposing we're only talking idle here). Thus, the "anticipate", which opens the IAC *just before* the A/C clutch closes, preventing the stumble. Your problem is that the ECM isn't able to control the clutch, so it can't predict the load, thus a stumble each time it closes. Your IAC will catch it after a moment, though, 'cause it senses the drop in RPM. Hmm.

 

The only solution I can think of is to just shut off all the A/C diagnostics,

A/C Low Charge Diagnostic, Error 61

A/C Press Sensor Diagnostic, Error 66

A/C Press Sensor Diagnostic, Error 67

A/C Relay Diagnostic, Error 68

A/C Clutch Diagnostic, Error 69

A/C Clutch Relay Diagnostic, Error 70

A/C Evap. Temp. Diagnostic, Error 71

 

and possibly modifying the values for

Fan 1 On High A/C Pressure Threshold

Fan 1 Off High A/C Pressure Threshold

Fan 1 Disable MPH, High A/C Pressure

Fan 1 Enable MPH, High A/C Pressure

Fan 2 On High A/C Pressure Threshold

Fan 2 Off High A/C Pressure Threshold

Fan 2 Disable MPH, High A/C Pressure

Fan 2 Enable MPH, High A/C Pressure

 

and live with a little stumble. Can't hurt to try. If worse comes to worse, you can just revert the mask.

[Bartman]

Bart' date=' the only thing I saw for your ECM (the 16181333) was "IAC Offset for A/C Anticipate Vs. A/C Pressure". Seemed like an odd name, so I checked into it a little. Seems the idea is that the ECM maintains the "base" idle with or without the A/C on - it uses the IAC to do this, of course. However, there's a finite period of time required for the motor itself to respond to IAC changes, so even under perfect conditions there'll be a momentary stumble each time the A/C cycles on and off (and I'm supposing we're only talking idle here). Thus, the "anticipate", which opens the IAC *just before* the A/C clutch closes, preventing the stumble. Your problem is that the ECM isn't able to control the clutch, so it can't predict the load, thus a stumble each time it closes. Your IAC will catch it after a moment, though, 'cause it senses the drop in RPM. Hmm.

 

The only solution I can think of is to just shut off all the A/C diagnostics,

A/C Low Charge Diagnostic, Error 61

A/C Press Sensor Diagnostic, Error 66

A/C Press Sensor Diagnostic, Error 67

A/C Relay Diagnostic, Error 68

A/C Clutch Diagnostic, Error 69

A/C Clutch Relay Diagnostic, Error 70

A/C Evap. Temp. Diagnostic, Error 71

 

and possibly modifying the values for

Fan 1 On High A/C Pressure Threshold

Fan 1 Off High A/C Pressure Threshold

Fan 1 Disable MPH, High A/C Pressure

Fan 1 Enable MPH, High A/C Pressure

Fan 2 On High A/C Pressure Threshold

Fan 2 Off High A/C Pressure Threshold

Fan 2 Disable MPH, High A/C Pressure

Fan 2 Enable MPH, High A/C Pressure

 

and live with a little stumble. Can't hurt to try. If worse comes to worse, you can just revert the mask.[/quote']Thanks for all the info. If I shut off all the A/C diagnostics and connect my A/C system to the PCM via the A/C request signal, what will the PCM be able to do for me? It sounds like I would still have the momentary drop in idle.

 

I initially thought the PCM raised the idle at all times when the A/C is on, but it sounds like all it really does is temporarily raises the idle just before the clutch engages. In order for me to take advantage of this feature I would need to not only connect up to the A/C request signal, but use the A/C clutch control output to allow the anticipate feature to open the IAC before the clutch is actually engaged.

 

I think the only other feature is that the cooling fan is on whenever the A/C is on.

 

I can live with the momentary idle drop when I turn on the A/C and even not having the cooling fan turn on until the water temperature gets high enough. But I still have the potentiometers wired in for evaporator temperature and A/C pressure; so if I can get the PCM to do these two things for me I will reconnect to the A/C request signal.

[strotter]

Yeah, that sounds right to me, too. Without control of the Vintage Air clutch, your ECM can't predict the increase in load; but the ECM can't know when to trigger the clutch w/o input from the a/c pressure switch. Basically, you need to give the ECM total control over the a/c system - which means you'll have to "bastardize" the Vintage system, run the pressure switch to the ECM, run the controller to the ECM, whatever. If you were using a similar really, really similar to the stocker, that would be for sure a great solution, but the Vintage system may behave differently than the stocker, or at least differently enough that the ECM won't be able to operate it properly. Your call. I'd *suspect* it'll work OK, but you may have to get into TunerCat and change some constants somewhere - and I don't know if those locations have been determined for the $EE mask. You may have to find a hack and get into it with a binary editor. Time consuming, maybe. Personally, I like burning up huge amounts of time for teeny little benefits; it's a side-effect of working on a Hybrid Z project.

 

BTW, I was just thinking by turning off all the tests, you'd be able to avoid error codes.

 

P.S: about the ECM increasing RPM for A/C operation: most of them do, but not necessarily by a lot - the mask I'm working with now adds a whopping 25 RPM with the A/C on.