How do I Tune Idle when Running Out of IAC Count Range

[JamesLinder]

Now that I'm running the AC due to hot weather, I have noticed that the idle control motor range appears to be insufficient on the PTM 102mm throttle body to reach 950 engine RPM
with AC on and the car in gear. This is most likely because I now have longer duration camshaft (238 by 242 degree) that results in less intake manifold vacuum at idle, and I now
have a situation where it appears I'm exceeding the idle air control motor range. (I'm still using the stock 2000 GM version of the PCM program).

When the engine is warmed up, it idles fine at 950 RPM with the (1) transmission in park, (2) AC compressor off, and (3) idle control motor IAC counts at zero (minimum possible) and
with the throttle blade screw at maximum opening.

The throttle blade screw must be at maximum opening otherwise there is insufficient idle air for cold winter starts; the throttle blade screw must be at its maximum position with IAC
counts at the 310 maximum possible. Anything less than maximum throttle blade screw position simply will not provide the total amount of air needed for a cold winter start.

NOW with the AC compressor on and the transmission is in gear, the idle control motor runs out of IAC counts (310) at about 850 RPM, so it is not possible to reach the desire idle of 950 RPM
with AC on and car in gear.

Is there anything I can do other than switching to electronic throttle control throttle body? I strongly prefer not going to the trouble OR expense of converting my car from throttle cable to ETC.

Thank you very much for your help!

P.S. TPS and IAC motor are both new, both have valid voltage ranges and are working correctly. I just barely had enough range during winter, and with AC I have now exceeded range.

[Ben C]

I would drill the hole larger in the butterfly.

[JamesLinder]

Thanks for your feedback, however I do not understand how this would help in specific situation. I will explain what I mean below and you can let me know.

It appears to me that drilling the hole larger would allow the throttle blade screw to be backed off to allow the blade to "less open" a little more at idle, however
I do not understand how this would give the IAC motor a larger control range? There will still be zero IAC counts at idle and 310 counts before reaching the
desired RPM, since the IAC circuit is still what is controlling the incremental idle air available to increase engine speed.

On these "throttle cable" throttle bodies the IAC motor is what provides all the extra air required when putting car in gear, when turning AC on, when power
steering increases torque required when steering, and cold weather idling, etc. I can see where enlarging the inlet port to the IAC circuit inside the throttle
body could allow that circuit it to flow more air at 310 counts than before, however I sure need to make certain this is the right approach before grinding on
a fairly expensive component like this.

[mowton]

Increasing the size of the hole on the blade will essentially increase the

inlet port to the IAC circuit inside the throttle

as this air will be included in the IAC port air during closed throttle idle. It will allow you to close the bade more thus giving the IAC stepper motor more "dynamic" range to adjust from cold to hot. After you drill the hole, you will do another IAC scan at operating temp and reset the IAC steps to 40-60 with that cam. You want to make sure that the T{PS mv are in the 400-600 mv range as well. If you still don't have enough make the hole a bit larger and repeat the process.

Ed M

[JamesLinder]

Wow, I did not realize this. Thank you very much for these details, all the information, and especially for your advice! That should do the trick now that I understand how it really works!

[mowton]

You are welcome...that and a bunch of other tuning stuff (40+ pages on Adaptive idle) is contained in our Tutorial :-)

Ed M

[Frost]

Best not to target 40-60 steps with anything but a stock TB. Often those 102mm units end up closer in the 90s range. The rub with drilling with is you can still have too much hot and it will end up closed. Of course this would most likely be hot idle/Park, but it's something to watch out for as well. There is window of operation. Big cam/compression sometimes has to make a concession on cold or hot running; defer to proper warm operation and do what you can with it cold.

[mowton]

Best not to target 40-60 steps with anything but a stock TB. Often those 102mm units end up closer in the 90s range. The rub with drilling with is you can still have too much hot and it will end up closed. Of course this would most likely be hot idle/Park, but it's something to watch out for as well. There is window of operation. Big cam/compression sometimes has to make a concession on cold or hot running; defer to proper warm operation and do what you can with it cold.

Steve, even if you readjust the IAC Steps vs effective area table for the increased effective area of the larger TB and increased hole? That would realign the Steps to effective area and give you back the full dynamic range you had with the stock TB.

Ed M

[Matt_lq4]

If you had an iac count of 125 for example, what would be the problem with that high of a count even if your afr is stoich?

[mowton]

If you had an iac count of 125 for example, what would be the problem with that high of a count even if your afr is stoich?

Because the IAC idle steps are tuned at full operating temp. You need to have sufficient range to allow cold engine starts where the IAC is going to have to open quite a bit. Setting to 125 instead of 60-80 (or even 40) will eat up some of that required head room, With a larger cam you need more air at cold start so that is why I push it down to 40-60. You adjust the blade/set screw to reach these settings...if you cant attain these without TPS (V) exceeding say .8 volts then the procedure is to enlarge the hole so the blade can be closed and still yield the same effective area/airflow. That also why you need to re calibrate the IAC steps vs effective area table to match the larger TB if so installed. The stock setting assumes about a 12 mm squared value...larger throttle bodies will yield larger effective areas so you move the step count (essentially shift to the right until Dynamic airflow and Idle desired airflow match....at least that's how I do it.

Ed M

[Frost]

No I'm not saying this is wrong...

I used to stick to the count range window but for non-OE, you can slide things around but you still don't really KNOW how much airflow you are getting by step since the passageways vary. Often you have to do "what works" or find the happy medium. It's not uncommon though, I was more pointing out (and it's not clear) to "realize" that you have a window to work with. There will be times that both ends don't cover all of real life. When you run the blade open (or drill) and add airflow, that airflow still has to come out once warm. On a stick car for example, it's very easy to adjust until you get good cold operation only to find out that at hot idle the IAC is almost fully shut trying to dial it back. Then, you have to close the blade a bit to get counts out of single digits when hot but you find that when it's cold, there still isn't just enough there to start with no pedal... It's easier for an auto car because being in-gear gives you load so you can run the blade further open than a stick car and not run out of room hot from having the blade open further.

All of working with the IAC in general is an exercise in ranging with aftermarket TBs in many cases. And, since they HAVE to work properly warm, you are usually left with tougher cold start start/running until 310 counts + TB hole+ area around blade = happy needed airflow.

[mowton]

Understand, its a balancing act (with compromise sometimes) with big cams and TB's :-)

Ed M

[SSUte01]

I've found very few aftermarket cable throttle bodies that come close to the standard GM throttle body in terms of 'window size' to cover both ends of the spectrum. Holley and BBK have been the best but both IAC paths come with some compromise. The above process is very generous of motown to articulate for all and sundry for free, hats off. IAC setups are typically the most challenging with a decent cam and aftermarket everything with a slow PCM.

More often than not you end up with having to compromise the cold end by having to pedal a little so that the hot end is controlled by the IAC. Even making the hot end less efficient through fuel and spark to make it require more IAC and make the cold end more efficient to require less IAC, most aftermarket IAC paths that I've played with, which would be dozens, don't cut the mustard. To OP, At the end of the day, as Frost said, your window is only so large, you need to choose where the compromise is with came, aftermarket setups. It is quite frustrating and very difficult to get the end customer to understand when the internet tells them that even the craziest setups idle like stock. It shits me.

[LSxpwrdZ]

Opening the throttle blade hole up will only reduce the amount of IAC counts globally. It's my understanding that P/N warm idle is already around 0 counts so increasing the blade hole size would definitely give more headroom per IAC opening however it'd probably make the car idle high in warm P/N condition.

I suggest modifying the actual orifice of the IAC passage. I've found that the air path in aftermarket throttle bodies leaves a lot to be desired. Opening up these passages will allow more airflow through as the IAC opens up. I have found the average airflow increase on a FAST or NW throttle body is only a 120 IAC window, meaning over 120 counts no more airflow is increased. The PTM throttle bodies are a little better with about a 180 count window. The eBay china TB's are the worst of them all.

Another "trick" is to use ignition timing to help cold idle. I usually add 6-10deg to the ECT spark adder table in the colder idle regions to help idle the car faster with less IAC counts. While this helps it's still usually not enough to help fully remedy the low airflow IAC problem.

[mowton]

Opening the throttle blade hole up will only reduce the amount of IAC counts globally. It's my understanding that P/N warm idle is already around 0 counts so increasing the blade hole size would definitely give more headroom per IAC opening however it'd probably make the car idle high in warm P/N condition.

I suggest modifying the actual orifice of the IAC passage. I've found that the air path in aftermarket throttle bodies leaves a lot to be desired. Opening up these passages will allow more airflow through as the IAC opens up. I have found the average airflow increase on a FAST or NW throttle body is only a 120 IAC window, meaning over 120 counts no more airflow is increased. The PTM throttle bodies are a little better with about a 180 count window. The eBay china TB's are the worst of them all.

Another "trick" is to use ignition timing to help cold idle. I usually add 6-10deg to the ECT spark adder table in the colder idle regions to help idle the car faster with less IAC counts. While this helps it's still usually not enough to help fully remedy the low airflow IAC problem.

James, not sure how opening the hole reduces the IAC count globally. If you needed more air to idle hot and you had to increase the number of steps to open the blade then you would lose dynamic range to handle the cold start....enlarging the hole/closing the blade a bit and getting back those steps would then increase the range...no?

I like the timing idea, never thought about that :-)...I did see than Dan M use timing in idle control recently...

Ed M

[JamesLinder]

Gentlemen,

I wanted to circle back to you guys and let you know how the problem with insufficient IAC range was actually solved. It turned out to
be fairly clean and simple, and 100% successful.

It turns out that the PTM throttle body has a separate port which I’m told is for the PCV valve hose connection, which I never used since
I have a crankcase vacuum pump (sealed system) for better sealing of rings and elimination of crankcase pressure against the back-side
of the pistons at high RPM. (Even if I ran a PCV valve, I would not connect it to the throttle body, but to the intake manifold.)

After examining the way the PCV and IAC ports had been bored into the throttle body, I chose what appeared to be the simplest way to
implement James Short’s recommendation, and increased IAC air range by increasing IAC inlet port area. This was done using a 3/16 inch
drill bit to create a hole connecting the bottom of the IAC port opening (where the IAC motor inserts into the throttle body) to the bottom
of the PCV port (not being used, it was already sealed with a plug where it exists on the passenger side of the throttle body).

When the throttle body is mounted on the motor, the hole that I drilled would be from the driver’s side of the throttle body toward the
passenger’s side of the throttle body, so that the two ports now have a 3/16 inch hole connecting them. The hole was of course drilled
with the throttle body off the car to allow for all drill metal to be cleaned up before remounting.

Upon re-installing the throttle body and before making any adjustments, the car which had idled at 950 RPM before now idled at a steady
2000 RPM due to the increased IAC air flow achieved by adding the PCV port to the IAC system.

Then, the throttle blade screw, which had been screwed in as far as it would go, was now backed off until the idle in-park, AC-off returned
to 950 RPM and IAC counts returned to the range of 60 to 70 counts.

Now with the car in-gear and AC-on it only takes about 150 counts to maintain a 950 RPM idle, whereas before this modification the full
310 counts would only yield an idle of 850 RPM under the same conditions. I now have plenty of IAC range to spare and the car maintains
a very steady idle when switching the AC on and off in both park and in gear, which of course was not true before the modification.

Gary, I think it was the fact that I have the additional torque required by the crankcase vacuum pump that put the IAC range requirement
of my set-up out of reach of the stock PTM 102mm throttle body. If not for that vacuum pump (in addition to power steering and AC torque
load), I believe the stock PTM throttle body IAC port would probably have been sufficient; it is rather large and well designed to begin with.
After running a vacuum pump for several years, I would never run a performance motor without one, especially and LS motor.

Thank you everyone very much for your input and advice! James, once again you nailed it dude!

Sincerely,
Jim Linder

[JamesLinder]

Gentlemen,

I wanted to circle back to you guys and let you know how the problem with insufficient IAC range was actually solved. It turned out to
be fairly clean and simple, and 100% successful.

It turns out that the PTM throttle body has a separate port which I’m told is for the PCV valve hose connection, which I never used since
I have a crankcase vacuum pump (sealed system) for better sealing of rings and elimination of crankcase pressure against the back-side
of the pistons at high RPM. (Even if I ran a PCV valve, I would not connect it to the throttle body, but to the intake manifold.)

After examining the way the PCV and IAC ports had been bored into the throttle body, I chose what appeared to be the simplest way to
implement James Short’s recommendation, and increased IAC air range by increasing IAC inlet port area. This was done using a 3/16 inch
drill bit to create a hole connecting the bottom of the IAC port opening (where the IAC motor inserts into the throttle body) to the bottom
of the PCV port (not being used, it was already sealed with a plug where it exists on the passenger side of the throttle body).

When the throttle body is mounted on the motor, the hole that I drilled would be from the driver’s side of the throttle body toward the
passenger’s side of the throttle body, so that the two ports now have a 3/16 inch hole connecting them. The hole was of course drilled
with the throttle body off the car to allow for all drill metal to be cleaned up before remounting.

Upon re-installing the throttle body and before making any adjustments, the car which had idled at 950 RPM before now idled at a steady
2000 RPM due to the increased IAC air flow achieved by adding the PCV port to the IAC system.

Then, the throttle blade screw, which had been screwed in as far as it would go, was now backed off until the idle in-park, AC-off returned
to 950 RPM and IAC counts returned to the range of 60 to 70 counts.

Now with the car in-gear and AC-on it only takes about 150 counts to maintain a 950 RPM idle, whereas before this modification the full
310 counts would only yield an idle of 850 RPM under the same conditions. I now have plenty of IAC range to spare and the car maintains
a very steady idle when switching the AC on and off in both park and in gear, which of course was not true before the modification.

Gary, I think it was the fact that I have the additional torque required by the crankcase vacuum pump that put the IAC range requirement
of my set-up out of reach of the stock PTM 102mm throttle body. If not for that vacuum pump (in addition to power steering and AC torque
load), I believe the stock PTM throttle body IAC port would probably have been sufficient; it is rather large and well designed to begin with.
After running a vacuum pump for several years, I would never run a performance motor without one, especially and LS motor.

Thank you everyone very much for your input and advice! James, once again you nailed it dude!

Sincerely,
Jim Linder

[JamesLinder]

I was using the "Cat Lightoff" tables to add about 3 to 4 degrees additional timing during cold weather starts last winter, since I do not have catalytic converters on my
car to warm up, and I run open-loop continuously anyway so it would not matter if I did. I replaced the negative numbers in the "Startup Correction" table with positive
ones as high as 12, and used factors between 0.20 and 0.35 in the corresponding "Multipler" table in the cells for 68 degrees and below. Now that I have the additional
IAC air flow range, I will probably have to zero out the Multiplier table so that no timing is being added this next winter regardless of temperature.

[JamesLinder]

I will share the reason I chose not to increase the size of the hole that came in the PTM throttle body blade. My opinion is that this should ONLY be considered if one runs out of IAC air
at MAXIMUM idle load conditions for your configuration (1) with the throttle body blade adjustment screw at maxium opening position, AND (2) there are still plenty of IAC counts "to spare"
when the motor is at MINIMUM idle load conditions (typically when the motor is fully warmed up, the car is in park or neutral, and the AC is turned off).

In my case, when I ran out of IAC counts at MAXIMUM idle load (AC-on and the car in-gear) where I was logging 310 counts but still not reaching the desired idle RPM with throttle blade idle
adjustment maxed out, whereas I had zero IAC counts at MINIMUM idle load conditions (AC-off, motor warmed up, and car in-park). I was very concerned that if I drilled the hole in the blade
any larger so that I could achieve 950 RPM idle under MAX idle load, I would have an idle of around 1050 RPM under MIN idle load, and with no IAC counts remaining to reduce the idle speed.
It might have worked without this happening, however it seemed a lot less risky to just increase the size of the inlet port(s) feeding the IAC bypass motor.

[JamesLinder]

Ed, I'm enjoying the hell out of your amazing tutorial package. I pretty much own all the training materials available, and yours is by far the most comprehensive and detailed I've seen yet.
All the material is truly professional quality and presented with thorough explanations of what is truly occurring when making changes. I have found that knowing how it all fits together and rules
of thumb are a lot more valuable than having a recipe list of changes to make. One of the most valuable features of your tutorial is that it provides an excellent presentation of the evolution of the
GM PCM, including the changes GM has made in the way the various PCM's control LS motors and precisely how this impacts the approach to tuning for each version of their PCMs. I would suggest
that you consider showing what you have to James Short, if he has time to look it over, simply because any feedback he may provide should prove valuable to your product. Have a great holiday
weekend my friend!