Question: IAC - How many steps at idle in acceptable?

[TedC]

At idle, in closed loop, with engine hot, what should be the acceptable range of steps that should be reported by a scanner? The FSM talks about setting up minimal air (which I undersatnd) and that the IAC should be positioned a calibrated number of steps (counts) from the seat during controlled idle operation. Yet it doesn't state any accepable step range under these conditions.

 

[bradfordsvettes]

I believe you are looking for approx 20 with a/c off any lower and you may nave a vaccum leak ,wrong pcv, or throttle misadjusted. be sure to clean the throttle body of deposits before settingthen check your tps setting Steve

 

[JAKE]

Right. In the low 20s is correct. Much higher than that also minimizes the IAC's ability to raise the idle in response to additional load on the engine.

You can adjust the IAC count by adjusting the minumum airflow adjustment on the TB. The more you open the butteflies using the set screw the lower the IAC count will be.

Jake

 

[redvett]

GM says with engine at operating temp 195F and A4 in "D" with ac off 15 -30 counts.

 

[c4c5specialist]

Hi there,
Wait a sec.
Back to symptoms, depending on engin modifications, if a minimum air rate set is done AND your using an ACDelco IAC motor, that reading in COUNTS could change.
Now, that being said, L98 is usually 25-35 counts, depending on if the AC is on or off, low voltage demand or hi voltage demand, automatic or standard.
Please keep the variables in mind, but NO L98 should be less than 20 at idle speed of 700.
Allthebest, c4c5

 

[jonszr1]

I was wondering if anyone knew what the counts should be on a zr1 .i have 3 of them and they all show 0 counts when fully warm. the counts when i first fire them up is 20-25.

 

[c4c5specialist]

HI there,
You have some type of vacuum leak or your scan tool is not interpreting the data correctly. What type of scan tool are you using?
What is your fuel trim data showing????
Allthebest, c4c5

 

[TedC]

This is the exact infomation I was after. I could not find it in the 1989 FSM. Excellent job guys!!!

 

[TedC]

Here are my readings:

Coolant TPS MAF Integrator Block Desired-RPM Actual-RPM IAC-steps Injector

In park, closed loop, 5 readings
215.1 0.54 7 124-127 129 625 610-649 13 20.3 1.9

In drive, closed loop, 5 readings
215.1 0.54 7 128-131 129 575 553-568 17-18 19.9 2.4

If I use the information redvett provided:
"GM says with engine at operating temp 195F and A4 in "D" with ac off 15 -30 counts."
Then the number of IAC steps are in the proper range.

If I use the infomation c4c5specialist provided:
"Please keep the variables in mind, but NO L98 should be less than 20 at idle speed of 700."
If I increased the RPMs to 700, then the number of steps always exceeded 20. That included Park or Drive. So, again I look to be within range.

What else did I learn?
If I wanted to increase or decrease the IAC step count, it was just a matter of turning the minimal idle screw. It doesn't take much adjustment to add or subtract 5 steps. Next, I wanted see what happened to minimal air RPM and TPS settings by adding 5 extra IAC steps to my above readings. With no more than 1/8 counter clockwise turn of minimal idle screw, TPS voltage dropped from .54 to .52 volts, and minimal idle RPM dropped from ~450 to ~425. So there is a direct linkage between the minimal air RPM setting and related impact on IAC counts.

 

[Hib Halverson]

DO NOT use the min. air adjustment to control idle speed.

If Min. air is set properly but the IAC counts are out of range during curb idle, you need to find the cause of that.

And yes, as long as everything else in the system is operating properly, there will be a relationship between min air and IAC. More min air...less IAC counts.

 

[TedC]

Hib, You are correct. I did not use minimal air adjustment to control idle speed. I used minimal air adjustment to add 5 extra steps to the IAC and then went back to through the process of checking minimal air RPM. The net effect was that adding 5 extra IAC steps reduced minimal air speed by ~25 RPM. The 1989 FSM recommends minimal air RPM of 450 +/-25 RPM for the automatic. Do you have a recommnedation on which is better? Higher air speed with lower IAC counts, or lower minimal air RPM with higher IAC counts as long as they are all with in GM specifications?

 

[c4c5specialist]

HI there,
So, the variable could be something as simple as the throttle body gasket leaking, an incorrect PCV valve, or a runner bolt being loose.
Your Block learn is at 129, which is almost perfect. So, a gross vacuum leak is not a problem here.
That being said, your so close, I really woudnt concern myself too much unless there is a stalling problem.
Allthebest, c4c5

 

[TedC]

Thank you for sharing your expertise. Engine runs great at either minimal air at ~450 RPM and 18 IAC steps in drive or minimal air at ~425 RPM with 23 IAC steps in drive. This was a learning process for me, trying to understand scanner data, and making sure minimal idle was properly adjusted. It is so much quicker to adjust TPS voltage using a scanner. Confirmed that actual voltage at the TPS matched what the ECM was seeing.