Ok, in that case, here's what's happening.

At the initial installation of the IAC, the PCM (after being reset) drives the pintle valve assy fully into the bore at a slow rate of speed. This is the touchdown measurement that the PCM uses as a baseline to position the STEP HEIGHT in terms of motor steps. It then STORES the MOTOR STEPS in memory. The PCM then positions the IAC based on a preprogrammed value for the number of motor steps based on a fixed motor step value that it assumes is to be in check (providing the IAC is working in terms of form, fit and function)

When you go to START the engine, at time of KEY <ON> event, the PCM prepositions the IAC to X- number of motor steps to obtain a preset gap of X-amount.

Now, if you've got a high idle condition this can be due to IAC gap is larger than what the PCM assumes it to be. This can be a result of corruption of motor step data values or, some other anomaly. This means, it's letting more air in than it should thus, the reason for the high idle. It also should be mentioned that a VAC leak would also induce high idle but, it would lope pretty bad under VAC leak condition.

When you step on the pedal even slightly, the throttle plates open up and, the IAC valve is supposed to close x-amount* depending on the TPS position. When the T-plates are fully shut, it's supposed to open again. In open loop mode, it is not controlled by the PCM thus, it remains open for the entire duration of open loop warmup mode only. This is not to be confused with open loop modes other than warm-up mode.

*Closed loop mode

What may be occurring is, the IAC valve assy is either sticking in the rear of the bore (doubt it), the PCM is losing control over this component or, the TPS reference voltage at the PCM input is not accurate thus, it remains in the closed position.

What could it be? Well, it could be a bad connection at either the IAC to PCM Input or, TPS to PCM input or, the TPS could be faulting thus, not returning to it's home position in terms of it's output voltage. Remember now, the TPS is the control for the amount of opening at the throttle plate so, if the TPS input is reading high, guess what - the PCM reads that as if the throttle plates are still open thus closing the gap at the IAC thus funking the whole thing up.

What you really need here is the proper tool to measure the motor step value while operational and, for this, you would need an DRB scan tool to tap into the PCM and see what those values are. However, you can measure the TPS voltage and, to a lesser accuracy, you may be able to actually measure the IAC as well. I'd have to take a look at the schematic and wiring diagrams to see which wires are the control wires and which ones are the reference and power wires.

Since this is a voltage driven / controlled system, the voltage that is seen at the PCM input directly relates to the position of the IAC motor position. There are four wires on the rear of that sensor harness and, by applying a positive voltage and, conversely, a negative voltage, the pintle valve assy will move in and out respectively.

Since there's no control sensor used to tell the PCM the actual distance of the pintle valve, it does NOT know where it is. The only thing it references is the touchdown measurement stored in memory during initial installation or, during a PCM reset. From time to time, I've witnessed corruption on this measurement at the memory level after a bad reset.

One other thing is, in order for the stepper motor to move x-amount of distance, it's assumed that full operational voltage is reaching the stepper motor. If the voltage is dropping or, there's a partial short somewhere in that CKT causing a voltage drop, the stepper motor is not going to move one motor step to the full amount and, over time, it gets a little bit further off from HOME position until, it's way off and, since the PCM does not know where it is, it has no way to correct this until the PCM is reset.

I think this may be where the problem is rooted judging on your description however, with something else in mind, I have another question for you.

Does the engine LOPE at idle or, is it solid at high idle?

CM

 

at high idle it runs smooth as glass and when it was idling normal right after i put the IAC in it ran great at normal idle, do you think its possible that i happened to get a bum IAC?

 

Depending on what brand you bought, it is possible. And you did clean the bore in the TB good and make sure the old oring isn't still in there right?

 

cleaned it out untill it was as good as new, checked for cracked wires, the i ac seems to be working, just not at the right time. if i come to a stop sign it will idle around 200? maybe lower, but then if it doesnt die, usually does, but once in a while it will kind of surge and then bring itself back up to around 500ish. i have noticed when its going to die, it will surge like i said then just eventually go so low in rpm that it looses it and dies sometimes it will just hang at around 200 lights in the das go dim when it does this, but as soon as you crack the throttle it lights back up and all is well, the truck runs and drives great down the road, now if only i could get it to idle

 

The IAC motor does not sound like it is not working correctly and, for some unknown reason, it's choking off the air supply.

Now, here's a quick test:

Remove the NEG (-) BAT cable and wait about five minutes to ensure a complete wipe on the memory.

While waiting, remove the IAC motor and inspect the pintle valve assy. It should be wobbly -not rigid. Do this by placing the end of a finger on the very tip and gently, GENTLY rock it from side to side. Do not push on it. This ensures that it has the ability to self align into the bore on the rear of the TB.

NOTE: DO NOT PUSH ON ASSY

Now, re-seat it and torque to spec. Re-attach NEG-BAT lead.

Place a stethoscope on the motor body of the IAC. Have an assistant turn the KEY <ON>. You should hear the stepper motor engage the piston into the bore and then back it out.

Start the engine and note the following:

-press on the accelerator SLIGHTLY and return to home. Note idle condition

-tap the accelerator hard and note idle condition.

After three minutes - it should be in closed loop mode.

-tap the accelerator hard and note idle condition.

<END>

THere's a quick and dirty test. Try that and see what results you can obtain from that.

NEXT TESTS: TPS VOLTAGE VERIFICATION TESTS

HOME POSITION VOLTAGE VERIFICATION TEST

-Backprobe the TPS and feed it to a DMM

-Place DMM in window

-Note starting voltage at idle

-Tap accel and release. Note voltage at idle of TPS

Repeat last step five times to obtain more data.

-Hit throttle hard and let it snap closed.

-Note voltage at TPS at idle.


LAST TEST: TPS VOLTAGE THROUGH RANGE

-With engine <OFF>, slowly run the TPS through it's full range of travel.

-Note output on the DMM.
----NOTE: It should be smooth, steady increase of voltage up to WOT. From WOT, slowly bring it back down to IDLE and note any anomalies or erratic voltage readings during test.


CM

 

the only problem with doing this test is that it will sit and run and idle and act normal all day long , it only acts like this when its in gear

 

WTF? Well now, that sure puts a WHOLE new spin on the failure modes now. Was this mentioned before? Did I miss this important piece of information?

CM

 

sorry guys, im trying to post on here at work, and well being a mechanic i can only be in the office for short periods of time or the boss man gets a little edgy, i thought i mentioned that before but after going through and re reading all my posts i must have forgotten to, but yes it only does it when in gear and right after i let off the gas to come to a stop it will coast like normal, but when i come to a complete stop, it starts doing all that i said earlier

 

That sure does change things. There is now a possibility the torque converter is bad.

 

+1 on the TC going out.

Now, I've got to totally re-think this from a whole new perspective and, in order to do that, I must verbally map this out here.

So, in this case, we've been presented with a new piece of information. We now have a new CONDITION to where, the anomaly occurs and that is, ONLY WHEN IN GEAR.

So, now, in order to get a clearer picture, we need to ask some questions here:

Q: Is engine idle affected when putting it into N (neutral) or not?


Q: Is engine idle affected when sitting IDLE, and IN GEAR?


Q: Is engine idle ONLY affected when, IN GEAR AND UNDER LOAD?

Let me know, thanks.

CM