Few questions:
1) When adjusting the IAC per instructions on this site, I jumper A and B on the ALDL to let the IAC pintle extend fully, then turn the ignition off, remove the jumper, unplug the IAC and start the car to adjust. My problem is when I do that it idles at like 3,000 RPM. That leads me to believe that the pintle is retracting instead of extending when I jumper the A and B and turn the ignition on. Any thoughts on this?

2) I know in the Bin there is a table for RPM vs. coolant temp. to dictate what the idle should be. My question is what does the ECM use to calculate what the idle is currently at. Thanks -John

 

Your order of operations if off:

- jumper A&B
- ignition on for 20-30 seconds
- UNPLUG IAC
- ignition off
- jumper out
- start car
- set min air

You have to unplug the IAC while the key is on or it does indeed retract.

Q 2 does not compute. Your ECM knows what the idle speed is because the ECM always knows the engine speed.

 

Well question 1 has been answered. Thanks
For question 2 obviously the engine doesn't "just know". In the previously mentioned, say that at coolant 78* idle should be at 750. Does the ecm just know that with the IAC at this adjustment idle will be at 750 or does it have some sort of feedback such as MAF voltage that calculates what the idle is at? Does that clear it up at all. I am doing my best to explain my question over the internet.

Afterthought: say the car has a vacuum leak, does the car register this high idle and try to close the IAC as much as possible to compensate or not? Thanks -John

 

The ECM knows the engine speed because it takes a reference pulse off the dizzy every time it fires the coil. So whether you're at idle or 6500 rpms, the ECM knows exactly how fast the engine is going.

As for how to control the idle speed, simple as if the rpms are too high it closes the IAC, and if too low then it opens the IAC.

In a MAF car a vacuum leak big enough to effect engine speed significantly is generally big enough that you'll set a MAF low error. This is because the ECM has a range values that it knows it 'should' see under certain conditions. Once you have a heavily modified car, say big heads and cam for example, now you might be moving a lot less air at idle so you change the values in the chip so the ECM doesn't flip out thinking you have a vacuum leak.

In a SD car, a vacuum leak accomplishes the same function as the IAC. I once completely butchered a runner gasket on mine putting it back together in a hurry in the dark. Ended up with a giant vacuum leak. Ran fine, but the leak was so bad the car wouldn't even idle down under 1000rpms, but i had to drive it that way. Didn't even set a code. A couple of days later when i had the time i pulled the plenum and fixed it.

in a nutshell.

 

That in a nutshell is what i was looking for. Thanks for taking the time to help. I've been more about looking at the why lately not just the how. Thanks -John