Initial Setup of TBI, TPI and MPFI systems. Min Air Adjust, TPS set and Fuel Pressure

[1project2many]

Here's a homegrown way to set minimum air that I learned with carbs and still works today. Get a strip of paper about 1/4" to 3/8" wide and maybe 5" or more long. Open the throttle and insert the paper into the throttle bore past the throttle plate and shaft. The paper should be perpindicular to the throttle shaft. Release the throttle and gently try to pull the paper. If the paper has a slight drag then the throttle angle is ok. If it's too tight you risk stalling and a sticking closed throttle when the engine's cold. If it's too loose you risk having a high idle the ecm is unable to control. Ensure IAC counts are 30-35 and tps volts are below .75 and you're usually good to go.

If you want to set minimum air while the engine is running there are a few ways to do it. You can remove TPS or use a substitute TPS to fool the ecm into thinking the throttle is closed, then increase engine speed slowly until the ecm / pcm tries to command the IAC all the way closed. Disconnect IAC, close the throttle, and check your RPM. Or you can get an IAC test tool and use it to drive the IAC closed. Or you can get a special rubber plug designed to plug the IAC passage (known as a dog turd to GM dealership techs).

[JeepsAndGuns]

Or you can get an IAC test tool and use it to drive the IAC closed.

I have often wondered if there was a way to "bench" test a IAC. Any more info on this tool? Is it electrical or what?

[EagleMark]

I have often wondered if there was a way to "bench" test a IAC. Any more info on this tool? Is it electrical or what?

I've done that on my test bench, the plunger extends all the way and pops out!

[JeepsAndGuns]

So basicly 12 volts has to be "pulsed" to either extend or retract it? Like grounding one wire, then quickly touching and releaseing the other to a hot wire?
I have a couple spare ones from a old dodge tb, I may play with them, if I burn it up there is no loss...lol

[EagleMark]

My test bench is not running, I'm in the middle of building a harness or I would check for you. Pretty sure 12 volts will burn it up. It's a 5 volt signal... IIRC

[JeepsAndGuns]

Ahh, good to know.

[88ragtop]

This article explains pretty well how the various types work: http://www.megamanual.com/ms2/IAC.htm. Just read around the megasquirt configuration stuff that you dont care about :)

- Frank

[1project2many]

A stepper motor like the IAC need to have power and ground applied to different terminals to make the armature turn a complete revolution. Look at the gm wiring diagram for your system and you'll see two connections for coil A and coil B. Each of these needs to be connected to 12V in turn on order to make the IAC move. When the ECM shows IAC counts, that's how many times the ecm has applied power to one or the other coils to make the IAC move. I have an old tool made by Thexton but I couldn't find it in their current tool lineup. Here's a thread that gives a homegrown way to check the IAC: http://www.chevytalk.org/fusionbb/sh...96c4b37436442/ There's also a mention of an OTC3320 IAC tester but I really think those old tools are only going to show up in Ebay auctions or swap meets now.