Setting base idle with '7427 PCM

When using the '7427 how do you get the IAC to zero out? I've followed the procedure you can find a 100 places online: jumper A&B on the ALDL, turn the key on, and then go unplug the IAC which should lock it in with the pintle completely closed. but I don't think that's happening. SES light is blinking fast, so I think it is in diagnostic mode, but no sound is coming from the IAC and I don't think it's closing.

This method always worked on my 7060 PCM, but I've found threads on here and elsewhere that make it sound like GM changed the diagnostic procedure when the 7427 came out. But those threads did not give me info about what that diagnostic process is. I'm probably missing something obvious.

I did find a recent thread with the 20 lbs paper trick (Dave W). Cut a 1/2" wide strip of 20 lbs normal paper. Set the throttle stop screw so the paper drags when pulling it out from between the throttle blade and the bore. I did that and drove it and it seems to be idling pretty good, but I didn't have the computer out there to log IAC counts. Will do that tomorrow. Maybe they will be low enough that I don't need to further mess with base idle.....

I recently found out the '7427 doesn't drive the IAC pintle in when you in jumper A&B. I plugged the hole in the top of the throttle body with some modeling clay and set my base idle. The best way to do it is once fully warmed up, in closed loop and in gear your IAC count should be 5-30 counts (FSM), but most guys here will say 5-15. I shoot for 10 myself.

I remove the air filter, unplug and remove the IAC valve. Pull the spring away from the pentil and screw it out a little at a time, (1/8)inch. Reinstall the valve by hand (not tight). When the pentil is seated the valve will not be threaded completely into the throttle body and you will see the pentil stick out at the chamber (takes a couple times). I use a old one. Don’t screw it out to far. Once it’s seated start it up. If the plates are closed to far it will barely run, open them to 10 counts. If the idle is high, close them until it barely runs then open until you get counts around 10. Do it on a warm engine.

Before I had an IAC driver to close, I used to do it the hard way playing with the throttle blade and tps, it works but you have to unplug/reset the 'puter between changes so it knows tps voltage again, otherwise you are chasing your tail.

Thanks all for the tips and tricks. I know now that the main goal is to ensure the IAC port is blocked when doing the adjustment. I will either use the modeling clay or the spare IAC screwed out manually to do so, haven't decided which. I have both available.

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Originally Posted by In-Tech

Before I had an IAC driver to close, I used to do it the hard way playing with the throttle blade and tps, it works but you have to unplug/reset the 'puter between changes so it knows tps voltage again, otherwise you are chasing your tail.

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i don't think unplugging the PCM is required, a simple key off, wait 10 seconds and restart will "zero" the throttle position. i do this a few times just to make sure i'm in idle mode. I still think setting by IAC count is the best method and requires no messing with the IAC.

EDIT: I've done the same procedure on older ECMs as well with no i'll effect, as long as the idle flag is set good to go....

Dont bother with blocking IAC port.

Otherwise pretty much same stuff, I aim for 10-15 steps hot engine no load (A/C, headlights etc). Also try to keep TPS around .55-.65 volts to keep auto trans happy. Just turning off the key for 15-20 is enough for PCM to shut down and look up new TPS voltage key on.