I looked in other treads and didn't see this exact problem. Have a 2002 blazer LS 4.3 automatic 4x4. 74K miles. RPMs increase by itself when in gear without touching the accelerator. Only happens when you start rolling. If you press on the accelerator to increase speed, it increases speed normal then takes over. If you brake and slow to a stop it idles down and returns back to a normal idle (about 600-700 rpm). when in park idle is normal. turning on A/C it increases rpm briefly then returns to normal idle like it's supposed to. If I start it and unplug the IAC all runs normal.....no more self increase in acceleration.
Items changed/cleaned:
new IAC, new MAP, find no vacuum leaks anywhere that I can find with propane torch, new fuel filter, TPS ohms good, cleaned throttle body, cleaned MAF....
Only code I get is 507 (idle higher than normal).
I'm wondering: did I get a bad 'new" IAC, is there a reset for the new IAC, perhaps there is some kind of internal transmission modulator valve vacuum leak..... I'm at my whits end. Hope you guys can help.

 

Come on guys.....you got nothing for me?????

 

When you say that the RPM's increase when you put it into gear... How high do they go? Will it do it if you go into gear and then into neutral? Does your motor move when you put the engine in gear - possibly pulling on the throttle cable? Do you have access to a scan tool capable of monitoring IAC sensor counts or sensor position?

There are no vacuum controls to the transmission in your truck.

 

swartlkk, RPMs increase approx 300-400 when I put it in gear. It increases when put in gear then settles back down to 600-700. Doesn't increase when I go back to N.

Motors not moving. Throttle cable is not moving at all when I put it in gear. Again, If I unplug the IAC after I start it, it doesn't do this. I'm suspecting the ECM is out whacked.

A friend does have an IAC reader to watch the steps. I'll let you know what i find there. Thanks for the reply.

 

SOLVED!!! Just so everyone knows, the entire problem was the idle. Hooked up the Tec II idle was really almost 800 rpms. supposed to be 625. Turned the idle screw at the throttle cable down to proper idle and all works correct.