So I'm still having problems with the GTA. My question is what tells the IAC to move? I've had a buddy look at it with his snap on scanner and all looks good, then for no reason the IAC moves to 92 then 104 and idles around 1400. The IAC is new along with the TPS, temp sensor, ECM and tune up. I have checked for vacuum leaks, there are none. If I reset the IAC and then the ECM all is fine the first time you start it, the second time you start it the idle goes up. This car is my daily driver and right now I've reset the IAC so it would idle at 650, then I plugged the old IAC in and sat it on the intake. This at least gets me to and from work. Could my PROM be bad?

 

really nobody know's

 

I wouldnt know, my car idles good when cold, but after she warms up the idle is high.

 

You probably lost the pintle from the old IAC by leaving it out of the TB and connected to the ECM. So be sure not to use it in the future if it went missing. The ECM won't know that the IAC is unplugged. So no need to do that.

The ECM controls the IAC for two basic reasons. Idle speed control and throttle follower (TF). TF is when the throttle is being opened the ECM also opens the IAC (higher steps). Then as the throttle is being closed the ECM does the same with the IAC. This is for both anti-stall and emissions.

However, your issue is with idle speed. It can be TF causing this if the ECM thinks that the vehicle is moving or that the throttle is being opened. So check for that with the scan tool.

The other item to check with the scan tool is the coolant temperature (CTS) value. If the ECM thinks that the engine is colder then it actually is, it will command a higher idle speed. And open the IAC to do this.

If all that looks OK, then there is a good possibility that the ECM is bad.

RBob.

 

Some of this is way over my head. The temp is reading 84c which is 183f. I installed a 195f t-stat in it about 2 months ago, I guess it is opening early. I still don't understand the throttle follower. I'm not sure where to look on the datamaster program to see if there's a problem.

 

That small difference in temp (194 vs. 195) won't make any difference. Would take a much lower temp reading for the ECM to command a higher idle speed.

TPS readings would be the first place to look. Make sure it's getting down to the proper voltage in the throttle-closed postion (check what voltage in a service manual but it should be around .5-.7 V on most TPIs). Checking for vacuum leaks would also be a good idea.