1990Z71Swede
I'm Awesome
I don't know on the top of my head how to test them, But right now I don't think that matters much, as the issue( might not even be an actual issue) is that the ECM is not even trying. In one of your scan tool pics I can see an actual RPM of 1800, and a demanded RPM of 1000.
Now even if you had a massive Vaccuum leak or a stuck IAC, the ECM would normaly still try to Lower the RPM, it would obviously not succeed but you would see it trying by it lowering the IAC value into probably single digits. Yours is at 155 all the time. I think the reason for that is that the ECM has inputs that is telling it that the engine is in a state where open loop IAC control is what is demanded.
If code 15 DOES come back after you've erased it I think that has to be it.
But the way you describe how the CTS acts on the scantool it seems to be reading correctly, so thats weird.
Most ECM sensor/output input diagnostics is not that advanced.
The ECM would only trip a code if it senses a short to ground, a short to positive(depending on the type of output) or an open circuit. On some inputs like the 02 sensor it will also accept a faulty condition for a while before it sets the code. Now this is how most Engine Management systems work, The TBI ECM might be different, but I would be surprised if it was and for sure it would not be MORE sofisticated.
Edit: A quick and dirty test of your IAC would be to reset it and unplug it. I believe that should render in a fully closed IAC and will put your idle way below 1800 Rpm. But again the high idle is probaly just a symptom of something else and not directly related to the IAC.
Also if you look at your scan tool when its operating correctly at 600-650Rpm (if you can trick it into doing that again) Most likely the IAC number will be way different, but more importantly, hopefully at least one of the other sensor inputs are going to differ too(in a way that is not just related to the lower RPM). If so, you may have pinponted your problem.
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