I have a 91 k1500 with a 350. The truck drives great but will not idle good, especially under a load. I have a scanner and watch the iac just keep dropping. I have replaced it and still have the same problem. So after some tinkering I found if I unplug the coolant temp sensor it idles fine. So I replaced it and still the same. So is my problem there or is it the ecm. If it is the ecm do I need a new prom or can I use the old one.