Diagnose Speed Density ECM Controlled Engines - How does it work and what should I think about?

I have had a number of questions on the channel regarding how to troubleshoot or diagnose problems with OBD1 General Motors engines. Often these are the "speed-density" control logic that is used on engines without a mass air flow sensor. This is used on most of the TBI throttle body injected engines as well as some of the port injected engines. The Tuned Port V8 Camaros used this method after about 1989. I will try to explain how these systems work and what to look for. Some of my explanations and symptoms are drawn from GM service manuals. It is important to consider whether an engine is actually running rich or lean vs whether the computer thinks it is running rich or lean. In some cases the symptoms will not match the actual conditions.

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