TBI systems are brutally simple, they aren't that hard to fix.
There are 4 things that directly affect the air fuel ratio. They are the O2 sensor, The CTS, the MAP sensor and fuel pressure.
So far, I haven't heard anyone talk about the MAP sensor, or making sure that the vacuum line going to it is connected right. If the ends of the vacuum line are brittle, cut them off so that they hold onto the MAP sensor - or just replace them with new. If this line is disconnected, the engine will run really rich.
Make sure the vacuum line to the PCV valve is connected, this will cause the engine to idle high and/or cause it to surge. Make sure that the hose to the brake booster is connected also and not cracked. Your engine has a bad vacuum leak, and the above items are the most likely culprits.
CTS - Domestic manufacturers use a temperature of about 200 degrees to determine when the engine goes into closed loop. To make sure of this, you should be using a thermostat of 195 degrees - lower temperature thermostats won't cause the ECM to go into closed loop. If the ECM isn't going into closed loop, the ECM won't read any data from the O2 sensor, and won't make changes to the fuel mixture.
You should check the fuel pressure. GM didn't read fuel pressure via a sensor on the TBI fuel injection systems, but they really should have, because it comes up a lot on TBI systems. The GM spec for tbi systems is 9-13psi. You will need an adapter to read it, like this:
https://www.ebay.com/itm/TBI-thrott...978587?hash=item3d26fad81b:g:US0AAOSwQItUJEQZ
The O2 sensor should be on the drivers side exhaust manifold at the back in the collector, except in certain applications. TBI systems only used one, and no O2 sensors after the cat.
A scanner probably won't help you until you fix the trouble codes and/or vacuum leaks. It doesn't help you to know things like the O2 crosscounts, if the system is running rich all the time.
This is a troubleshooting guide for TBI systems:
http://www.73-87chevytrucks.com/techinfo/TroubleshootingTBISystems.pdf
