[This message has been edited by 2.5 (edited 05-10-2018).]

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1987 Fiero coupe; Blue; 2.5L; 5-spd
1987 Firebird Formula; Blue; 5L; 5-spd

Sounds like the problem is probably still there, so:
Check for vacuum leaks and also check the condition of the wiring and connectors. Any broken connector clips that might be intermittent? Frayed wiring?

The ECM calculates how much fuel it thinks the engine should need based on the various sensors. Major inputs include engine RPM, the coolant temperature and the manifold air pressure.
The O2 sensor reports a voltage between 0.0V (lean) and 1.0V (rich). The ECM tries to control the fuel so that it keeps switching above and below 0.45V. If it's above 0.45V, then the ECM starts trimming fuel until it goes below 0.45V. Then it does the opposite, and so on. There are limits to how far it can trim or add fuel.
If the O2 sensor reading stays above 0.45V for too long, and the ECM cannot trim the fuel any further (due to it's programmed limits), then it sets the "rich exhaust" code.

So if you get a rich exhaust code, if means either a bad O2 sensor, maybe a bad connection to it, a leaking fuel injector, or a completely different problem that has confused the ECM so that it grossly miscalculates the fuel mixture. If the calculation is bad enough, it won't be able to trim enough fuel to compensate and so the "rich exhaust" code is set.

A vacuum leak could do that, especially if it's local to the MAP sensor (so the sensor sees more air pressure than what's actually in the manifold). So give close attention to the vacuum hose leading to the MAP - take it off and make sure it doesn't have a pinhole leak. Cover the other end and blow in it.

I don't remember the pinout, but one of the wires leading to the MAP sensor should have 5.0V on it. I'm assuming one of the other pins is ground, so you should find 5.0V across those pins with a multimeter. Sorry I don't remember the pinout details, but the point is that if you're not getting the proper 5.0V supply then it could mess with the sensor readings. There might be better info about this test on the forum somewhere.

I think you can measure voltage across a different pair of pins to get the MAP's current pressure reading. But I don't know what voltage you should expect to see at particular conditions. This might be documented somewhere.
It's much easier to verify good sensor readings with WinALDL, but lacking that, probing with a multimeter can partially substitute. The difference is that when you probe with a meter, you can't be sure that the ECM is seeing the same thing you are seeing (it won't catch wiring problems, etc).

Engine RPM comes from the same signal that eventually gets to the tach (does your tach work?)

The coolant temperature sensor (CTS) is separate from the sender that drives the dashboard gauge, so a working dash gauge doesn't mean anything. You can test the CTS by unplugging it and measuring the resistance across it's 2 pins. This chart shows how many ohms of resistance there should be for a given temperature. Make sure you're measuring the right sensor (not the dashboard sender).

[This message has been edited by armos (edited 06-10-2018).]