Simply put a rich system code means too damn much fuel, lol. If the pressure regulator is pumping 90psi thent he injectors can't hold it back and it seep/rushes into the cylinder. It can't all be burned so you get black smoke and a rich code. Kick back the extra fuel and your code will go away. Seen it all to many times. Transman
If you only have 1 02 sensor the code defination you related - rich left side? is not correct.
suspect (will check code#) problem 02 sensor circuit.
PS - Found this list
11 1986-88 models, Engine not cranked since battery was disconnected.
11 On 1989 models, no distributor reference during engine cranking.
12 Memory Standby power lost.
13 MAP sensor pneumatic circuit.*
14 MAP sensor electrical circuit.*
15 Vehicle speed/distance sensor.
16 Loss of battery voltage sensor.*
17 Engine running too cool.
21 Oxygen sensor circuit.
22 Coolant temperature sensor circuit.*
23 Throttle body temperature sensor circuit.
24 Throttle position sensor.*
25 AIS motor driven circuit.
26 Peak injector current not reached.
27 On 1986-87 models, internal problem in logic module fuel circuit.
27 On 1988-89 models, fuel injector control problem.
31 Purge solenoid circuit.
32 On 1988-89 Calif. models, EGR diagnosis.
33 A/C cutout relay circuit.
34 On 1988-89 models, speed control solenoid driver circuit.
35 Fan control relay circuit.
37 On 1986-87 models, shift indicator lamp circuit (manual transmissions); Part throttle unlock (automatic transmissions).
37 On 1988-89 models, part throttle unlock solenoid circuit.
41 Charging system excess or no field circuit.
42 ASD relay circuit.
43 Spark interface circuit.
44 On 1986-87 models, battery temperature out of range.
44 On 1988-89 models, loss of FJ2 to logic board.
46 Battery voltage too high.*
47 Battery voltage too low.*
51 Oxygen feedback system stuck at lean position.
52 Oxygen feedback system stuck at rich position.
53 Internal logic module problems.
55 End of message.
62 On 1989 models, EMR mileage accumulation.
63 On 1989 models, fault Code error.
88 Start of test.
Lets check for a vacuum leak, coolant sensor is working, If there is a pressure regulator on TB check vacuum hose for fuel,
(looks like older choke pull off) ....no fuel in hose is what you want to see....
Part throttle unlock .....is this an automatic trans??
My read on code 37 is "Baro read solenoid circuit. Monitored by SMEC when ignition is on. Check engine light is illuminated on Calif. models. displayed if solenoid does not turn on and off at correct time." This comes from Shopkey5. Transman