Hello, My name isXXXXX was a Chevrolet technician in 1983 and worked on many of these cars. The basic computer controls the fuel delivery to the carburetor and the timing control to either increase the timing or retard the timing. This is controlled by various sensors, the main components are the O2 sensor and the coolant temp sensor. Here is the diagnosis for the codes you have now.
Code 13 Check and test the Throttle position sensor which is mounted on the carburetor on the front of the carburetor, it has a three wire connector going to it. Check for loose wires or connectors. also check the wiring going to the O2 sensor which should be in the right side exhaust pipe near the exhaust manifold. If you find no problems with wiring and the connections are good, I would replace the Of sensor [the Of sensors typically last about 5 to 8 years].
Code 15 is for the coolant temp sensor which is mounted in the thermostat housing or in the intake manifold close to it. Check to make sure the cooling system is full and the cooling system has the proper 195 degree thermostat, these engines were designed to run at least 195 to 210 degrees. Lastly, check the wiring to the sensor and also the connector these connectors were bad for hardening and cracking and causing a loose connection. I would replace the coolant temp sensor and the connector to it if you can locate replacements in your area.
After you have completed these repairs you need to disconnect the positive battery cable for a few minutes and then reinstall the cable and start the car and let it idle and warm up then drive the car normal till it warms up to relearn the basic block learn of the engine to the computer [ECM].
This is a very basic feedback computer system in this car, it is not hard to understand or to work on, you should have no problem working and maintaining the system, I would pick up a Haynes manual for your year car and it has the basic trouble codes and diagnosis for the basic trouble codes.
Yes, the engine light being on will cause the computer to stay in open loop when it has a problem. This means the system is in what is called Limp Mode the fuel to the carburetor is richen up and the timing control is retarded the engine will run really bad and it will use a lot of gas ,which could cause the catalytic converter to fail. So it is important to maintain this system so as to not cause more problems.
I hope this has helped to answer your question and provide you with some knowledge of the way the computer system works in your car.