91 305 TBI, pulled codes 34 and 42 from the ECM, what is the best way to troubleshoot these codes?

 

Does it consistantly trip those codes? If so, the first code indicating a high vacuum at the map, which is either an idicator that the sensor is bad or the there is a short or some other fault thats causing the voltage at the output to be low or zero. If you ahve data logging software, you can hook the map up to a handheld vacuum pump and check it. Otherwise, just replace it and see if that works. The second one probably means that the knock sensor or the module is bad, or the timing is too far retarded. You can test it by watching the timing with a timing light and running the motor at >900 rpms. Hit the block somewhere near the sensor with a hammer and see if the timing retards or not. Having a manual would help as well. The good manuals from helms (and even chiltons) have diagnostic flowcharts in them.