So this has been a problem code for a few members here. The Ignition bypass is a 1 wire connector near the distributor, Connected the computer can control the timing advance, Disconnected the timing is fixed and doesn't change. When setting the initial distributor timing you need to disconnect the IC connector.
Recently a member here found that the secondary Ignition wires(spark plug wires) being to close to the primary wire( small distributor wires to ignition control module, and coil) caused the ecm to set code 42, and run poorly from the secondary electromagnetic interference.
Since none of these TBI engines have the original spark plug wires, and even the AC Delco/GM wires are only a close exact fit, careful attention needs to be taken when routing spark plug wires.
There are two computer grounds attached to one of the thermostat bolts, should be a studded bolt, these ground sometimes get treated roughly when the t stat is serviced. Inspect them for breaks, and nice clean tight connection to ground. The ECT (engine coolant temp) sensor is also just to the passenger side of the t stat housing, this sensor plays an important part in how well the engine runs. This sensor also sees somo rough handling, check the connector pins in the plug are nice and tight and making good contact on sensor pins.
