My 1994 chevy 1500 will start and run untill the thermostat opens then a couple mins later it dies and wont start
I would try first replacing the temperature sending unit, as the computer (OBD1 in that one) relies on its information from cold to hot on how to adjust the other sensors to make the motor run smoothly. Also you may want to check the timing. When doing this you need to unplug the brown with black wire under the glove box. This controls the advance on the timing.
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