What would cause my 1991 Oldsmobile 98 regency elite car just shut off after it was running for 30 - 45 minutes ? It was like I turned the key off but I did NOT !!

scan the computer for codes. you will have to use the paper clip trick to do it, as this car is OBD1. google how to scan 1991 OBD1 oldsmobile. there are good youtube vids. if you get no codes, check all fuses and relays. make sure the battery and connections are good and test the alternator. if all that checks out good, test the fuel pressure. if all checks out good and no engine codes, google how to test the injectors. had a 91 chevy do something similar and it ended up being bad injectors.

check the ignition module. it sits under the coil. it probably got hot and melted some of the inside parts.