I have a 1990 firebird and when I put the car in gear it shut off I replaced quiet a few things such as the intake gasket and tge gaskets for the carburetor. What could be wrong.
Not enough info, but if it starts and the shuts down when you put it in gear, I would suspect something in the ECM sensing will not allow it to maintain a high enough RPM. Have you scanned it to determine if there are any codes kicking out? If it is a carb'd engine, you may want to see if the carb itself just needs some attention like the idle speed adjustment.
there is also a device in the tranny known as the tcc or torque converter control solenoid. it "locks up" the torque converter at highway speeds so that it doesn't shift so much. If it malfunctions, the converter cant unlock and it will die when you put it in gear. the plug in for it should be on the front drivers side of the transaxle. google its location on your car and unplug the connector. if it "fixes", then that's whats wrong. also, you can drive the car like this if the repair is to expensive, but you will sacrifice some fuel mileage.
I should have also said, to scan the computer for codes. it is an OBD1 car so you can google how to extract the codes from the computer on GM OBD1 cars. it involves counting flashes of the check engine light and a paper clip. not hard.
Looking for a Used Firebird in your area?
CarGurus has 757 nationwide Firebird listings starting at $3,490.
Search Pontiac Firebird Questions
Pontiac Firebird Experts