Car fires up an shuts off - 1994 Buick Park Avenue

it might be or the relay

Usually, an engine that backfires thru the exhaust is a sign of ignition problems.
Assuming you have the standard 4.3 V6, you should have a flat distributor cap and standard rotor.
I would start there.
The cap could be shorting out inside and making the coil fire the wrong plugs at the wrong time.
It could also be other issues with the distributor or the ign module.

The emergency fuel shut off switch is present to prevent gas fires in the event of an accident. It is an impact switch and disengages power to the pump after an accident. There has been situations where an accident caused fuel lines to break, the key was still supplying power to the pump, and a car fire resulted. This switch prevents such fires. Thanks and please rate.

check the starter relay, most likely a loose connection...

chwck the oil level

you need to determine if your missing spark at the plugs or fuel pressure is below 50psi first

check the cam sensor and crank sensor, also is the distributor turning,possible distributor gear pin broke

check main relay

sounds like the coil is bad

have the electronic module checked and maybe replaced to correct this problem