Fuel pump relay keeps blowing on my 1996 Cadillac STS, why?

If the fuel pump relay/fuse blows immediately the it shouldn't be too hard to diagnose. I would start by removing the power wire off the fuel pump. (If not sure which is power remove both) replace/reset relay again and see if it blows. If not blown then your pump has gone to earth ( it's bad). If it still blows then the wire between fuse and pump is going to earth. (not so easy to trace).

Do you have normal compression, fuel pressure, and spark ?

Usually a sign of a faulty pump, or a problem in the wiring between the relay and the gas tank.
You could unplug the harness at the tank to verify.

Test the electrical circuit . Testing Electric Fuel Pump Problems with Voltage Drop Test
GM Fuel Pump Relay Testing

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Hi,

I'm Ben and -- hopefully -- I can provide some meaningful assistance.
Sorry but the answer is no.

1991/1992 Part #REPB314517 Fits the following 1990-1992 Cadillac Seville submodels:

1991-1992: Base; 8 Cyl 4.9L; "FI, Naturally Aspirated, VIN B"
1991-1992: STS; 8 Cyl 4.9L; "FI, Naturally Aspirated, VIN B"

1994/1996 Part #E3913M Fits the following 1994-1996 Cadillac Seville submodels:

1994-1996: SLS; 8 Cyl 4.6L; "FI, Naturally Aspirated, VIN Y"
1994-1996: STS; 8 Cyl 4.6L; "FI, Naturally Aspirated, VIN 9

Check that the fuel pressure is the proper fuel pressure for the car some have low pressure systems and some have high pressure systems. if you put a low pressure pump on a high pressure system the pump will overheat and blow the fuse because it cant keep up.

Fill tank and put in a GOOD fuel system cleaner. (STP, Gumout, etc..) Running it out of fuel could've have damaged pump, fuel actually "cools" pump, running out and having to re-prime might have damaged pump. Try the cleaner first (pump replacement is not cheap) and consider changing fuel filter soon also.

Is the gas tank above 1/2 tank now?

A worn / aged fuel pump will fail when it's hot and the fuel level is low (example: below 1/4)

Park your car in a cool area.

Start the car when it's cold outside,

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Check the engine fuel pump pressure.
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Replace the fuel pump relay,fuel filter,fuel pump and fuel pump stainer at the same time.
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try to relase the fule pump conector and use new fuse then turn switch on if its blow the fuse again thats mean there is short in the wire harness or in the fule pump relay

RELAY