Does the 91 ls come with a fuel pump ecu? If so where is it located? I checked behind all of the carpet in the trunk and couldn't find it.

 

IIRC, the fuel pump is taking orders from the fuel pump relay. It is the relay that takes orders from engine ECU. You should be able to test the pump by bridging the load side of the relay sockets, and you should be able to determine if the ECU is correctly ordering up some fuel pump by test lamping the control terminal sockets in the junction block.
Colin
(p.s. and always start fuel pump quest with a look at the fuel pump fuse)

 

I guess the -91 LS400 have a Fuel Pump ECU because the -94 have one. See picture below where it's located. Not the best picture but the best one I could find.

 

FYI

From Alldatadiy.com

"PURPOSE
The Fuel Pump Electronic Control Unit, located in the left rear of the passenger compartment, controls fuel pump output.

OPERATION
During idling and light engine load, the fuel pump demand is low, so it receives a lower voltage (about 9 volts) from the Fuel Pump Electronic Control Unit. During starting conditions, high speed and load conditions the Fuel Pump Control Unit delivers a full 12 volt signal to the Fuel Pump, which causes an increase in output. This signal is switched by the Engine Control Module."