LOW VOLTAGE ON A FEW WIRES WILL NOT RUN

Have you looked over there?

Thye fuelpump relay is located to the right of the brake booster. this is for a 1991 gmc jimmy (s-15)

I have enclosed a diagram to help locate the relay.

The most common reason for no fuel pressure on these trucks is not the relay but rather the fuelpump itself. the pump is located inside the fuel tank.

There is also a problem with the wiring to the fuel pump, you will need to locate the top of the tank and then off to side of tank is wiring .
This wiring comes off the top of the tank, you will need to check and there is connector problems..
there is one or two ground wires that come off wiring to top of tank which run to frame, check to make sure the grounds are good. if you end up with a bad pump you will see a notice to rewire a new fuelpump connector. hope this is helpful. good day.

If it shakes try replacing the distributor cap, the pin in the center wears down and jumps causing eratic engine shake (get the genuine GM one it seems to be more precise and smoother running engine) also make sure your spark plug wires aren't broken, or breaking.

Sounds to me like you need the alternator repaired,low voltage makes the engine work poorly and it will quit when the voltage gets low enough that all the sensors are out of range,by then you don't have enough power in the batteries for a restart.

-Drivers side starting at the front of engine: 1-3-5
-Passenger side starting at fornt of engine: 2-4-6

Sounds like the relay is not shutting off fuel pump. Turn the key to on position ( not start ), pump should only run for a few secounds and you will hear a click and pump should shut off, if it keeps running relay is bad. If it works like it should than fuel regulator (if there is one) would be the next posability.

sounds like your drawing moister change the dis cap prob has a hair line crack in it!!!

Check the fuel pump.

cHECK THE FUEL PRESSURE AT THE RAIL. iF LOW i WOULD CHECK THE INLINE FILTER AND THEN A POSSIBLE PUMP

I do not know the codes but have been told "vacuum leak"? Mechanic thought gas cap might be loose or the O ring in cap may be dry-negative. Hope this may help some.