1998 buick century cylinder 5 misfire

P1189 Buick - Engine Oil Pressure Switch Fault PO301
Possible causes
- Faulty spark plug 1
- Clogged or faulty fuel injector 1
- Faulty ignition coil 1
- Fuel injector 1 harness is open or shorted
- Fuel injector 1 circuit poor electrical connection
- Ignition coil 1 harness is open or shorted
- Ignition coil 1 circuit poor electrical connection
- Insufficient cylinder 1 compression
- Incorrect fuel pressure
- Intake air leak
PO300 is just telling you that you had a misfire.

P0304 is engine misfire #4 cylinder Several things can cause a misfire #1 electrical which you have covered #2 Mechanical Test engine compression is should be around 150 cranking and 120 running . internal valve train components can cause bad compression and the valves then self. #3 fuel injector can be the cause suggest swapping with a know good cylinder and see if misfire follows injector.

My 96 century with a 3.1 v-6 gave me fits for years shooting a 171 code(running too lean) I wound up changing the fuel filter and the problem went away. If it's been a while since the filter was changed, try it.

The list is long. Bad spark plug/s, wire/s, distributor cap, camshaft, valve/s, piston/s, gasket/s, vacuum leak, fuel injector/s among other things. Some diagnosis will be needed.

Crank Position Sensor ? (located behind the harmonic balancer)

adjust plug 5 and intake valve

If I am not wrong these vehicle have a big issue with the coils/ignition module so what I would do first is replace the #6 coil first mind you there are two cylinders to a coil on this vehicle, so replace the coil first and then see what happens. You can get a used coil from the junk yard for 5 buks.

running rough, could be a few possible things besides spark plugs, and wires!
air filter, fuel filter, fuel injectors, fuel pump, coil pack, Etc.
Id start with the air and fuel filters!
Good luck, Hope this helps!

YEP DIRTY FUEL INJECTORS WILL CAUSE MISFIRE.

Check for trouble codes and check spark control module and coils