I just purchased an 04 Grand Prix GTP. I was aware of a misfire upon purchase so I simply factored repair costs in with the price. I've replaced the ignition coil, spark plug and spark plug wires and still have a misfire (p0304) The only other possible cause was a bad or clogged fuel injector for that cylinder but I don't wanna keep spending money to still have the same problem....I took a good look under the hood and noticed that one hoes had a screw in it. My assumption was this was a quick fix for the previous person...what is this hose and where does it go?

I believe that one goes to your HVAC controller inside the dash. You might find the hose hidden somewhere to the right of the alternator.

What do you think the purpose of putting a screw in that hose was?

The hose probably broke somewhere causing a vacuum leak. They pulled the hose off and stuck a screw in it to block off the vacuum leak.

Does your heating system change modes correctly? Meaning, from floor to vents to defrost?

Yes it works fine...I thought it might be playing a part with cylinder 4 misfire.

Hard to believe that the HVAc works OK, if that is the vac line to it.....you have a vac solenoid box, and the different modes would not work....all that part is, is like a vac distributor...

Even so, that plugged hose should have nothing to do with a misfire....

You changed all the wires? All the plugs(used A/C Delco)? Replaced just 1/4 coil?

Did you check coil output by inserting a spark tester at the end of #1 wire, and then #4 wire?

If spark is ok, and compression is ok, then you either have a bad injector, or you could have a vac leak between the SC'er and the lower intake.....

With engine running, take a can of carb cleaner, and spray around the entire joint of the SC'er and the lower intake......if you get a change in idle, the gasket is leaking.....

If ok, then with engine running, disconnect the injectors one at a time, and reconnect....should be a noticeable drop on all of them....if disconnecting #4 has less of an effect, you can pull the fuel rail and exchange #4 injector with another(Have new upper/lower O-rings handy for the injectors)....if the problem moves to another cylinder, you know it is the injector...

I've changed the that spark plug, wires, coil and that fuel injector

If you have replaced everything, it'* possible the issue is mechanical, and your next step should be compression testing.

It could be a low voltage to fuel system bank 1 causing lean measurements or ---Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1) scan for codes .. po171 with a po174 together point to a peleneum boot intake leak..code P0131 may mean that one or more of the following has happened: Faulty o2 oxygen sensor Short to voltage on O2 signal circuit High resistance or open on O2 signal circuit Possible Solutions Replace faulty sensor Repair short, open, or high resistance on o2 signal circuit

What does any of that have to do with a misfire?