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Help!!!! 2004 v6 3.9l Mustang is throwing code p0304 and is running horribly

2786 Views 3 Replies 3 Participants Last post by  SilverStreak00
My car has been running really rough, no power and throwing code p0304 for a cylinder 4 misfire. I have already changed the spark plugs, wires and fuel filter. I am thinking it may be a bad injector?!? When it first started acting up i figured it was my plugs. I was planning on doing them anyway as it was time for the scheduled maintenance. After changing the spark plugs and wires the car was much better but still running rough so I put injector cleaner in the gas tank on my next fill-up. It ran better and the check engine light went away so I thought it was just a dirty injector. It wasn't long though until my car started doing the same thing. That is when I changed the fuel filter thinking maybe I got bad gas. It made a difference and helped some but still the problem was there just not as bad. So... Again, I added injector cleaner on my next fill up and it seemed to help, the problem didn't completely go away but helped. The next tank without cleaner ran horrible again and threw the code once more. I once again added cleaner to my tank when I filled up yesterday. I don't want to just keep blindly throwing $$ at it as I've already done plugs, wires and fuel filter..... There wasn't any fluid on the plugs when they were changed if that helps....
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I have a 2004 3.9L, the codes codes they give although maybe helpful, are generic. Did you gap the plugs, as plugs out of the box are off, some a lot? When you changed them, were they clean or covered in deposits, my 3.9 has been using oil, so mine get so much deposits, that it fouls the plugs. Mine would run perfect several days in a row, then missing and running bad, then running good for several days, I thought I had gotten a bad tank of gas, so I did like you, but it turned out to be 3 fouled plugs, and I think I only had about 10,000 mi on them. You also could have a bad coil pack (it is actually like 3 coils in one package), but each coil fires 2 cylinders, or the module. What I would do, is pull the sparkplugs, one at a time and check them, if okay, then put it back, if any are bad, then replace them all. My 2 cts. Good Luck.
The plugs that were pulled were worn but no deposits from oil or coolant... which was a good sign, but I really don't think the plugs were the problem. especially since it is the same cylinder code both before and after changing the plugs and wires. Coil pack possibly but why would injector cleaner make it run better if it was a coil?
Highly doubt it's the coil, but you can test the resistance across the towers with voltage meter and check the OHM specs. There are also cheap spark testers that can be used to check output as well to that cylinder.

Considering you feel a difference after fuel treatments/cleaners leaves the injector to be suspect. You can try to unplug the #4 injector with the motor running (i suggest cold) and see if it runs any different. If it were me and I was 100% certain the plugs/wires/coil test out fine, then I would pull the upper intake off and swap injectors, say #4 with 5 or 6 and see if the misfire code follows to that cylinder. This would point to the injector for sure.
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