P0304 code on Ford Mustang?

About two weeks ago I got a check engine light on my 2010 Ford Mustang with 60,000 miles. I hooked it up to a code reader and got P0304 which is fourth cylinder misfire. Since the car was at 60,000 miles anyway, I went ahead and replaced all the spark plugs and wires, but the issue remained. I then went to the dealer who suggested it was the coil and replaced it. The problem has just returned. So… What now?

I would remove each spark plug wire one at a time from coil pack terminal.Start engine, hold plug wire with insulated pliers about a 1/4-1/2 inch away from coil post and make sure every terminal is firing good.A new coil pack can be bad.Since you addressed the ignition system already, vacuum leaks or injector issues is usually the only other cause for misfires. Make sure the injector wiring harness for #4 is getting power.Use a noid light or test light and connect it to the injector connecter.Noid or test light will flash if harness is working properly. You can also hold a screwdriver against the injector while holding the handle to your ear.It should make a repetitive clicking noise if its working.Kits are sold that include new orings, pintle caps, and injector filters that you can buy to service all 8 injectors. The #4 injector might just be dirty, if it is the part causing the misfire. If ignition parts and injector isn't causing the misfire, check for a vacuum leak around the #4 cylinder.

Possible spark problem or fueling fault. If its regular and you can hear it misfiring you could try spraying try some eazystart or carb cleaner into intake. If it stops misfiring while your spraying but returns when you stop then its fuel related, as jj said, new injector? Could also be coil again, or plugs or even head gasket leaking between cylinders? You can run it in the dark to check for bad leads, as you can see even small sparks better. You could move plugs around to see if misfire moves cylinders, and clean them/replace to see if it remedies. Colour of plugs can also give you an idea of cause, on faulty coilpacks you normally get 2 misfire not one because they are normally "wasted spark" systems meaning they spark to ignite fuel and again when exhausting. What this means is 1&5 spark together then 3&4 then 2&6. Coil fault will mean both pairs not working eg. Cyl 3 and 4.

I agree with JJ, fuel injector could be the culprit. You could also try some combustion chamber cleaner (like Seafoam, but there are far better brands in aerosol cans, like Mopar). Could be a fouled intake valve.

If it's none of the above, you're looking at a possible Burnt exhaust valve or a blown head gasket.

How about you now look into the fuel injector.