Check engine code P0304

[Gary Mick]

Check engine light lit the other day. OBDII reader said P0304 (intermitant misfire of #4 sparkplug). Vehicle has 46k miles so I bought some NAPA premium sparkplug wires and a new set of plugs (NGK Iridium) and installed. Light has come on again, twice. Any suggestions on what to check next? Thx

[bobs turbo pt]

welcome to the board gary!! maybe a sensor ?? just a thought [:I]

[Hooligan]

Did you use dialectric grease on the plugs before you put the boots on?? That may have something to do with it.[:I]

I changed my plugs & wires with 500 miles on it because I could feel a vibration while idling. I used Champion 7440 gapped at .040 and performance wires from my dealer. I have had NO problems since.

[quicksilverdon]

Welcome, Gary.
First thing to check- that boots are fully seated - push hard 'till you feel/hear a click - they're tough. A little dielectric grease inside the boot helps.
Next I suggest you pull wire off # 4 (both ends) and swap with # 3 to see if it's wire or plug ( if you still get 304, it's plug, if it becomes 303, it's wire). It happened to me (only with plug 2) - after I swapped, light went away! I suspect I may not have had wire seated at one end or the other...

[Karshry]

After going through this hassle several times, my service garage finally figured out the problem. Initially, they replaced the plug wires and coil. After that failed within hours, they twice replaced the coil, finally using an OEM coil. When it failed again, they replaced the plugs. Fine, until two days later it happened again. Today, it turns out that the rough idle and No. 4 cylinder misfire is due to a leak in either the upper or lower (or both) intake manifolds. They are being replaced tomorrow at a 50 percent discount from my servicer. Oddly enough, at high speeds on a 500 mile trip this fall with the check engine light on, it ran wonderfully.

[quicksilverdon]

Thanks for the post - yeah, there have been a few with intake gaskets - usually the car idles poorly with that, and trick is to spray starting fluid or wd 40 along intake seams - if idle smooths out, it's gasket.
Another possible cause can be a clogged injector.

[elmeyer]

Mine was the lower intake gasket. Idled rough, but anything above idle ran great.

[dsmturbo]

I am getting this code when ever I fuel up, and have replaced the gas cap. I am thinkng I have a bad injector (leaking) or intake gasket.

How hard are the injectors to remove on a 2005 GT?

[Carl48]

Mine occassionally idles rough at start up (no real pattern to it) and sometimes the check engine light comes on for a while. The code is usually 0304. New plugs and wires seemed to help a little but did not completely solve the problem. It happened again today...real cold here...suspect the plastic intake manifold may be the culprit.

[dsmturbo]

Mine is not the plastic intake. I will try and swap wires again.

I don't understand why this code seems to appear every time I fuel up though?