Code P0304

Welcome to 2CarPros. It sounds like the oil worked into the area between the plug and the rubber of the connection. That would only take a few times for the spark current to travel through the oil and carbonize it. From then on the spark follows that path to ground instead of through the plug. It's an issue called carbon tracking. The oil is leaking out of the seal between the valve cover and the spark plug tube. It's a rather common thing on engines designed this way. The repair is to remove the valve covers and replace the seals and valve cover gaskets as they are shown in the images below. The problem is that to do that you have to remove both upper intake plenums to be able to get the covers off to change those seals. Those are items 2 and 8 in the last picture. It's not a real difficult job if you have the tools. If the rest of the vehicle is in good shape I would do the proper repair. If on the other hand you don't want to bother with it or just don't have time at the moment you can try a quick fix. For that you clean the areas where the tube seals against the cover with some carburetor cleaner on a rag, clean into the tube as far as you can and let it dry. Then apply a thin coat of RTV sealer over the seam area and a bit into the tube. The oil in the cover shouldn't be under pressure, instead it just seeps out. The RTV will cure and usually stop the seepage for a while.

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