So here's the skinny on the #6 spark plug fouling. You are correct in that it does involve the EGR valve! However, not in the way you suspected. Because of the EGR valve's location near the right center edge of the intake manifold, exhaust heat is concentrated in that area of the intake manifold flange. All that heat bakes the intake manifold gasket causing it to lose pliability and subsequently shrink ever-so-slightly. At that point, oil vapor wafting in the valley pushes passed the gasket predominantly into the #6 and #8 intake runners. The hot oil vapor partially burns in the combustion chambers, coating the spark plug insulators with hard ash deposits. The effective repair solution is to replace the intake gaskets using Fel-Pro or similar aftermarket gaskets, sparingly smeared with some high-temperature Ultra Copper silicone sealer. The precious metal spark plugs may work for a while but eventually, gasket replacement will become necessary. So, now you have the rest of the story.
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