Have you tried to tighten the valve cover bolts? Does the gasket feel soft or hard? If soft and bolts not tight try tighting a little then check. If that does't work then replace the valve cover gasket. When you do this clean the valve cover and the head where the gaskets seats. This should solve the problem.
Bill

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94 dodge cummings,86 ford 6.9,75 mbz 240d[ sold], 82 mbz 300sd,83 mbz 300sd