I had a similar problem PCV problem in a Jeep I had (return line was plugged). My 300D has a little leakage around the hose leading to the airbox (I have the new style flat one) and I spoke to a mechanic who said there is a rubber fitting or baffle of some sort inside the valve cover that might need to be re-epoxied back to the top of the valve cover.... but thats what he said... maybe it'll help you move in the right direction.

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1983 300D 326,000mi (RIP)
1982 Corvette Collectors Edition

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