Probably a good idea to get on to the last owner if possible, to see if the deposit is perhaps a result of an 'historic' leak, now resolved, maybe? I had a bad coolant leak from a badly installed intake manifold assembly, which caused a misfire even after the leak was resolved and everything dried up. The misfire persisted as a result of the leaked coolant having rusted the long spring inside the COP rubber tube, making for a poor or intermittent contact with the plug.
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