Evening there The deposits are caused by the engine drawing partially burnt exhaust into the inlet manifold via the exhaust gas recirculating valve ( EGR ) .
It is not a fault ( as such ) but a stupid idea aimed at cleaning up exhaust emissions ( not a bad idea ) . On engines driven slowly & on short journeys the effects are far worse , literally cutting off air flow due the gunk build up in the inlet ports .
Careful cleaning to remove the mess without scratching the metal below is easily done & pays huge dividends
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To stupidity & beyond
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