I went to top up my oil this morning and the oil filler cap was stuck tight. After some force the cap came unstuck, this has never happened before. I noticed around the under-side of the cap a strange yellow gunk build up. I have attacked some pic's. Could anybody tell me what this stuff is and should I worry? I have a 1999 ML430.

David.

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It looks to me like your cap is not sealing well. The ML has been known to collect condensation around the cap, forming a goo-like substance. Never seen yellow before though. If the goo made it past the gasket, I'd replace the cap.

IN the winter that type of build-up isn't abnormal.. Driving for 40-50 miles on a freeway at speed should elimate that build-up.

Heat in the engine will evaporate the WATER that condenses in that filler neck.

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A clogged PCV valve will reduce crankcase ventilation and cause that emulsion, a combination of oil and water.

But most likely is you are driving short trips and don't get the engine thoroughly warmed up, which would drive the condensation out.

Jim in Phoenix