I think the runaway likely came from a quantity of WD 40 laying in a low spot in the intake, which got swept into the cylinders when the engine finally started. The developing starting problem could have come from low compression. Typically an engine with compression problems will start easily if it has recently run as the oil is still on the rings, helping compression, but will start hard after standing long enough for the rings to become dry. Typically the longer it stands the harder it will be to get it started. This is of course a long distance guess, and I don't know what happened to the engine when it ran away.

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'97 E 300 D