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Think about it: you boost on an engine with a known lean condition, forcing even MORE air (lean = less fuel, more air) into the engine, increasing compression, etc. A lean engine is a hot engine, hot air expands, you have more air than normal, the air has to go somewhere, you are exceeding the pressure capabilities of the cylinders, etc. The excess pressures force oil past the rings, which increases the crankcase pressure. The oil filler cap isn't made to handle any serious pressures, although it does manage most of it. The result? Some air pushes out the seal, and oil comes with it. The end result is a wet oil spot around the filler cap.
As I said, this is a SYMPTOM of another problem, and it is ONLY APPLICABLE if you have a turbo engine and see the wet oil spot after trying to boost under said condition. If you want to test it clean the oil spot and go take a drive. Do one drive without any boost at all, check it, and than do another with boost.
The jiggle test is absolutely worthless, so I don't know why you are even bothering. If you have an oil stain around the filler cap its pretty obvious you have excess crankcase pressure, so why would you bother confirming the obvious? What you need to analyze is WHEN you have excess crankcase pressure and then trace it back to a single cause.
In my case this was a bad AMM or Knock sensor connection (I reseated both when the it became fixed), though I favor the AMM.
good luck,
rt
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