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Welcome, I'm Chris (aka Moose), my normal schedule online is around lunch and after dinner past midnight.
As an engine runs there is going to be some compressed air and fuel squeeze by the rings and cylinder walls on the compression stroke. Engines use a PCV system that allows fresh filtered air to be sucked into the engine from a vacuum line connected to the intake and PCV. This pulls vapors out of the crank case. So there will be some slight pressure with the oil cap removed but anything excessive means you either have a clogged or faulty PCV system or the engine is worn out and has excessive blow by.
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