The non-vented caps are used with a charcoal vapor canister on the newer-model vehicles. The canister stores the vapors while the engine is stopped, then vents then through the intake system on startup. Older fuel systems either have a tank vent, or a vented cap, or both. I'd check for a leaky fuel pump, putting raw gas into the crankcase.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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