How do you find the source of a gasoline smell from a 1996 Mercury Villager the smell is coming in through the vents when the engine is warm yet the check engine light is not coming on?

I would say you have a fuel leak. It can be at the fuel rail or any of the injectors or connections. This is a very dangerous situation. Have this repaired ASAP. A very slow leaking fuel system will not set the check engine light.

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