When was the last tune up ? Were the plugs changed ? However, I'll bet it is due to dirt in the injectors. In the past I used injector cleaners, but when I had my pathfinder the service advisor recommended that they NOT be used due to the chemical harshness on the gaskets and rings. Was it a ploy to get them to do it ?.

Pool gas mileage is also affected when the outside temperature is below -15c. Don't know if you are experiencing those extremes but believe me the gas mileage of a SUV at -40c ain't pretty.

Smell excessive fumes ? Also check the fuel line with the motor on and off. With it on may be the line from the pump onwards, due to pressure.

Hope my two cents helps

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