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Dear Beka964,
Hope you're well. I suggest that the engine coolant temperature sensor - or the ECU - has failed. These readings - which show an irregular impact on fuel efficiency as temperature falls - from OK at 68F to poor at 40F to excellent at 14F (or lower), mean that the engine computer is getting erroneous signals as to how hot is the engine.
When ambient temperature is 40F, the computer thinks the engine is "cold", and makes the mixture richer, so that there will be adequate power. When the ambient temperature is 14F, the computer thinks the engine is "hot", and makes the mixture leaner. Your mileage improves sharply.
I'd start with the engine coolant temperature sensor (or whatever is the correct name for this part on a Grand Voyager), because this part is not hugely costly. I'd guess this sensor's resistance should vary with temperature. By immersing it in water of known temperature - or putting it in a freezer, in which a thermometer lets you know how cold it is - you should be able to read the resistance, with a Volt-Ohm-Multimeter, at varying temperature levels. If there is no pattern, then the sensor has failed. Resistance should vary uniformly with temperature.
If a new engine coolant sensor makes no difference - then move on and look at the fuel system computer.
Hope this helps.
Yours faithfully,
Spook
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