Hello and Welcome to justanswer...
There are of course many reason that any given vehicle can have poor fuel consumption and it doesn't take much to send it off...
Reasons can be from any one of the sensors affecting the engine management system which has a detrimental effect on running, even un felt - but enough to cause trouble with mpg... to problems with the brakes on the vehicle slightly binding on one of the wheels - to problems in between with the tyres... pressures , tread , even tyre type... and even to usage, terrain , where you live - how its used, driven etc etc..
A lot of the above you can probably rule straight out but do think about them as there may be one or more having an adverse effect..
Going back to sensors on the engine - most will put the engine management light on if there is a fault with any one of them - but one of them won't always - and that is the ECT sensor... This is the Engine coolant temperature sensor..not the one for teh gauge - but the one that sends signals to the cars Ecu.. main computer - re. the temperature of the engine.
The Ecu then sends commands to the fueling and ignition systems according. If the sensor is faulty and thus sends incorrect , or no info to the ecu as above, the fueling etc will be incorrect and thus do as you describe resulting in poor mpg.
If you remember the old auto and manual choke systems.. (on petrol engines - but the principal is the same ) ..it works like that , only electronically.. If and when the choke was in the wrong position for the temp of the engine, the engine performance would suffer and if it were not adjusted right , slightly "On".. then increased Mpg ....- exactly the same.
So, without the aid of plug in diagnostics at the moment and the results thereof - its well worth renewing that sensor. Its usually located on the front right of the engine head - under the plastic cover... I will give you a pic / drawing below which shows its location for you ... identified by a red dot !
Other than the things mentioned above which should all be considered - I would suggest renewal of that sensor and try it from there... IF you were still to have trouble after that , then proper diagnostic tests would need to be done to check the rest of the engine management system through.... but the Ect sensors are a common issue....
Here is the drawing / location for you for the Ect sensor..
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