My Volvo XC90 has suddenly started to consume about 30 per cent more petrol than it used to. The car runs normally and I do not feel any power loss. But I did notice recently that the engine temperature is lower than usual. I have serviced the car, changed the spark plugs and air filter, but that did not help. I took it to another workshop, which told me that it could be the throttle-position sensor or oxygen sensor. I agreed to have the throttle sensor cleaned and oxygen sensor replaced. Unfortunately, the problem persists.
Considering what you have done so far, the temperature sensor is probably faulty.
The engine temperature is the biggest clue. It suggests that the sensor is faulty and sending the wrong signal to the electronic control unit.
All fuel-injection systems supply more fuel to the engine when it is cold and will continue to do so until the engine warms up. As long as the electronic control unit believes the engine is cold, extra rich fuel is injected, causing an increase in overall consumption.
Replace the temperature sensor and the fuel consumption should return to normal.