I have a 2012 Diesel (1.6 112 HP) Ford C Max.

Last Monday, I noticed that fuel consumption jumped by 40% ! In the beginning, I was blaming some cold weather that day. But then, the temperature returned to the normal (16-18°) and the consumption is still very high.

The only habit that changed is that I just put in the tank what I need for a week (1/4 of my tank's capacity). So the vehicle's tank is at his lowest on Friday/Saturday. And I enable cruise control whenever I can even in town.

How to explain it ?

Edit Didn't notice but my car doesn't heat anymore. Can it explain high fuel consumption ?


  • Check engine light on?
    – JPhi1618
    Oct 12, 2017 at 19:57
  • It's off when I turn the engine on.
    – Bill
    Oct 12, 2017 at 20:09
  • Do you notice a lot more exhaust smoke from the vehicle? Is there a lot of soot around the tailpipe? Oct 12, 2017 at 20:46
  • Is it running rough?
    – elrobis
    Oct 13, 2017 at 19:13
  • I cannot imagine such a big difference from just running with a partially empty fuel tank, but it is trivial to fill it back up. Fuel usage probably stays that high indicating another cause. Which probably means you do NOT want to drive the car until you know what causes it. Take it to a garage and ask them to check what is wrong. Or, if you have an ODB reader (ELM327 are dirt cheap!) do it yourself. The cost of the checkup (both in time and money) may be much lower than continuing to drive with a vehicle with an unknown and possibly damaging fault.
    – Hennes
    Oct 14, 2017 at 20:13

1 Answer 1


The short version: The combination of the increased consumption and the lack of heat makes me think the thermostat has failed and is stuck open.

The longer version: Car engines have a thermostat in the cooling system - the idea is that when the engine is warming up the thermostat should be closed, this prevents the coolant from flowing through the car's radiator meaning it heats up faster. Once the engine is up to temperature the thermostat is supposed to open and allow coolant to circulate through the radiator which prevents it from overheating. Thermostats can fail either "open" or "closed" (meaning it is stuck in one of the positions and doesn't change as intended), if it sticks closed coolant never flows through the rad and the car overheats very quickly, if it fails open then the car will take a very long time to get up to temperature (exacerbated in a diesel engine since they take ages to heat up in the first place) meaning that on most "normal" journeys the engine never gets up to the right temperature and when the engine is below "proper" operating temperature it will run inefficiently which presents as higher fuel consumption and you will be getting very little (if any) heat from the heater.

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