How to set idle because it it too high

There are a lot of factors that may contribute to high fuel consumption. Does the engine run ok during normal operation? Rough idle? Check engine light on? Have the oxygen sensors scanned for operating information to determine if the fuel system is operating effeciently by a qualified service center. These sensors monitor the effeciency of the fuel system

O2 sensor. (oxygen sensor) im 45% sure.

engine consume lot of fuel.clogged air filter.low tire pressure.faulty emission control device.dirty fuel injectors.heavy foot on accelerator.lot of short stops like driving in the city.running AC will consume gas.high mileage engine low compression assume lots of fuel.slipping transmission.weak ignition.driving up steep grades or mountains burns more fuel.

obviously does, more RPM needs more gas

The coolant temperature sensor adjust the air to fuel ratio depeneding on the temperature of your engine. If it is bad it will run your engine too rich(to much gas, not enough air) and it will run rought at idle and definatly have black smoke, good-day! its worth a try ! In some cases you can unplug the sensor and the PCM will compensate by running a default air/fuel ratio and in some cases allowing the car to run without the sensor, I'd just replace it.

It the temperature sensor is faulty the ECM will think that the engine is constantly cold so it will keep the fuel to air ratio high constantly = the fuel consumption will be high. Your mechanic is probably right, and replacing the sensor is a relatively cheap job - less than 50€.
BTW to identify your exact model just look on it's ID badge, it's inside the engine bay .

Usually plugged fuel filters and/or bad pumps cause loss of power at the high end (full throttle) when you are sucking down the fuel not so much at the low end.

Rough idles can be a lot of things, usually something to do with the fuel mixture or timings. Dirty injectors, distributor problems, streatched out timing chain/belts... those are a few common rough idle issues.

Has the fuel economy gone down? Does it still have good acceleration?

Peace,

Sounds like a vacumn leak. Gasket under intake plenum may be leaking. Be sure the pcv valve has the o-ring around it in the correct place also as it will set a code.

system is probably running "open loop".

use a scantool to get system codes.

You likely need a tuneup, i.e. O2 sensor(s), fuel injectors (system) cleaning.

look and listen for a vacuum leak