Clio 1.2 16v Poor fuel economy

[jonhlemon]

Hello guys,
In may i bought a clio 1.2 16v (2001). On the beggining i was getting about 7l/100, getting about 600km on a full tank.
In july i made a relatively big trip (300km) where i pushed the whole time averaging 140 km/h and It has since here my fuel economy has decreased.
In august me and my friend changed the timing kit, thermostat and all the belts, and it has been since that the fuel economy has been dropping,getting these days about 500km from a full tank, like 8,5-9l/100km. The timing is correct, but since changing the thermostat the temperature rarely stays in the middle, and it always going up and down (never going past the middle). My last service was about 5000km ago, where i changed all my filters and oil( i didn't change the spark plugs tho). I think i've may have a small leak on the muffler, but it was always like there since i got it. The brakes ain't sticking i think. There are no errors on the obd port (i have one of those small ones)
Where can i start checking to see the problem?

Thanks!

 

[Krarl]

Start with spark plugs, leads and coil, then clean out the breather pipe and the inlet as these get full of oil and can cause restriction on the D4F engines.

Then just run some cataclean through it and take it for a good hard blast down the motorway, that should clean any crap out

 

[Martin_172]

Re visit the thermostat, it should be holding s consistent temp, is it the correct thermostat? Is there any air pockets in the cooling system?

 

[jonhlemon]

Start with spark plugs, leads and coil, then clean out the breather pipe and the inlet as these get full of oil and can cause restriction on the D4F engines.

Then just run some cataclean through it and take it for a good hard blast down the motorway, that should clean any crap out

I'm gonna change the coil and spark plugs on the next service, but as it seems the problem is the thermostat, that is making my car running rich..

 

[jonhlemon]

Re visit the thermostat, it should be holding s consistent temp, is it the correct thermostat? Is there any air pockets in the cooling system?

I think it's the correct thermostat, the casing was the same, i think it's just defective. It's a magnetti marelli one. No air pockets

 

[Gus]

Lambda sensor may be deciding to put more fuel in.