1.4 hdi consumes coolant - the level decreases after a trip.

Hello,
i notice that my citroen c3 pluriel 1.4 hdi consumes cooling liquid.
And i tell 'consumes' because it apparently does not loose liquid.
I checked under the car, and on the floor: there are not spots on the floor.
And inspecting under the bonnet, i see that all is dry.

But, in the coolant tank, i see that the level decreases after a trip, when i hold a speed greater than 100 km/h

I measured the level, doing the same trip of 50 km, with different speeds:
The consume seem proportional to the load of the motor:

90 km/h: no coolant is consumed
100 km/h: 1-2 mm coolant is consumed
120 km/h: 1 cm coolant is consumed
130-140 km/h 2-3 cm coolant are consumed

This means that must use the car with speed not greater than 100 km/h,
else i have to refill the coolant after some days of use of my car.

My car has many km (450000)

I am not sure, but seems that when i hold an high speed (130-140 km/h), seem to smell some water vapor coming from the air vents of the car...

What could be the fault?

Thanks!!

Car heater matrix leaking? It's located behind the heater control panel.
Are the carpets wet?

thank you,
i have this doubt too.
i have to check it.
what i have to dismount to see this heater matrix?
thank you!

Sorry I don't have photos, but if you put your hand behind the heater control panel, towards the front of the car you should be able to feel if the carpet is wet, shine a torch there and on the LH side you can see the feed and return lines of the coolant.
See here, the heater matrix is to the left of the electrical connector:

Any pin prick sized leak and let the coolant out, but because its a small hole, the escaped coolant can evaporate very quickly and not get noticed as a pool or puddle.

Thank you to all!
I will check.

Someone told me also to check the EGR valve
Seems that if there is a small hole in the valve,
it aspires the coolant.

What is the function of this valve?
Where is it located, exactly?

Thank you!

Hello,
any suggestion?
Thank you!

I don't see any connection between the coolant and EGR but here's a detailed answer I just posted on the EGR.

The EGR stands for "Exhaust gas recirculation", and It's main function is to mix Exhaust gasses into the fresh Air/Fuel mixture during the combustion stroke.

This in turn cools down the combustion and dramatically reduces NOX emission which is a lethal gas.
There are claims it improves performance but i don't think this is true.
VolksWagen's Latest scandal was right around this issue, since a poor Air/fuel ratio gives better economy and performance but creates far more NOX which will fail an emission annual test.

Here are the full instructions for the EGR removal in case you feel like diving in anyway..