Citroen Xantia coolant temperature is extremely hectic!!!

[August]

Hello, my 1999 Citroen Xantia 1.8 16v manual started to act extremely strange. The engine coolant temperature rises nicely and gradually until around 80C then all of a sudden it picks up speed and in like 20-30sec rises extremely fast and reaches around 100-105C and then after couple of seconds drops too normal temperature in like 10 seconds and doesn't act up for the rest of the journey(doesn't reach higher than 90C). It seems to be doing that only when starting the car after it was sitting for quite some time, for a day or so aka. when the engine is completely cold.

[white exec]

Sounds like a 'sticking' thermostat, rather than one that operates smoothly and progressively.
Probabably best to replace it first, then go from there.

[August]

Was thinking of that, but it was replaced 10000km ago... Could it be deffective?

[white exec]

It's possible, but would be my first suspect, and needs eliminating.

[August]

Hmmm... How about the blue sensor on the housing near the thermostat. Can it be acting up and showing false readings?

[white exec]

The blue sensor does feed the gauge. It could be misbehaving, or maybe just a poor contact.

[August]

It is quite strange, because when that "thing" happens the temperature gauge moves fast.. unnaturally fast.. thats why im thinking of a sensor.. but sticking thermostat is possible aswell

[August]

And also it happens just once.. once the temperature rises quickly and drops quickly it stays on a normal operating temperature and doesn't act up or anything.

[admiral51]

It is not unknown for replacement thermostat to be defective straight out of the box.
Do the cooling fans both kick in when the temp gets to 100+ ?
If the temp gauge is normal for the majority of the time in normal use then i would also suspect the thermostat being a little sticky

Colin

[admiral51]

Posted my reply as OP was posting so does look like sticky thermostat if guage is reading normal for the remaining time

Colin

[August]

They do kick in when the temperature acts up and reaches temperature of fans kicking in, but temperature doesnt stop climbing and reaches quite above the fan engagment temperature... After few second it drops almost instently to around 80c and ofcourse fans stop working.

After all that non sense happens.. temperature reaches Reaches "fans temperature" nicely and progrresivlly and when they do turn on it doesn't go higher and drops off in minute or so

[admiral51]

Another random thought is maybe the coolant needs changing, not sure when it was last changed but sediment can stop a new thermostat working correctly.
If the guage is showing the right temp then the sensor is doing its job, if it was the sensor it would ( hopefully) be playing up on a random basis.
This seems to be a thermostat being weak to open but once open doing its job and the fans kicking in means that the rest of the system is working as intended.

Colin

[August]

The thermostat and the coolant has been changed 10k km ago.. but if its the case... Will do that again

[xantia_v6]

Maybe you should invest in a cheap IR (infra-red) thermometer so that you can measure the actual temperature at the thermostat, to localise the problem.

[white exec]

Trapped air in the thermostat (water outlet) housing could cause gauge sensor (or maybe the thermostat) to be erratic, but not a place where air usually gets trapped. Is there a bleed screw (or expansion/venting hose) located there?