Revs too high when warm, too low when cold.


I have had problems with my 2000 registered Sirion for a few months.
When the engine is warmed up the rev counter shows 2000 revs per minute whilst idleing. When cold the revs are very low and the engine sometimes cuts out.
My garages diagnostic machine identified a faultly coolent temprature sensor, they replaced the sensor which appears to have helped, but not solved the problem.
The car now idles at around 1200 rpm when warm and 600rpm when cold.
Would there be any benefit in replacing the coolent or does anybody have any idea what the cause could be?
I don't think the coolent has ever been changed.

Poor idling

Not being a mechanic I'm not sure how it works but the Sirion runs at revs when cold, sort of an anti-stall device then idle revs drop when warm. Some of the guys on here should be able to help and I will be interested too to see how it works. Sorry I can't be of direct help but hope you get it fixed.


Two options:

Internal corrosion may have built up around the bottom of the sensor hole allowing no, or very little water to be in contact with the sensor. Remove the sensor and poke the hole out, change the coolant anyway and use a cleaning agent to flush the system out, follow with copious amounts of water down every water pipe you can remove.

Has the vehicle got an airlock, a slight airlock may mean the sensor is actually sat in air rather than water, this will affect its readings and the engines idling speed.