[Mk2] [Turbo] Strange coolant temp problem

[TheRoadWarrior]

Not been around on here for a while so hi all!

Backgound; i've got a stack system in my car which senses coolant temperature with a sensor probe tapped into the coolant neck bolted to the engine (Not the one with the thermostat in, the one on the other side of the engine where the standard coolant sensors are)

I had some clutch work done at a reputable independent recently, when i picked the car up i noticed something strange with the coolant temperature- it seemed to get up to temperature ok (84degs) but if i boosted the car the temperature would drop down to perhaps 50 or 60 degrees very quickly, like in a second or 2. Also when driving at higher speed on a motorway (~70) it would struggle to maintain the standard temperature, however if i slowed down to around 60 it seemed to maintain around 80degs. I checked the coolant level and it was right at the top of the filler neck. They told me they'd not touched the coolant system and i've got no reason to disbelive this.

Now to me this sounded like the thermostat sticking open, however i'm not a believer in coincidences so i figured it might be something related to the clutch work i'd had done- perhaps the sensor was damaged some how? On the other hand it has been colder recently which would show this sort of problem up.

Anyway, to be safe i decided to change the thermostat and gasket which i did today. The previous one came out fine (It'd been in there less than a year) and looked as new, the gasket wasnt damaged at all. However the problem seems to still be there... it'll maintain 84degs fine under normal town type driving, but when i boost the temperature rapidly drops down.

I'm wondering could this be a problem with air in the system? Perhaps under boost air is forced around the coolant sensor leading it to see a much lower temperature?

I'm worried in the cold weather the car is over cooling so i need to sort this, but what can i try? I've bled the system as best i can.

[JJ]

Realistically, I dont see an issue in this as it proves that when you're ringing it, its getting the flow of coolant through it....

Things you need to consider that the coolant around the rad is chilled a lot more this time or year, so you're going to get colder coolant passing through the block.... you're not really going to get a static temp from the coolant coming out of the engine when kicking it.... which means your probe in the neck will read whats being pulled / pushed through.. more stable at cruising speeds etc...

Did you fit this during the temp probe at the start of spring / summer... coolant temp will be more stable ....

If you had air in the system, this would be noticable in the filler neck on a cold start up.. if you've got a large cavity or needing to top up....

Look at the temp issue in another way :

ie. If you ran a water flow of 10 ltrs per min through a boiler ( inlet ) the outlet would be heated to 50 degrees... if you increased the flow to 20 litrs per min, the outlet temp would suffer and would decrease.... Depending on how long the systems been running, eventually, the whole system would be heated ( more stable temp as a whole ) as it doesn't have a cooling medium.

Coincidence with the clutch work or with the seasonal changes ?