Technical Overheating

[stocky]

So i went for a drive a while back and the water temp light came on after about half an hour. Managed to get home and checked the rad air vent. It had been set in the open position. I removed the temp rod in the bottom of the radiator and tested it. It didn't move in boiling water so i ordered a new one at great expense £170.
Tested it when arrived and it worked ok. Fitted it and reset the air vent. Been for a couple of rides since and noticed the plungers not opening the bent when the cars hot. Removed temp rod and checked again, opens about an inch when water comes to boil.
My question is why is it not opening once cars up to temp or does it only open in extreme circumstances?
Im worried that if I go for a decent drive the car may overheat, anyone any ideas please.
What should I check next?

 

[stocky]

So today I drained and flushed the rad and block added stp flush, ran for half an hour, drained and throughly flushed. All the time running, high temp light never came on and thermostat never opened. I'm assuming this means I don't have a problem now?

 

[summersc]

Is your engine behaving like it’s over temp? I am wondering if your temp sensor is acting up.

 

[stocky]

No seems to be running fine this after half an hour

 

[summersc]

Hey @stocky did you ever get to the bottom of this?

 

[stocky]

Not fully, been for short drives since and its not done it but when I've looked under the vent hadnt opened, must be as it didn't need to as I know it now works. When i got the car it was jammed open. Got an mgb so it's been put away for winter now.
It was defo over temp when it happened, I flushed rad through, put in the new sensor and changed oil. My thoughts were that as it was jammed open a new sensor wasn't going to solve but it's been ok so far.