Another temp gauge problem!!!!!

[turbo.jon]

Hi

I have done a search and cant find the same thing im having so here goes:

Few months ago my temp gauge started playing up would go up to 90 then drop right down to 0 and would be all over the place then it packed up all together. so i done the normal route change temp sender to see if that sorted it which on the test run seamed to sort it but next morning was back to playing up.so last weekend fitted a new thermostat which again was fine to start with now im getting the following. i drive to work in morning which is 10 miles away it heats up fine sits at 90,have hot air from heaters but when i get to put my foot down on open roads temp gauge goes all over the place but never over 90. i have checked the hoses and the one with temp sensor in is very hot and the other one connected to thermostat is cold/warm.all parts fitted are gen vw any ideas as its driving me mad

[Hulmie]

Check if the temp sensor is clipped in properly. If not it may unseat itself when you put your foot down. Also check the wires that connect to it to make sure theres no splits in them.

[turbo.jon]

been out having a look and sensor is connected fine to me it seams like thermostat is not opening which is why hose is not as hot as the others.If i let car sit there running it will go up and sit at 90 but when i check hoses the thermostat one is still cold or slightly warm.now dont no if this is whats happening car heats up to 90 thermostat should open but is not,temp gauge is reading 90 as the pipe the sensor is in is at 90.i then get on open road because im then going faster the rad is cooling the water to much as its not cooling whole system as thermostat stuck? or could it be an air lock but i have hot air in heaters ?

[turbo.jon]

was bugging me so just been taken thermostat off to test (only took 10 min  ) boiled the kettle dropped it in and nothing does not open also tested my old one and that only opens a tiny bit.so now got go find a new one from some where thats open 

[Hulmie]

did you buy a genuine VW Thermostat mate?