On Alfa number 15
AO Platinum Member
Join Date: Dec 2005
Location: United Kingdom
The temperature sender doesn't make the thermostat open, it opens on its own due to heat.
It sounds to me as if your thermostat is stuck open though, you will need a new one. It is a very common problem.
The job is more difficult than on most cars, the thermostat is part of a housing that attaches to the cylinder head on the right hand end.
You need to remove several hoses before you can remove it, but as long as you remove the original clips and replace with jubilee clips it isn't too difficult.
The sensor itself screws into the thermostat housing, and will need to be swapped over to the new stat. They only cost around £20 all in from somewhere like shop4parts, where as the thermostat will be around £40 all in.
You will never be cool driving a Vauxhall.