Had this problem for a while, but me and my friend have done a lot of trouble shooting and research, so i think now would be an appropriate time to post about it.
I recently swapped my s4 rx7 fc N/A with a Turbo II motor. we have wired everything up, the car runs and drives perfectly, however the temp gauge isn't reading right, as in it will only go up to about a quarter then stop and not go up any further. We have checked the gauge itself and it works fine, and the sender has a strong contact with the wire, which brings the problem to the sender itself. We're not sure if the t2 sender has a different resistance to the N/A sender, but we think that may be a strong possibility.
Any advice is appreciated. sadly I cant just put the N/A sender back in because the t2 sender has a slightly larger thread size. so any ideas I can use to get around that would also be appreciated.