My coolant temps on my VVC160 Spyder started going crazy. Going up to 120 on the gauge for a bit then dropping quickly back to 80, staying there for a few miles then quickly back up to 120, rinse and repeat. It never boiled over and I got back home without any additional drama though. I suspected the 'stat was sticking shut for a while then finally opening and maybe also sticking open too. My previous engines have all been traditional 'stat and have always sat rock solid at around 90. This engine came with the PRT and also sat steady at around 90 until this episode.

The later MGTF specs say the correct PRT is rated at 87 degrees. The only replacements I could find (other than expensive NOS genuine ones from Rimmers) are all rated at 82 and are listed generally for Land Rovers but "also fit MG TF". I went for one of those and fitted it today. It seems to have solved the erratic cooling issue but now my gauge is steady at just under 80 rather than 90. I guess running a bit cooler isn't a big problem but just wondering what everyone else is using and what temp readings you are seeing?

This isn't much help, but the first thing I'd do is not trust the gauge. I've been chasing V6 temperature puzzles and for the past couple of months have used an OBD dongle with my phone, which shows huge variation between gauge temp and OBD temp.

Can anyone confirm - where is the sender for the gauge, for either engine type? Does the ECU tend to have its own sender?

Both senders are in the same casting where the coolant flow exits the head. Blue connector is for the gauge, black (sometimes brown) is for the ECU. This on the 4cyl VVC, not sure where they are on the V6

It is common for separate senders for gauge and ECU.