300Tdi Temperature Issue

My recently purchased Defender 300 Tdi seems to have a cooling issue.

After 15 minutes of steady driving the temperature gauge shoots from the middle into the red where it stays for about a minute before returning to the middle. This gauge fluctuation continues to happen until I shut off the engine. The engine does not seem to be overheating and the expansion tank does not appear to be especially hot. There is no steam or coolant loss. The expansion tank was recently replaced by the previous owner. The fan does run constantly from start up to shutdown.

Any ideas what I could do to remedy this?

Sounds like the temperature sender is either intermittently shorting out or more likely you have a chaffed wire to the sender. If it shorts to earth (or the engine block) the needle will go to full scale. Fan always runs as it is driven by the engine rather than being electrical. When its cold turn the ignition on and get someone to watch the gauge while you wiggle wires, if it starts moving you will be closing in on where the chafe is.

When I got mine it would do that when I truned the lights on.

Earth was dodgy.

Cheers