Over heating or sensor + how to check fan and water pump working?

Not sure if the Landy is overheating or if it's just the sensor.

I fitted a numerical Tim temp gauge and a new thermostat (the previous thermostat was stuck open).

Currently touring Skye and the west coast. It's currently 26 degrees and it's very hilly and the land rover is packed full of stuff.

It normally sits at around 80 and when working hard goes up to just below 100. That's all fine. But it now also suddenly shoots off the scale, I.e. to 120 then suddenly drops back to 100. A few times its shot up to 120 for a few seconds then dropped to 80 all in about 5 seconds.

It suddenly jumped to 120, so I pulled over, I opened the expansion tank carefully, I know you shouldn't but I felt that if it really was at 120 it would boil over and I'd know for sure. Nothing boiled over and the water in the expansion tank was just slightly warm. The engine was hot but not 120 degrees to the incredibly accurate instrument that is my hand.

It just seems like a dodgy sensory to me, because I would have thought that the engine warms up and cools down a bit slower than these sudden changes? I would have thought if it was really over heating then the temp would rise slowly and then take a while to cool down. Surely it can't rise to 120 then drop back down to 90 within a matter of a few seconds?

If it is overheating I'm trying to work out what the cause is. The radiator top hose is nice and hot and slightly pressurised and the bottom hose is cool, with a temp gradient down the radiator. That suggests the radiator and thermostat are working and also the water pump otherwise water wouldn't be circulating?

The fan spins when the engine runs.

How do I check the water pump and fan are definitely functioning correctly?

Going to check the earths of course and I don't have a thermometer otherwise I'd check that way.

Thanks for advice in advance. If I don't reply to questions or advice quickly it's because I'm on the west coast with little signal! It's heavenly btw!

Sounds like a dodgy gauge or wiring as I wouldn't expect large temperature changes to happen In a matter of seconds and then go back. If the heater works then that is a good indication that the pump is working. I would suspect a chaffed wire as that will cause the temperature reading to increase when shorting to earth.

Wire might have melted on the head ?, chaffed where it goes through the bulkhead ? Worth a quick look.

TIM mechanical or electric gauge? either way not sounding good so your heater is your best "off grid" gauge - keep heater on full, if it's blowing hot/warm all's good - if it goes stony cold pull over! Good luck and have fun................

Thanks for the input everyone. It's a TIM mechanical gauge. The heater is producing hot air, only issue is I think motor in the heater is kaput as it sort of just puffs out hot air from all vents and the power knob does nothing.

I'm managing to keep within normal limits at the mo. It's probably been above 100 degrees for a cumulative total of 5mins. Let's say that is a true reading, is that long enough to have done any lasting damage? Should I prophylaticly change the head gasket when I get back?

Will post photos when I'm back! Anyone in Scotland make the most of this weather it doesn't happen often!

Weird................. irregularities with mech gauges are mighty rare - simple because they're mechanical [basically work on laws of physics] are you certain the "line" has not been cut or kinked anywhere? be sure nothing can make occasional contact with exhaust manifold etc.? How old is the gauge? maybe you've got a dodgy one...................... when back at base whip out the sender/bulb, stick it in a cup of recently boiled water along with a thermometer you trust and see if they agree..............
5 mins or so above 100 degrees wouldn't worry me - good luck

ps. lazy fan doesn't matter - weird thing is when a motor is boiling heater is COLD!!!!

Ta very much. Yeh heaters hot and the pipes too it are as hot as the engine.

Thanks for all the help!