Td4 erratic temperature gauge readings.

Hi all, I have a td4 55 plate. When I bought my freelander the temperature gauge never went above quarter and I got no heat from the heater ( short test drive and I never noticed) so I fitted an inline thermostat mod and all seemed to be ok as the needle sat just below half and I had plenty of heat from my heater which was brilliant as it was winter. But as the summer months came along my troubles started and the temperature gauge started shooting up to the red line very quickly( within about 4/5 seconds) and it was very random I could drive 5 hours up to Scotland and not have a problem but a 20 mile drive and it would keep shooting up, as soon as I put my heater onto full the temperature drops just as quick ( 6/7 seconds). I never lost water until last week and needed to put a litre in to top it up.
As you can probably tell by my description of the problem I am no mechanic but I fail to see how an overheated engine can cool that quick?
Any advice would be greatly appreciated.

Hi and welcome to the forum
Best to ask in the freelander section of the forum.

Welcome to the forum

Freelander section over ere:

https://www.landyzone.co.uk/forum/land-rover-freelander.9

hopefully small issue like temp sensor or stat. Guys n gals in FL section will know

Welcome to LandyZone

I have no idea that how to measure erratic temperature with gauge along with the exact reading of the meter. Can you please make it a little easy for me and summarize this. I have Essaygeeks for those students who find assignment writing a quite difficult task. You can grab this for amazing content.

Take your modded stat out see if it cures.

as the engine temp builds up from cold the temp under the stat eventually causes it to open but then as circulation begins the stat should close again slightly to regulate temperature. it doesn't sound like yours is closing until the temp is much cooler, maybe even after switch off.