2.5 n/a D temperature fluctuation

Sudden temperature changes in engines in my experience have always been due to thermostats. maybe yours is opening late. Try taking it out and go for a drive and see if it now runs cool. If so then its not a serious problem causing it to get hot. If you do replace the 'stat give it a flush and new coolant too, while you are at it!

My 2.5 currently has a thermostat which is stuck open and the gauge hardly registers anything until i go at 50 or 60 for 10 minutes or so.

have your radiator fins rotted away?

My first two ideas are either a sticking thermostat or a sticking gauge. You might want to also check for air locks or low fluid level.

When it's hot, turn it off and see if the gauge falls smoothly back to the cold position. If it does then the gauge is probably ok.

My 2.5 n/a runs around half on the guage all the time, slightly below half on regular 50mph ish, if I'm gunning it (65-70 on motorways) the guage will be slightly over half, ditto if I'm driving enthusiastically off-road or on windy backroads.

I would expect you to be running hotter at 70mph because a 2.5 n/a is working very hard there.

Faced with your symptoms I'd overhaul the cooling system, drain and flush the block and radiator, new thermostat, nice new 50/50 coolant mix (10.8 litres) and if that didn't solve the behaviour and the radiator didn't look too rough (missing fins etc) then I'd look at the sensor for the temp guage.

in my humble opinion, if following my advice kills your engine it's not my fault, etc etc etc.

Regards. Mark.

Thanks for the advice chaps. It only started doing it this week. Temp gauge is okay. Fluid level is okay, no fluid loss or airlocks. Probably the thermostat then, which was my initial thought. Many thanks for your help everyone. Very much appreciated and reassuring!

Stew.

I have the same problem and as mentioned previously my radiator fins have rotted away to nothing. It does not leak, but the temperature gets near red at speed. Slowing down, the temp comes down. Yours Gmacz

Found the fault chaps. Me bloomin water pump is knackered!!! I pick up the replacement in the morning!!

Cheers, Stew.

I changed my water pump when I had the timing belt done, the old one was starting to get a little noisy, when I tool it out the fins were about half the size of the ones on the replacement... :oO

Regards. Mark.