TD5 cooling puzzle...

Eyup all!
This is a bit of an odd one.
Er indoors has a 52plate Disco2 that she loves. To be fair, it is a nice motor. Had all the usual bits, 3 amigos, injector seals etc.
Last night we were out for a meal in a country pub, as you do on a summer saturday night & the poor old Disco threw a bit of a wobbler.
Booting it uphill & the temp gauge dropped to blue. Panic stations! no water thinks I. Stop PDQ & lift the bonnet. Water ok.... Odd. Turn the heater to hot & it was. Went on steady & had our meal & let the old gal cool down. Checked under the bonnet again & still got water. Raised the header tank & cracked the bleed, got a few bubbles but nothing to speak of. Set off home with the heater helping us to sweat off some calories we had just put on.
This morning, went to it, checked all good & hit the button. Sparked into life without a care in the world. Stopped it & did a full rave recomended bleed. Started & waited. Temp does not move but heater warms up. Set off on a test run & gauge doesnt move but heater hot. Get back & turn off. Have a brerw then go for another look. Turn the key & temp says normal & heater is hot..... start up & temp drops but heater stays just as hot as when the gauge read normal.
So. After much head scratching & getting far to many splinters under my finger nails from doing so, its time to call on the collective genius`s in Landyzone.
Any ideas? Thermostat? Temp sender? Head gasket? water pump? I would even go as far as changing all the wheel bearings if i thought that would fix it.... Can you feel the desperation here?...
Thanks all in advance.

if its not loosing coolant or has any air and all hoses and rad are reasonably equally hot after its warmed up, it sounds more like and electrical issue, id start with sender

+1 for the temp sensor... better that than it's wiring somewhere, put a new one to be 100% ruled out and if no joy we'll speak then

Right ho! New temp sensor it is then..... Once i find it that is.

it's in the elbow where the top hose meets the engine

Perfect! Will order one first thing & report back A.S.A.P.

Right then!.
Temp sensor fitted & all good....sort of... Within a couple of mins of start. Temp gets to first notch above the blue. Sits there for about 10 mins then shoots up to normal horizontal. Damned odd....
Anyways, thanks all thus far!

That sounds about right, especially if you are not travelling. Can be even longer in the cold weather and you have the heater on.
Griff

Temp gauge will rise when coolant temp reaches 85degrees or some such figure.
Mine does the same as yours
Sits above the blue till up to temp then shoots to half way

that's how the gauge is conceived to work:
1. at 40*C goes to the first level and stays there up to 70
2. at 70 goes straight to the middle and stay there up to 120 !!!
3. at 120 goes up to the red zone
that's why a coolant gauge is important, cos without it you can drive it at 115*C for long without even knowing it and that's not healthy at all.

Interesting stuff indeed! Guess an old style capilary gauge is in order. Thanks all for your help with this one.

it's enough to fit one like this http://www.ebay.co.uk/itm/190995352666?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT , i have it and it's quite accurate(checked against tester), no need for any fitting of cutting hoses, i drilled into the elbow and cut thread in it then used HT loctite no leak at all...off course clog the elbow with something as to not let anything fall into the engine while you work on it
BTW...you need this too http://www.ebay.co.uk/itm/Instrumen...g-Pod-Black-/291478201806?hash=item43dd7465ce , or other kind of pod as you like, the market is full