Having just replaced my TD6 main thermostat (as the Temp.Gauge had not moved above the 10 to position for the past six months) I was feeling some what smug as all returned to normal in the Temp. department. The Gauge climbing to mid position and staying there on a local 10 mile test run. Sadly my euphoria was short lived. Yesterday I visited Family which involved 150 miles on the Motorway. As soon as 60 to 70 mph was reached the temp. gauge started to drop and settled just above the Blue for the entire motorway part of the trip. Once back to trundling speeds (30 - 50mph) the gauge returned to the mid position and all seemed normal. Am absolutely at a loss as to what might be causing this problem. The Viscous Fan seems to be operating correctly (although it's impossible to check what's going on in the higher rev range). Could change the temp sensor and or water pump?! Any ideas please. . .

Hi sarn, my temp gauge never goes much past 10 to neither, it may sometimes reach 5 to but very rarely gets to 12 o'clock position, maybe only in the summer or uphills. I believe the egr thermostat maybe the problem as other's have had success when changing it.
Was going to have a go at mine when the weather gets better.

^^^ +1  .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

^^^+2 on the EGR 'thermostat'. Have seen quite a few threads on this subject so would be interested to see a definitive answer. My temp gauge is all over the place at the moment especially with the weather we're having at the moment, never gets above 1/2 way (12 O'Clock,) but in the summer it stays steady and the viscous fan runs (like a jet fighter's afterburner, or so it sounds!) when it gets above 2 O'Clock.

A real puzzle, but if the engine is running cool, is it a problem?

Thanks guys but should have stated ERG thermostat and cooler system completely removed on my RR and exhaust manifold port blanked,along with relavent coolant pipes either side of the water pump.

That is more than likely your problem, if you have removed the EGR thermostat and just linked the pipes together there will be a constant flow to the rad, if you still have the stat it might be worth putting it back in just to see if that cures it.

I haven't messed about with any of the EGR system other than removing the EGR it's self, and mine runs at a normal temp all the time. I'm not sure what symptoms a stuck shut EGR stat causes and whether it's enough to over heat the engine, but stuck open will cause over cooling  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

Hi miggit just to clarify I actually blanked off the two coolant stub pipes connected to the EGR thermostat / exhaust cooler. I would have hoped this would have the same effect as a permanently closed EGR thermostat. I did not make the mistake of linking them.

In which case you is showing the signs of an open / missing thermostat

If an engine never gets hot then the only thing that can cause it is a defective thermostat, or something has bypassed it. Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

New main thermostat fitted - all fine running round locally,gauge at 12 o,clock. Only when I hit the motorway and wind it up does the temp gauge start to drop to almost in the blue. Once slowed down to B road speeds all fine again !!!

Still recon it's the thermostat, as that's the thing that regulates the engine temp. I suppose that there is a possibility that the viscous fan has seized up and it's engaged all the time, that could be causing the engine to get over cooled at speed, but the thermostat should be able to compensate for that.

The way the system is supposed to work is the thermostat restricts the flow of water from the engine to the rad until the water is over 88, then it opens up letting the water in to the rad so it can be cooled. When it's dropped below 88 the thermostat closes allowing the engine / water to heat up again. It really does sound like yours has jammed open, possibly not fully, but open all the same. And it's restricting the water flow enough for the temp to get to 70 when there is little or no air flow through the rad, but at speed there is too much cooling being done by the rad and the temp drops. I realies that you have fitted a new thermostat, but just because it's new doesn't necessary mean it's working correctly.... I've fitted dud spark plugs before.... unless you dipped it in some hot water and saw it open and close there is no way that you can 100% say it's working

And while I do recognise that it's a major source of , it is however the most likely cause  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

I did give an uneducated warning about this over here: http://www.fullfatrr.com/forum/topic36076.html, which was shot down by others, but seem to have been proven right  .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

But Wait, I have discovered more

There is another thermostat.... for the oil.... it can cause under heating issues too

http://www.fullfatrr.com/forum/topic30370.html Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

Has anyone considered the fluctuating coolant temperatures experienced by the OP might be down to low coolant level or air locking of the system. The symptoms he describes are exactly what used to happen to diesel Discovery 1's when they suffered low coolant level due to leaking head gasket. Not suggesting head gasket is in any way leaking but the system appears to have been refilled after work on it.

Thanks to everyone who contributed to this discussion - A lot to think about here with so many possibilities. Think the first thing I will try is swoping the main thermostat for a 100% original LR part, then leave off the viscous fan and test run the car under all conditions. If all ok will replace fan and re run test having made sure system is fully bled of air. Will report back in due course unless I have committed suicide.

Just in case you didn't know there are 3 bleed screws for the coolant system, the header tank, the coolant rail and the EGR thermostat, D'OH!

Just in case you didn't know there are 2 bleed screws for the coolant ..........  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!