Coolant leak and Overheating?
 

the thermostat seal is weeping something terrible, losing about 500ml for 50-100 miles, so keeps needing topping up until the part arrives. My partner uses the car for work -im just the tech!- and didnt top it up at all last week, then complained the heater was cold and I had to top it up about a litre and a half.

What worried me is that before topping up the temperature gauge was at 9 oclock and not red light, but the dash test said the coolant was 125degrees c.

Surely at that temperature the needle should have gone beyond 9 o clock?

If you let the temperature go like that a few more times you are going to RUIN your engine.--:eek:

At that temperature the light was probably just about to come on or maybe the light itself has failed. You are heading for a much, much greater bill than just replacing that leaking seal.----------:duh::duh::duh:

KEEP that rad topped up or soon you will be walking or travelling on the bus.---:eek::eek::eek:

As I thought, my partner was watching the needle but as it hadnt changed didnt think anything was wrong. What controls the needle out of interest?
Since seeing that temperature I have firmly made it clear that the coolant must be topped up everyday before coming home until the thermostat is replaced...

You have a first class spanner wielding magician (shiner)living just down the road buddy :D
Regards
Chris

The gauge is supposed to begin rising above 'normal' once the coolant temperature exceeds 115C. So yes, you would have expected some significant shift at 125C (OBD). The OBD and the gauge use the same data so it's not that one is right and one is wrong. But exactly why it didn't move I haven't a clue at present. The red light will pop on at around 127-130C according to the MGR thermal profile. I doubt a few seconds at 125C is anything to worry about. On a diesel I think you're pretty safe. Most regular head gaskets need rather more to exceed service specification. I once had a new head fitted to a Rover and there was a thermal sticker on it for warranty purposes. It permanently changed colour when heated. The in-warranty level was 140C maximum.

TC