[roller]

I was sitting in a traffic jam this morning for about 45 minutes and noticed that the temperature guage dropped down to one bar.
Once i started moving again it very quickly went back up to its normal operating temperature
Spoke to my local dealer and they could not give an answer.
Could it be that the engine is so efficient on idle that it cannot produce enough heat to keep the temperature up?

[Johnno]

There's a couple of things you can check,

check coolant level, any signs of coolant leak and make sure the cap is tighten

did you have the heater on when this happened ? and if so did the heater go cooler, possible that the thermostat may have stuck open.

also it could have a air lock.

[Shambles]

Can't say mine has ever done that - dropping BY a single bar yes, but not DOWN to one bar

If the engine were as efficient as we'd all like, it would stay at the optimum operating temperature - 5 bars in my case.

I wonder if your pressure is dropping too low...

[roller]

Just been out and checked water level OK and no visible leaks and coolant level OK, will just have to wait and see

[Johnno]

How long was it from leaving your house and hitting the jam?

If it was only a short distance what could of happened is that the thermostat hadn't quite reaches it's operating temperature and while you was sat there the thermostat opened and the cooler coolant from the radiator enters the engine and that is why the temp gauge goes down.

You'll just have to keep a eye on it. Remember from the first time you start the engine  and the first recirculation of the coolant it will be a lot cooler than if you have been driving it for awhile.

maybe someone else can think of something

[roller]

I had done about 20 miles through london when i hit the jam, it was strange how quickly it warmed up again though

[eye30]

Stop start techology in play?

[roller]

it hasn't got the stop start plus it was 0 degrees so had the engine running the whole time

[Phil №❶]

At 0 Deg & at idle, not a lot of heat being produced, which is good as less fuel being consumed.

[Dazzler]

Running cold.. Definitely not a ford then

[PhireSideZA]

My 2c:

Looking at my OBD readout, my temp gauge only starts moving (I have a needle style gauge though) when the coolant nears the 50*C mark. So, my thinking is that the coolant running over your engine temp sensor could have been around 50 - 60*C which sounds quite likely given the cold weather outside? AFAIK the engine coolant sensor sits after the thermostat on the radiator side, so your thermostat could have been nearly closed and the coolant around the radiator side could have been chilling due to the ambient air temp?

It does sound a bit strange as the thermostat is supposed to keep the coolant in the idea range of around 85 - 90*C to optimize fuel atomisation (in petrol engines at least, not so sure about diesel) and to prevent excess unatomised fuel from making it's way past the rings and diluting the engine oil.

I am no mechanic so I could be wrong, this is what I have understood though

[Phil №❶]

In a Diesel engine, combustion can only occur if the air Fuel mix is raised by compression to the flash point of the fuel. So regardless of ambient temps, firing will only occur if this condition is met. Not sure exactly what that temp is, but the engine won't run unless that is reached inside the combustion chamber. However, the heat produced travels to cold parts of the engine and can be dissipated if it's quite cold, at idle.

Once driving, larger quantities of fuel are required to move the vehicle and revs increase, so there's more work being performed, which enables correct temp to be achieved and excess heat to be dealt with by the cooling system.