Temperature gauge problem 2009 Peugeot 207

[regsmyth]

Ok, so an update.
I got a new thermostat, like the one in the video J2007 linked to.
I watched an Autodoc video on it, and got stuck in...
It went quite smoothly till trying to pull off the last connection, which took over an hour!
I then managed to drop the 3/4 spring clip while putting in the new thermostat, it disappeared into the depths of the engine and it took another hour to find it.
Eventually all was reconnected, and I took it for a gentle 5 miles. Gauge began to rise and settled at 90. Perfect.

However this morning I got a warning "Anti Pollution Faulty" when I started it. I have no idea what the issue is, but I guess it now needs a diagnostic from a garage.
Went for a longer drive with some hills, and was surprised to see the gauge drop down as low as 70 on hills or with hard acceleration, then rise back to 90 in "normal" driving. I'm not sure if that's normal, (in my car it stays at 90 all the time) and I don't seem to be losing any coolant. Heater is working fine, and fan cuts in when idling when gauge rises to about 95.
So one problem solved, and another arises. Who'd have thought?

 

[johnny2007]

Temperature sensor?
It's usually more towards the top of the engine and cheap.

 

[Harry Bloomfield]

The heater does blow hot air, although I'm not sure it's as hot as it should be.
I think the symptoms I'm getting are indicating a "stuck open" thermostat.

Simple to test basic operation. From cold, start the engine and run it whilst continually feeling the top hose with your hand. It should remain cold, then quite suddenly get hot - that means it is working. If instead, it just gradually gets warm, the state has failed open.

 

[carmanmemoranda]

Before doing anything, let the car cool overnight and recheck coolant level.

Could just be an airlock. Think there might be 2 bleed valves n that system ? If so, have you bled them ?

 

[Harry Bloomfield]

Went for a longer drive with some hills, and was surprised to see the gauge drop down as low as 70 on hills or with hard acceleration, then rise back to 90 in "normal" driving.

That would suggest to me, that either the coolant level is too low, or there is an airlock - either way, it is allowing the sensor to not be in the coolant.

 

[regsmyth]

The thermostat I fitted is new, and it comes with a new sensor.
There is a bleed valve in the thermostat and I have checked this a couple of times, and it's not got air in it.
There is also a bleed valve on the heater matrix pipe - checked and bled.
The thermostat allows water into the top radiator pipe, but not the return from the radiator, so the top pipe does get warm even when the thermostat is closed.
The fluctuating gauge reading is puzzling.
Coolant level after a good drive today is still good. Will check again tomorrow.

Thanks for all the input.