Overheating/faulty thermostat/temperature sensor?

[JoeyD]

Basically, my car gets up to normal operating temperature fine - stays at half way when cruising at 60 etc. But when I slow down to 2nd/1st/Reverse gear the temperature gauge randomly goes up, sometimes all the way to maximum (with engine STOP lights on the dash) but then as soon as it's gone up, it returns back to normal temperature.
I'm fairly certain the car isn't overheating - not even close to doing this, I'd just like to know where the fault lies...

Any help appreciated!

(1.2 16v BTW)

 

[JoeyD]

More info: when it does "overheat" the in car heater goes stone cold. And also the temperature gauge sometimes ventures below normal operating temperature... so would that be a thermostat problem?

 

[MicKPM]

Nope - the fact the heater temp rapidly changes also leads me to think you've got water pump problems.

 

[JoeyD]

I'll check for any leaks etc, but surely it would be more prone to overheating when the car's being worked hard, rather than doing 10/15mph in 1st..
It seems really inconsistent too, this morning it 'overheated' for 2 seconds and sorted itself out, then this afternoon it was completely fine.

 

[regi james]

if its changing that quick you could have an earth fault or blocked coolant hose causing the temp sender to give an incorrect reading, try put load on the electrics like opening an electric window and see if it moves

 

[JoeyD]

Will give that a go!

Could it be affected if I've gotten snow or moisture in a connection somewhere?
I'm more convinced it's electrical tbh - the engine doesn't feel like it gets hot enough to be actually overheating...

 

[regi james]

it sounds like an electrical problem apart from the heater going cold unless u have climate control?

 

[JoeyD]

Nope, just a regular heater.
It goes cold for about a minute - then back to normal...

 

[JoeyD]

There are a few white water marks around the engine, but that could just be from salty roads tbh...
Surely if the engine was actually overheating the temp. gauge would go up and stay up? Not go up, immediately come back down and repeat a few minutes later...

 

[JoeyD]

Coolant level has gone down quite a lot! Looks like its water pump time.

Thanks for your help!