Fixing all your keys
AO Silver Member
Join Date: Mar 2001
Location: United Kingdom
County: Tyne and Wear
Re: Erractic Engine Temp
If you can see rad corrosion then it will be a lot worse than it looks.
However, what you describe is normal for this time of year, if a little more exaggerated on your car, possibly a sticky thermostat as ZF says, but it is working ok. The temp drop is caused by your thermostat opening. The temp sensor is right next to the thermostat opening so it is sensing a sudden rush of cold water.
So long as after this the temp rises up to normal and stays there with only minor fluctuations then everything is fine - although start to think about changing that rad.