I drive a 99 T Ford Focus 1.8 petrol and have noticed that when I drive somewhere (after engine has reached normal temperature) and I then switch off, when I turn the igntion key so that I can see what the position of the needle is on the temperature gauge I notice a slight fall even though you'd expect the engine to be the same temperature as it was 5 seconds before you switched the engine off !
Also, if I wait say 10-15 minutes after switching the engine off before checking the position of the temperature gauge needle, then this time the needle has fallen quite a long way from the centre towards the cold end.
I drove my sister's Ford Fiesta (56 reg, 1.25 petrol) and tried the same experiment by checking the needle position just after switching the engine off and found that the needle was still in the same position as when the engine was on. Secondly I checked the position of the needle about 35-40 minutes after turning the engine off and found that the needle had dropped to half way between normal (centre) position and the cold position. Both these observations are different to what I experience with my Focus, particularly the second where even after 10-15 minutes my needle has fallen to nearly the cold position.
How can this be explained technically and as I had the car for 3 years and cannot remember noticing this before does it indicate some problem that needs attention and what other symptoms whould the car have e.g. rapid or slow coolant loss, excessive oil consumption and are there any definite tests that can be done ?
Also I was on a coach and noticed that the temperature gauge needle whilst the engine was at normal operating temperature was bang on the centre mark, between cold and hot. This contrasts with my own car in that on mine the needle is slightly under the centre position and never reaches it even when at full temperature.
Is there any reason why a car that previously had a guage that was bang on the centre mark to drop because of a fault ?