Following a gearbox change on my 1.6 K-Series, I was seeing much higher readings than usual on the coolant temperature gauge. From starting the engine the temperature gauge would climb to around 90C, stay there for a while and then head on up to around 115-120C at which point the fan would cut in and bring the temperature down to about 100C. Even with a constant flow of air at motorway speeds the temperature wouldn't drop much below 95C. I reviewed lots of useful threads which boiled down to it likely being an airlock, thermostat, sender or gauge issue. This in particular was a very useful thread: /forum/techtalk/coolant-temperature-question and I can confirm that gauge 71229 is indeed a suitable replacement for 71164. However, the one very interesting "feature" I noted (which nobody else seems to have reported) is that the gauge would read around 10-20C higher with the engine running than with the engine stopped. I still don't have a good explanation as to how that might happen so answers on a postcard please! Anyway, to cut a long story short (and after some very astute advice from Rob Clay) the issue was traced to a missing earth connection between the engine mount and chassis on the right hand side of the engine. I'm kind of surprised that was the only symptom, but once the earth strap was reconnected readings returned to normal throughout the temperature cycle, so big sigh of relief! Other useful things I found out: It's quite normal for my IR thermometer to read 103-107C on the water rail of a K-Series next to the temperature sender when the fan comes on, and 93-97C when the fan switches off. IR thermometers tend to read low on shiny surfaces, so stick some masking tape on them before take a reading.