Howdy RR Hive Mind,

My 2002 P38 4.0 Thor has very recently developed a wild water temp fluctuation. Suddenly the gauge (that has remained stoically at mid point the whole 5+ years of owenership ) flies into the red - with the red warning light coming on almost before it gets up there. A few mins or sometimes even seconds later it then immediately returns to mid-point. I can prompt this instant cool down with some extra revving or accelerating, but not always, so it's not necessarily the reason it changes back.

I'm thinking it must be the sensor - as the immediate return on the gauge seems unlikely if actually getting that hot - but wanted to run it past 'the experts' before charging into a parts search.

Thanks

Assuming you haven't been losing coolant and regularly topping up so you've ended up with an air lock that is moving around inside the system, then it could be a sender problem. If you have diagnostics, even a cheapo Chinese generic code reader that can do live data, use that to monitor the coolant temperature. The earlier GEMS used two separate sensors, one to report coolant temperature to the engine ECU and one to drive the dashboard gauge. Your later Thor has a single sensor that is actually 2 in the one housing. An OBD reader will display the temperature according to the engine ECU, not the reading according to the gauge sender. That way you will be able to see if it really is getting hot or just the gauge telling you it is when it isn't.