Coolant temp gauge going off the scale with power off

[Kai Foyen]

I was recently flying at 3000ft with a passenger and decided to drop down to around 1000ft fairly quickly to get a better view of the coastline so came off the power to do some side-slipping. When I did, the water temp gauge climbed incredibly quickly off the scale. Alarmed by this, I came back on the power and it just as quickly dropped to normal again. It rose and fell inversely about the same speed as the rev counter.
Keeping an eye on the water temp and egt gauges, I flew back to the airfield. I tested coming off the power a couple more times over 15 minutes with the same result, although there was never any increase in egt.
I landed safely but, whilst on the ground, I noticed that the issue had disappeared. I have recently flushed the radiator and changed the coolant but have flown for 2hours since then with no issues.
The engine is a rotax 582 grey-top on a Quantum.
Any ideas for this engine novice?

[Chris Boswell]

Hi Kai
I fly a Quantum 582 grey top etc, I noticed a while back on mine that when I’m descending with power off/reduced power etc that coolant temp gauge fluctuates higher then returns to normal once power back on, prob not off the scale though, think all you can do is make sure no leaks or air in system, expansion chamber at correct level and cap fitted tight not worn, they all can be bit fickle and you tend to get to know your own aircraft and it’s own quirks,maybe try more gradual descents under power to see what gauge is doing and observe different readings for different scenarios, maybe someone else will come up with a more technical answer but hope that helps a bit!
Chris

[Pete Twissell]

If the temp gauge is moving as quickly as the rpm counter, I would suspect an electrical issue. It is unlikely that the coolant temperature could really change as quickly as that.