carritts

Gareth, I tested it in boiling water. It opened fully and closed again. Not massively scientific,  but good enough. 

Thermostat is good. I've decided to change the sender as it's cheap and looks pretty straightforward to fit. I'll keep you posted.

Thanks Gareth, I'll ponder some more as a thermostat change is a pain, but I'll check the removed thermostat. I don't lose any coolant.  Cheers 

Thanks Gareth, I've checked the pump out by disconnecting the return feed and monitoring flow. It looks good as there is flow and flow increases with revs. I am now at a complete loss. Bugger.

Thanks for the response. I can't recall the makes, but to have exactly the same issue with both thermostats indicates it's not a thermostat issue. Also, given that the temperature reading is stable after the initial overheat and that the issue is intermittent,  I'm very doubtful that it is a sender or gauge issue. The only plausible cause is as I've stated and this was suggested by a local mechanic.  I was really looking for someone with a similar experience and a solution?

Thanks again. 

I have an annoying overheating problem. Often, the car warms up from cold very quickly and goes straight into the red zone after approximately 5 minutes. It then stays there for the next 10/15 minutes before dropping down to normal temperature and not budging,. It doesn't do this again in the same day; despite being sat for several hours. I've done the obvious and changed the thermostat (twice). I was advised that it could be the water pump. Apparently, some have a plastic impeller that can slip on its shaft. This sounds feasible. Has anyone else had this issue? It's a big job to change, so I don't want to do it unless someone has had the same experience? Thanks.