Yesterday was hot for coastal Maine [80 + F] when I drove to Spitfire to a shop on the mainland for a radiator flush and parts replacement.
The mechanic removed the lower hose to drain the coolant [he didn't unscrew the drain on the block] and it looked quite awful, as you can see in the photo. He then used a garden hose with strong water pressure to flush out the cooling system, including opening the heater valve, and ran the hose for quite some time. He also cleaned out the overflow bottle. All parts of the system included a lot of brown silt.
He removed the thermostat and I decided to have him install a new 180 F one, as well as a new, 13 psi cap. He put in a fresh coolant mix, as per the owner's manual, and started up the car, running it for quite some time to get it warm and to circulate out as much of the air as possible.
When he was done I left and drove the car about 10 miles at 40 - 60 mph, with a lot of stop and go traffic. The gauge indicated running up to 3/4, which activated the fan, and then reducing it to a point between 1/2 and 3/4. At no time did it get above 3/4, and it always dropped whenever I started moving at speed. This was an improvement from what I noticed on my last long trip - and I never had to open the heater valve to reduce the temperature level.
An unrelated bonus was that no matter what the temperature, the car again returned 30 mpg in mixed driving.
The real test will come this weekend when I head to give a presentation in Vermont at a Land Rover rally. I'm taking the Spitfire because of the impossibility of getting the Land Rover on and off the ferry during this busy summer season. That's about a 4 hour drive each way and the mechanic did mention his concerns about a head gasket problem causing the higher running temperatures. The good news is that I'll be in a convoy with two Land Rovers so I can always get towed in a pinch!