Investigating the Cooling Issue

TL;DR : If it sounds too good to be true, then it probably is

After the slight over-heating issues at the track day, I did eliminate the idea that it could come from the faulty oxygen sensor. Those were (are) totally independent problems.

With the oxygen sensor replaced once again, I came to understand and suspect that my car does not like cold weather. So I took the opportunity of a nice Spring weather to do various tests early in the morning or late in the evening with a cold weather (~5°C), and compare what happens in the afternoon when it's over 15°C.

On the first cold night, I got the coolant up to 110°C when briefly pushing hard. And got no issue in the next afternon.

On the next morning, we went back to the Fagnes with a mask covering the front grille. That somewhat improved things but the solution was not perfect either. It confirmed that the thermostat is "shocked" close as soon as it opens. The rest of the drive it must have remained closed as the cabin heater was mist likely cooling the engine enough. We were clothed appropriately to drive with the top open, with the heater and fan on full blast.

A good solution could be to discard the thermostat and replace it with a remote one on the hot side of the radiator. That Rover K-series remote housing from QED would fit nicely, replacing the "Y" piece of hose that feed the radiator and the return to the expansion tank.

House of Speed knows about the part but they don't keep it in stock. I did send an e-mail to QED and got no reply.

Anyway, recent blats with temperatures between 20 and 25°C confirmed I could hammer it like a mad man while keeping the engine at 82°C -  It does not need fixing before say next October.

The car would have 3 thermostat housings then : there's the standard R06A one in the cylinder head, not fully machined so I can't just fit a thermostat there;  there's the Caterham one on the radiator return; and there would be a hopefully 100% satisfying QED one on the hot side of the radiator.

As I heard that the expansion tank pressure relief cap could sometimes be an issue too, and as mine tends to spit quite a bit, I looked for a replacement one.

I first tried the easy and cheap way and received an allegedly legit' item with all the right part numbers written all over it, even in the plastic mould.

The valve was plastic and the expected rubber ring looked like hard plastic too. I got it on the car anyway, and drove a few times to work and back with it.

One evening, I decided that we should go for a cold drive again and see if there was any improvement. We were less than 2 km away when the coolant got already quite hot for the little power I had given it. I stopped, lifted one side of the bonnet, and discovered that the expansion tank was empty. We went back home right away, downhill with no gear but the engine kept idling to make sure I would somewhat cool the cylinder head and not blow the gasket.

With the sensor (on the empty thermostat housing) reading around 88°C I knew there was coolant in the cylinder head. With no coolant it would get really colder than that. I'd better see 115°C than 50°C at that moment.

There was no trace of spit or spilling around the cap. I can only assume the missing coolant got evaporated while gently commuting in the week (I cannot imagine it was stolen). I got about 3/4 liter of coolant in and the original cap back on the tank.

I didn't fit the hood back and we went on for over 2 hours late night driving, with the expansion tank in plain sight in front of me all the time. We got the usual slight over-heating when hammering by 5°C in the Ardenne area. No coolant was evaporated, spitted or otherwise consumed.