Further to my recent thread, post head gasket replacement with coolant level going UP and faulty coolant caps, things seemed to be going ok.
Today I was on the motorway and of course being glued to the temperature gauge, I noticed as soon as it started to go up above normal. Fortunately I was near an exit junction and was able to merge into the first lane and slow to 60 and put the cabin heater on full. As soon as I did so, the temperature dropped back down to normal. It did get to about 3/4 for a few seconds. I pulled over as soon as I could and checked the coolant tank. The level was normal but I could hear a slight hissing. I (safely) checked the coolant cap and it was very loose. I tightened it back up and drove home (about 15 miles) and everything was fine and the cap remained tight, although I only did a couple of miles at 70mph, the remainder being 50/60.

So can the coolant cap work itself loose like that? Perhaps I erred on not over-tightening it and it just needed to be tighter. Or could something else still be wrong and caused this to happen? I.e. will it happen again??

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