Bizarre overheating coolant loss issue.

Bizarre overheating coolant loss issue.

So the MGTF I bought at rock bottom had a proviso that it was overheating.

So I went through the system with a fine toothed comb.

First of all the inlet manifold gasket was replaced as half the bolts were only finger tight.

Then I found one of the plastic T-joints had split so was loosing pressure and I replaced that with a brass fitting.

Then I replaced the cap.

Then I bled the system ...TWICE.

This seemed to get rid of everything.

What the symptoms were, and still are, is that the stat sits around 94C on the motorway after a slow warmup (only a few minutes).

But then if I'm driving above 70 for any extended period of time it will sometimes shoot up to 120 and vapour lock.

At that point I'm having to add around 1-2 litres to refill.

Now this indicates to me either the cap is not working (it's brand new), there's an air leak somewhere causing pressure loss, another coolant leak somewhere I can't see, and/or the PRT is knackered.

After bleeding I can rag the car to pieces and no overheating but it seems to happen after about 1-2 weeks of long distance driving with no obvious signs of coolant loss in the tank.

Any ideas? I'm guessing another possible corroded plastic connector or a sludged up radiator causing insufficient cooling.

I have yet to flush the coolant system and replace the PRT (which I have a spare for already), and I really don't think it's HGF for various reasons, though it will end up happening at this rate through heat cycling if I don't sort this issue out.

Are there any common but difficult to get at areas of leakage (the under pipes look fine to me)

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Bizarre overheating coolant loss issue.