If you fill your radiator right up to the cap, you are actually overfilling the radiator. As the fluid in the radiator warms up, it will expand and pressure will build. When the pressure gets too high, the radiator cap will release the pressure and the coolant will leave the radiator through the overflow.

If you overflow into a sealed container, the pressure will build there and will cause the cap to burst (as you experienced).

However, none of this indicates that the car is overheating, or the coolant is boiling - this is exactly how the system is meant to work and protect itself from building up too much pressure in the cooling system.

After you've warmed the car up and let it cool, the fluid level in the radiator should be sitting at about the same height as the first row of radiator fins (or even just below). You should be able to see this if you look through the radiator cap.

I purchased my 89 Rover Mini Automatic in 2018 and since then I have had issues with coolant boiling and radiator overflowing. The radiator has been flushed, thermostat replaced and auxillary fan fitted etc. The engine doesn't seem to overheat but the coolant boils.

My engine runs at 4000 rpm @ 100 kmh, and it has been suggested that the engine is overheating the oil and thus the coolant, causing the issues.

My mechanic has suggested converting thev4 speed auto into a 5speed manual transmission to reduce thevworkload on the engine at speed.

Would this require engineer certificate or adjustment of rego detIls?

All thoughts or advice appreciated.

Cheers


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