| Right, first you need to confirm where the problem is coming from. You need to check the rad cap to make sure its working correctly and not damaged or the wrong one. You need to also confirm that the rad is in good condition and not gunked up which would be stopping it doing it’s job. You need to confirm the water pump is working for same reason. Check the oil for water and water for oil in case you have a blown head gasket. Check water for combustion gases (same reason). Do a coolant pressure test (Leak down) and compression test. Most garages should be able to perform these tasks but you are better of going to someone who is familiar with Z’s. The overflow isn’t designed to hold pressure so it’s not an expansion tank… just an overflow. Best of luck.
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