Hijet 1.3 16valve Overheating

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I too have caught the overheating bug. Started to happen after long journey when stuck in traffic. Engine runs well but still overheating after a while. So far I've replaced thermostat, 2 pressure caps, water pump which didn't need changing. I've used 2 bottles of "K seal." then I've tried flushing. I've bled it umpteen times. I've even removed the centre of the thermostat to ensure a good flow to rad. This seems ok. No cold spots on rad, electric fan kicks in ok but still it will discharge into reserve tank. Have suspected head gasket blowing into water port. No imullsion in the oil. No steam from exhaust apart from initial warm up. How common are head gasket failures and any tips please. I've rebuilt engines before but never one stuck under a bulkhead between the seats. Any other suggestion please.