wot causes engine 2 pressurise immediately?

just fitted cylinder head to a renault kangoo 51 plate diesel van and the engine pressurised immediately.it had been tested and was deemed ok.it had a fibre gasket b4 but we replaced it wiv a metal 1 would this make any difference,we let the engine idle for approx 20 mins thinkin there could be an airlock and waited till fan kicked in but still no joy.help please.