1.6 Focus Overpressurising

hi,

I bought a 1.6 petrol focus (high mileage zetec se engine) on the cheap with a head gasket fault. I removed the head, had it pressure checked and skimmed, replaced thermostat, belt, pump etc. Rad doesnt appear to be blocked.

On running it up, it does nt overheat, fan kicks in, gauge doesnt move above mid way. It emits a lot of white smoke if held at 4k rpm, but when driven, far less smoke is kicked out. When I switch of the engine you can hear the top pipe that runs from the back of the head to the expansion bottle bubbling for a good time afterwards. Also tried a combustion leak test on the expansion bottle. Ran fine with it on for about 5 mins (no colour change) then suddenly pressurised up, the colour changed as coolant got in to bottle (I appreciate these tests arent always foolproof). Didnt overheat though and ran fine 5 mins after. I doubt its related but I did notice that I have a very small coolant leak from the housing below the coil pack and have ordered a new gasket.

I suspect I am going to have to remove the head again due to the white smoke, but does anybody have any other suggestions first as to common faults as I've had little to do with this engine in terms of cylinder heads. Is there anything in particular I should be looking out for? are cracked cylinder heads a common occurance? (although I would of thought the pressure check would of showed that up?) I havent touched the egr valve but does it utilise the coolant system on this engine as it does on some other engines?

Sorry for such a long first post, hopefully once its sorted I can enjoy my focus, as I think they are great cars