Vauxhall Vectra smoking

If your talking about white smoke at exhaust tailpipe , sounds normal until engine reaches working temp and condensation dried up , especially in cold /damp weather .If not that , unburnt fuel .

I can be driving for half a day before it happens the temp can be at 90 then it can suddenly drop and it smokes any way of fixing problem ???

Have you had a look at this similar problem:

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If worried you should get a block test check carried out to eliminate head / head gasket . That's if the smoke is steam and any sign of coolant level dropping .

Could also try bleeding cooling system in case of air lock which can be another sign of head gasket .