Hi, so brought a old 51 reg 3 series petrol. Checked oil cap no milky residue or any signs of mixing coolant. No coolant loss or power loss. But white smoke for the first 5/10 minutes and a fuel smell in the car. Could this be head gasket problem? 

Hi Eyvon

Welcome to the Forum

You have none of the normal gasket indicators as you say in your post No mayonnaise in the oil filler cap No signs of oil in the coolant No coolant loss.

The white smoke can be condensation but the smell of fuel, that could be over fuelling or a leak or both. Possibly injector seals? or a fuel line perished I would start the hunt there checking the fuel system for any sign of leaks particularly where it can drip on the manifold

Dave