It may be that the mayo has formed because the car has been habitually used for short journeys only since its last
I agree that you should have a compression test and if that shows it to be ok, top up your coolant then give it a run with an engine flush (I used a litre of paraffin last time I needed to flush an engine, but if you're not comfortable with this, there are proprietary products that you can buy, so use your discretion); anyway, after giving it a run and while the engine is still hot, drain away the
and replace the filter; BE CAREFUL you don't burn yourself. If you go to a mechanic to do this, get them to blow some compressed air through the smaller pipes to make sure they are not clogged with coagulated
before they drain it (and they must do it while still hot or the sludge will remain).
I hope this is of some help, and BTW, nearly all cars emit steam and condensed water this time of the year; the question is how much yours is spewing out. Do you get any blowback from your radiator or header-tank when the engine is running and cap is off? That's another indicator that the head gasket needs replacing.
And you will also see
traces in the header with the coolant.